path: root/docs/refguide.xml

<indexterm><primary>man page</primary><see>reference guide</see></indexterm>
<indexterm class="startofrange" id="man-nmap1-indexterm"><primary>reference guide (man page)</primary></indexterm>
<refentry id='man-nmap1'>
  <refmeta>
    <refentrytitle>nmap</refentrytitle>
    <manvolnum>1</manvolnum>
    <refmiscinfo class="source">Nmap</refmiscinfo>
    <refmiscinfo class="manual">Nmap Reference Guide</refmiscinfo>
  </refmeta>
  <refnamediv id='man-name'>
    <refname>nmap</refname>
    <refpurpose>Network exploration tool and security / port scanner</refpurpose>
  </refnamediv>
  <!-- body begins here -->
  <refsynopsisdiv id='man-synopsis'>
    <cmdsynopsis>
      <command>nmap</command>
      <arg choice='opt' rep='repeat'>
        <replaceable>Scan Type</replaceable>
      </arg>
      <arg choice='opt'>
        <replaceable>Options</replaceable>
      </arg>
      <arg choice='req'>
        <replaceable>target specification</replaceable>
      </arg>
    </cmdsynopsis>
  </refsynopsisdiv>
  <refsect1 id='man-description'>
    <title>Description</title>
    <indexterm><primary>Nmap</primary><secondary>description of</secondary></indexterm>
    <web>
    <note><para>This document describes the very latest version of
    Nmap available from <ulink
    url="https://nmap.org/download.html" /> or <ulink
    url="https://nmap.org/dist/?C=M&amp;O=D" />.  Please
    ensure you are using the latest version before reporting that a
    feature doesn't work as described.</para></note>
    </web>
    <para>Nmap (<quote>Network Mapper</quote>) is an open source tool for network
    exploration and security auditing.  It was designed to rapidly
    scan large networks, although it works fine against single
    hosts. Nmap uses raw IP packets in novel ways to determine what
    hosts are available on the network, what services (application
    name and version) those hosts are offering, what operating systems
    (and OS versions) they are running, what type of packet
    filters/firewalls are in use, and dozens of other
    characteristics.  While Nmap is commonly used for security audits,
    many systems and network administrators find it useful for routine
    tasks such as network inventory, managing service upgrade
    schedules, and monitoring host or service uptime.</para>

    <para>The output from Nmap is a list of scanned targets, with
    supplemental information on each depending on the options
    used. Key among that information is the <quote>interesting ports
    table</quote>.<indexterm><primary>ports</primary><secondary sortas="interesting">&ldquo;interesting&rdquo;</secondary></indexterm>
    That table lists the port number and protocol,
    service name, and state.  The state is either
    <literal>open</literal>, <literal>filtered</literal>,
    <literal>closed</literal>, or <literal>unfiltered</literal>.
    <literal>Open</literal><indexterm><primary><literal>open</literal> port state</primary></indexterm>
    means that an application on the target machine is listening for
    connections/packets on that port.
    <literal>Filtered</literal><indexterm><primary><literal>filtered</literal> port state</primary></indexterm>
    means that a firewall, filter, or other network
    obstacle is blocking the port so that Nmap cannot tell whether it is
    <literal>open</literal> or <literal>closed</literal>.
    <literal>Closed</literal><indexterm><primary><literal>closed</literal> port state</primary></indexterm>
    ports have no application listening on them,
    though they could open up at any time.
    Ports are classified as
    <literal>unfiltered</literal><indexterm><primary><literal>unfiltered</literal> port state</primary></indexterm>
    when they are
    responsive to Nmap's probes, but Nmap cannot determine whether they are
    open or closed.
    Nmap reports the state combinations
    <literal>open|filtered</literal><indexterm><primary><literal>open|filtered</literal> port state</primary></indexterm>
    and <literal>closed|filtered</literal><indexterm><primary><literal>closed|filtered</literal> port state</primary></indexterm>
    when it cannot determine which
    of the two states describe a port.  The port table may also
    include software version details when version detection has been
    requested.  When an IP protocol scan is requested
    (<option>-sO</option>), Nmap provides information on supported IP
    protocols rather than listening ports.</para>

    <para>In addition to the interesting ports table, Nmap can provide
    further information on targets, including reverse DNS names,
    operating system guesses, device types, and MAC addresses.</para>

    <para>A typical Nmap scan is shown in <xref
    linkend="man-ex-repscan" xrefstyle="select: label nopage" />.  The only Nmap arguments used in
    this example are <option>-A</option>, to enable OS and version
    detection, script scanning, and traceroute; <option>-T4</option> for
    faster execution; and then the hostname.</para>

<example id="man-ex-repscan"><title>A representative Nmap scan</title>
<indexterm><primary><option>-A</option></primary><secondary>example of</secondary></indexterm>
<screen>
# <userinput>nmap -A -T4 scanme.nmap.org</userinput>

Nmap scan report for scanme.nmap.org (74.207.244.221)
Host is up (0.029s latency).
rDNS record for 74.207.244.221: li86-221.members.linode.com
Not shown: 995 closed ports
PORT     STATE    SERVICE     VERSION
22/tcp   open     ssh         OpenSSH 5.3p1 Debian 3ubuntu7 (protocol 2.0)
| ssh-hostkey: 1024 8d:60:f1:7c:ca:b7:3d:0a:d6:67:54:9d:69:d9:b9:dd (DSA)
|_2048 79:f8:09:ac:d4:e2:32:42:10:49:d3:bd:20:82:85:ec (RSA)
80/tcp   open     http        Apache httpd 2.2.14 ((Ubuntu))
|_http-title: Go ahead and ScanMe!
646/tcp  filtered ldp
1720/tcp filtered H.323/Q.931
9929/tcp open     nping-echo  Nping echo
Device type: general purpose
Running: Linux 2.6.X
OS CPE: cpe:/o:linux:linux_kernel:2.6.39
OS details: Linux 2.6.39
Network Distance: 11 hops
Service Info: OS: Linux; CPE: cpe:/o:linux:kernel

TRACEROUTE (using port 53/tcp)
HOP RTT      ADDRESS
[Cut first 10 hops for brevity]
11  17.65 ms li86-221.members.linode.com (74.207.244.221)

Nmap done: 1 IP address (1 host up) scanned in 14.40 seconds
</screen>
</example>

<!-- This para is a bit jumbled together for man page rendering reasons -->
<para>The newest version of Nmap can be obtained from
<ulink url="https://nmap.org" />.  The newest version of this man page
is available at <ulink url="https://nmap.org/book/man.html"/>.
<notbook>It is also included as a chapter of <web><ulink url="https://nmap.org/book/"><citetitle>Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning</citetitle></ulink>.</web><notweb><citetitle>Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning</citetitle> (see <ulink url="https://nmap.org/book/"/>).</notweb></notbook>
</para>

  </refsect1>

  <refsect1 id='man-briefoptions'>
    <title>Options Summary</title>

<para>This options summary is printed when Nmap is run
with no arguments, and the latest version is always available at
<ulink url="https://svn.nmap.org/nmap/docs/nmap.usage.txt" />.
It helps people remember the most common options, but is no
substitute for the in-depth documentation in the rest of this
manual.  Some obscure options aren't even included here.</para>

<!-- sortas="#" puts it before the entries that start with '-' in the options
     section. -->
<indexterm class="startofrange" id="nmap-usage-indexterm"><primary sortas="#">summary of options</primary></indexterm>
<indexterm class="startofrange" id="nmap-usage-nmap-indexterm"><primary>command-line options</primary><secondary>of Nmap</secondary></indexterm>
&nmap-usage;
<indexterm class="endofrange" startref="nmap-usage-nmap-indexterm"/>
<indexterm class="endofrange" startref="nmap-usage-indexterm"/>

  </refsect1>

  <refsect1 id='man-target-specification'>
    <title>Target Specification</title>
    <indexterm><primary>target specification</primary></indexterm>

<para>Everything on the Nmap command-line that isn't an option (or
option argument) is treated as a target host specification.  The
simplest case is to specify a target IP address or hostname for scanning.</para>

<para>When a hostname is given as a target, it is
  <firstterm>resolved</firstterm><indexterm><primary>DNS resolution</primary></indexterm>
  via the Domain Name System (DNS) to determine the IP address to scan. If the
  name resolves to more than one IP address, only the first one will be
  scanned. To make Nmap scan all the resolved addresses instead of only the
  first one, use the <option>--resolve-all</option> option.
</para>

<para>Sometimes you wish to scan a whole network of adjacent hosts. For
this, Nmap supports CIDR-style<indexterm><primary>CIDR (Classless
Inter-Domain Routing)</primary></indexterm> addressing.  You can append
<literal>/<replaceable>numbits</replaceable></literal> to an IP
address or hostname and Nmap will scan every IP address for which the
first <replaceable>numbits</replaceable> are the same as for the
reference IP or hostname given. For example,
<literal>192.168.10.0/24</literal> would scan the 256 hosts
between 192.168.10.0
(binary: <literal>11000000 10101000 00001010 00000000</literal>)
and 192.168.10.255
(binary: <literal>11000000 10101000 00001010 11111111</literal>),
inclusive.
<literal>192.168.10.40/24</literal> would scan exactly the same targets. Given
that the host
scanme.nmap.org<indexterm><primary>scanme.nmap.org</primary></indexterm>
is at the IP address 64.13.134.52, the specification
<literal>scanme.nmap.org/16</literal> would scan the 65,536 IP addresses
between 64.13.0.0 and 64.13.255.255. The smallest allowed value is
<literal>/0</literal>, which targets the whole Internet. The largest
value for IPv4 is <literal>/32</literal>, which scans just the named host or IP
address because all address bits are fixed. The largest value for IPv6 is
<literal>/128</literal>, which does the same thing.</para>

<indexterm><primary>address ranges</primary></indexterm>
<para>CIDR notation is short but not always flexible enough.  For example, you
might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or
.255 because they may be used as subnet network and broadcast addresses.  Nmap supports
this through octet range addressing.  Rather than specify a normal IP
address, you can specify a comma-separated list of numbers or ranges
for each octet.  For example, <literal>192.168.0-255.1-254</literal> will skip all
addresses in the range that end in .0 or .255, and <literal>192.168.3-5,7.1</literal> will
scan the four addresses 192.168.3.1, 192.168.4.1, 192.168.5.1, and
192.168.7.1. Either side of a range may be omitted; the default values
are 0 on the left and 255 on the right. Using <literal>-</literal> by
itself is the same as <literal>0-255</literal>, but remember to use
<literal>0-</literal> in the first octet
so the target specification doesn't look like a command-line option.
Ranges need not be limited to the final octets: the specifier
<literal>0-255.0-255.13.37</literal> will perform an Internet-wide scan for all IP
addresses ending in 13.37.  This sort of broad sampling can be useful
for Internet surveys and research.</para>

<indexterm><primary>IPv6</primary></indexterm>
<para>IPv6 addresses can be specified by their fully qualified IPv6
address or hostname or with CIDR notation for subnets.  Octet ranges
aren't yet supported for IPv6.</para>

<indexterm><primary>link-local IPv6 address</primary><see>IPv6 address, link-local</see></indexterm>
<indexterm><primary>IPv6 address</primary><secondary>link-local</secondary></indexterm>
<indexterm><primary>zone ID (IPv6 address)</primary></indexterm>
<para>
IPv6 addresses with non-global scope need to have a zone ID suffix. On
Unix systems, this is a percent sign followed by an interface name; a
complete address might be <literal>fe80::a8bb:ccff:fedd:eeff%eth0</literal>.
On Windows, use an interface index number in place of an interface name:
<literal>fe80::a8bb:ccff:fedd:eeff%1</literal>. You can see a list of
interface indexes by running the command
<command>netsh.exe interface ipv6 show interface</command>.
</para>

<para>Nmap accepts multiple host specifications on the command line,
and they don't need to be the same type.  The command <command>nmap
scanme.nmap.org 192.168.0.0/8 10.0.0,1,3-7.-</command> does what
you would expect.</para>

<para>While targets are usually specified on the command lines, the following options are also available to control target selection:</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-iL <replaceable>inputfilename</replaceable></option> (Input from list)
          <indexterm><primary><option>-iL</option></primary></indexterm>
          <indexterm><primary>target specification</primary><secondary>from list</secondary></indexterm>
        </term>
        <listitem>
          <para>Reads target specifications from
          <replaceable>inputfilename</replaceable>.  Passing a huge
          list of hosts is often awkward on the command line, yet it
          is a common desire.  For example, your DHCP server might
          export a list of 10,000 current leases that you wish to
          scan.  Or maybe you want to scan all IP addresses
          <emphasis>except</emphasis> for those to locate hosts using
          unauthorized static IP addresses.  Simply generate the list
          of hosts to scan and pass that filename to Nmap as an
          argument to the <option>-iL</option> option.  Entries can be
          in any of the formats accepted by Nmap on the command line
          (IP address, hostname, CIDR, IPv6, or octet ranges).  Each
          entry must be separated by one or more spaces, tabs, or
          newlines.  You can specify a hyphen (<literal>-</literal>)
          as the filename if you want Nmap to read hosts from standard
          input rather than an actual file.</para>
          <para>The input file may contain comments that start with
          <literal>#</literal> and extend to the end of the line.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-iR <replaceable>num hosts</replaceable></option> (Choose random targets)
          <indexterm><primary><option>-iR</option></primary></indexterm>
          <indexterm><primary>random targets</primary></indexterm>
          <indexterm><primary>target specification</primary><secondary>at random</secondary></indexterm>
        </term>
        <listitem>
          <para>For Internet-wide surveys
          and other research, you may want to choose targets at
          random.  The <replaceable>num hosts</replaceable> argument
          tells Nmap how many IPs to generate.  Undesirable IPs such
          as those in certain private, multicast, or unallocated
          address ranges are automatically skipped.  The argument <literal>0</literal>
          can be specified for a never-ending scan.  Keep in mind that
          some network administrators bristle at unauthorized scans of
          their networks and may complain.  Use this option at your
          own risk!  If you find yourself really bored one rainy
          afternoon, try the command
          <command>nmap -Pn -sS -p 80 -iR 0 --open</command><indexterm><primary><option>-sS</option></primary><secondary>example of</secondary></indexterm><indexterm><primary><option>-PS</option></primary><secondary>example of</secondary></indexterm><indexterm><primary><option>-iR</option></primary><secondary>example of</secondary></indexterm><indexterm><primary><option>--open</option></primary><secondary>example of</secondary></indexterm>
          to locate random web servers for browsing.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--exclude
          <replaceable>host1</replaceable><optional>,<replaceable>host2</replaceable><optional>,...</optional></optional></option> (Exclude hosts/networks)
          <indexterm significance="preferred"><primary><option>--exclude</option></primary></indexterm>
          <indexterm><primary>excluding targets</primary></indexterm>
        </term>
        <listitem>
          <para>Specifies a comma-separated list of targets to be
          excluded from the scan even if they are part of the overall
          network range you specify.  The list you pass in uses normal
          Nmap syntax, so it can include hostnames, CIDR netblocks,
          octet ranges, etc.  This can be useful when the network you
          wish to scan includes untouchable mission-critical servers,
          systems that are known to react adversely to port scans,
          or subnets administered by other people.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--excludefile <replaceable>exclude_file</replaceable></option> (Exclude list from file)
          <indexterm significance="preferred"><primary><option>--excludefile</option></primary></indexterm>
        </term>
        <listitem>
          <para>This offers the same functionality as the <option>--exclude</option>
          option, except that the excluded targets are provided in a
          newline-, space-, or tab-delimited
          <replaceable>exclude_file</replaceable> rather than on the
          command line.</para>
          <para>The exclude file may contain comments that start with
          <literal>#</literal> and extend to the end of the line.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-n</option> (No DNS resolution)
          <indexterm><primary><option>-n</option></primary></indexterm>
        </term>
        <listitem>

          <para><indexterm><primary>reverse DNS</primary><secondary>disabling with <option>-n</option></secondary></indexterm>
          Tells Nmap to <emphasis>never</emphasis> do reverse DNS
          resolution on the active IP addresses it finds. Since
          DNS can be slow even with Nmap's built-in parallel stub
          resolver, this option can slash scanning times.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-R</option> (DNS resolution for all targets)
          <indexterm><primary><option>-R</option></primary></indexterm>
        </term>
        <listitem>
          <para>Tells Nmap to
          <emphasis>always</emphasis> do reverse DNS
          resolution on the target IP addresses. Normally reverse DNS is
          only performed against responsive (online) hosts.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--resolve-all</option> (Scan each resolved address)
          <indexterm><primary><option>--resolve-all</option></primary></indexterm>
        </term>
        <listitem>
          <para>If a hostname target resolves to more than one address, scan
            all of them. The default behavior is to only scan the first
            resolved address. Regardless, only addresses in the appropriate
            address family will be scanned: IPv4 by default, IPv6 with
            <option>-6</option>.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--unique</option> (Scan each address only once)
          <indexterm><primary><option>--unique</option></primary></indexterm>
        </term>
        <listitem>
          <para>Scan each IP address only once. The default behavior is to scan
            each address as many times as it is specified in the target list,
            such as when network ranges overlap or different hostnames resolve
            to the same address.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--system-dns</option> (Use system DNS resolver)
          <indexterm significance="preferred"><primary><option>--system-dns</option></primary></indexterm>
        </term>
        <listitem>

          <para>By default, Nmap reverse-resolves IP addresses by sending
          queries directly to the name servers configured on your host
          and then listening for responses.  Many requests (often
          dozens) are performed in parallel to improve performance.
          Specify this option to use your system resolver instead (one
          IP at a time via the <function>getnameinfo</function> call).  This is slower
          and rarely useful unless you find a bug in the Nmap parallel
          resolver (please let us know if you do).  The system
          resolver is always used for forward lookups (getting an IP address from a hostname).
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--dns-servers <replaceable>server1</replaceable><optional>,<replaceable>server2</replaceable><optional>,...</optional></optional>
          </option> (Servers to use for reverse DNS queries)
          <indexterm significance="preferred"><primary><option>--dns-servers</option></primary></indexterm>
        </term>
        <listitem>

          <para>By default, Nmap determines your DNS servers
          (for rDNS resolution) from your resolv.conf file (Unix) or
          the Registry (Win32).  Alternatively, you may use this
          option to specify alternate servers.  This option is not
          honored if you are using <option>--system-dns</option>.
          Using multiple DNS servers is often faster,
          especially if you choose authoritative servers for your
          target IP space.  This option can also improve stealth, as
          your requests can be bounced off just about any recursive
          DNS server on the Internet.</para>

          <para>This option also comes in handy when scanning private
          networks.  Sometimes only a few name servers provide
          proper rDNS information, and you may not even know where
          they are.  You can scan the network for port 53 (perhaps
          with version detection), then try Nmap list scans
          (<option>-sL</option>) specifying each name server one at a
          time with <option>--dns-servers</option> until you find one
          which works.</para>

          <para>This option might not be honored if the DNS response
          exceeds the size of a UDP packet. In such a situation our DNS
          resolver will make the best effort to extract a response from the
          truncated packet, and if not successful it will fall back to
          using the system resolver. Also, responses that contain CNAME aliases
          will fall back to the system resolver.</para>

        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id='man-host-discovery'>
    <title>Host Discovery</title>
    <indexterm class="startofrange" id="man-host-discovery-indexterm"><primary>host discovery</primary></indexterm>
 
   <para>One of the very first steps in any network reconnaissance
    mission is to reduce a (sometimes huge) set of IP ranges into a
    list of active or interesting hosts. Scanning every port of
    every single IP address is slow and usually unnecessary. Of
    course what makes a host interesting depends greatly on the
    scan purposes. Network administrators may only be interested in
    hosts running a certain service, while security auditors may
    care about every single device with an IP address. An
    administrator may be comfortable using just an ICMP ping to
    locate hosts on his internal network, while an external
    penetration tester may use a diverse set of dozens of probes in
    an attempt to evade firewall restrictions.</para>

    <para>Because host discovery needs are so diverse, Nmap offers a
    wide variety of options for customizing the techniques used. Host
    discovery is sometimes called ping scan, but it goes well beyond
    the simple ICMP echo request packets associated with the
    ubiquitous <application>ping</application> tool. Users can skip
    the discovery step entirely with a list scan (<option>-sL</option>) or
    by disabling host discovery (<option>-Pn</option>), or engage the network
    with arbitrary combinations of multi-port TCP SYN/ACK, UDP, SCTP
    INIT and ICMP probes. The goal of these probes is to solicit
    responses which demonstrate that an IP address is actually active
    (is being used by a host or network device). On many networks,
    only a small percentage of IP addresses are active at any given
    time. This is particularly common with private address space
    such as 10.0.0.0/8. That network has 16 million IPs, but I have
    seen it used by companies with less than a thousand machines. Host
    discovery can find those machines in a sparsely allocated sea of
    IP addresses.</para>

    <para>If no host discovery options are given, Nmap sends an ICMP
    echo request, a TCP SYN packet to port 443, a TCP ACK packet to
    port 80, and an ICMP timestamp request. (For IPv6, the ICMP
    timestamp request is omitted because it is not part of ICMPv6.)
    These defaults are
    equivalent to the <option>-PE -PS443 -PA80 -PP</option> options.
    The exceptions to this are the ARP<indexterm><primary>ARP ping</primary></indexterm>
    (for IPv4) and Neighbor
    Discovery<indexterm><primary>Neighbor Discovery</primary><secondary>for host discovery</secondary></indexterm>
    (for IPv6)
    scans which are used for any targets on a local ethernet network.
    For unprivileged Unix shell
    users, the default probes are a SYN packet to ports 80 and 443 using
    the <function>connect</function> system
    call.<indexterm><primary>unprivileged users</primary><secondary>limitations of</secondary></indexterm>
    This host discovery is often sufficient when scanning local
    networks, but a more comprehensive set of discovery probes is
    recommended for security auditing.</para>
    
    <para>The <option>-P*</option> options (which select
    ping types) can be combined.  You can increase your odds of
    penetrating strict firewalls by sending many probe types using
    different TCP ports/flags and ICMP codes.  Also note that
    ARP/Neighbor Discovery<indexterm><primary>ARP ping</primary></indexterm>
    is done by default against
    targets on a local Ethernet network even if you specify other
    <option>-P*</option> options, because it is almost always faster
    and more effective.</para>

    <para>By default, Nmap does host discovery and then performs a
    port scan against each host it determines is online.  This is true
    even if you specify non-default host discovery types such as UDP
    probes (<option>-PU</option>).  Read about the
    <option>-sn</option> option to learn how to perform
    only host discovery, or use <option>-Pn</option> to skip host
    discovery and port scan all target addresses.  The following options
    control host discovery:</para>

    <variablelist>

      <varlistentry>
        <term>
        <option>-sL</option> (List Scan)
          <indexterm><primary><option>-sL</option></primary></indexterm>
          <indexterm><primary>list scan</primary></indexterm>
        </term>
        <listitem>
          <para>The list scan is a degenerate form of host discovery
          that simply lists each host of the network(s) specified,
          without sending any packets to the target hosts.  By
          default, Nmap still does reverse-DNS resolution on the hosts
          to learn their names.  It is often surprising how much
          useful information simple hostnames give out.  For example,
          <literal>fw.chi</literal> is the name of one company's Chicago firewall. 
          <indexterm><primary>DNS</primary><secondary>records as source of information</secondary></indexterm>
          Nmap also reports the total number of
          IP addresses at the end.  The list scan is a good sanity
          check to ensure that you have proper IP addresses for your
          targets.  If the hosts sport domain names you do not
          recognize, it is worth investigating further to prevent
          scanning the wrong company's network.</para>

          <para>Since the idea is to simply print a list of target
          hosts, options for higher level functionality such as port
          scanning, OS detection, or host discovery cannot be combined
          with this.  If you wish to disable host discovery while still
          performing such higher level functionality, read up on the
          <option>-Pn</option> (skip host discovery) option.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sn</option> (No port scan)
        <indexterm><primary><option>-sn</option></primary></indexterm>
        <indexterm><primary>host discovery</primary></indexterm>
        <indexterm><primary>ping scan</primary></indexterm>
        <indexterm><primary>port scan</primary><secondary>disabling with <option>-sn</option></secondary></indexterm>
        </term>
        <listitem>
           <para>This option tells Nmap not to do a port scan after host
           discovery, and only print out the available hosts that
           responded to the host discovery probes.  This is often known as a <quote>ping
           scan</quote>, but you can also request that traceroute and
           NSE host scripts be run. This is by default
           one step more intrusive than the list scan, and can often
           be used for the same purposes.  It allows light
           reconnaissance of a target network without attracting much
           attention.  Knowing how many hosts are up is more valuable
           to attackers than the list provided by list scan of every
           single IP and host name.</para>

           <para>Systems administrators often find this option
           valuable as well.  It can easily be used to count available
           machines on a network or monitor server availability.  This
           is often called a ping sweep, and is more reliable than
           pinging the broadcast address because many hosts do not
           reply to broadcast queries.</para>

           <para>The default host discovery done with <option>-sn</option> consists of an ICMP echo
           request, TCP SYN to port 443, TCP ACK to port 80, and an ICMP
           timestamp request by default.  When
           executed by an unprivileged user, only SYN packets are sent
           (using a <function>connect</function> call) to ports 80 and 443 on
           the target.  When a privileged user tries to scan targets
           on a local ethernet network, ARP requests
           are used unless
           <option>--send-ip</option> was specified.
           The <option>-sn</option> option can be combined with any of the
           discovery probe types (the <option>-P*</option> options) for greater flexibility.
           If any of those probe type and port number options are
           used, the default probes are
           overridden.  When strict firewalls are in place between the
           source host running Nmap and the target network, using
           those advanced techniques is recommended.  Otherwise hosts
           could be missed when the firewall drops probes or their
           responses.</para>

           <para>In previous releases of Nmap, <option>-sn</option> was
           known as
           <option>-sP</option>.<indexterm><primary><option>-sP</option></primary><seealso><option>-sn</option></seealso></indexterm></para>

        </listitem>
      </varlistentry>


      <varlistentry>
        <term>
          <option>-Pn</option> (No ping)
          <indexterm><primary><option>-Pn</option></primary></indexterm>
          <indexterm><primary>host discovery</primary><secondary>disabling</secondary></indexterm>
        </term>
        <listitem>
          <para>This option skips the host discovery stage altogether.
          Normally, Nmap uses this stage to determine active machines
          for heavier scanning and to gauge the speed of the network.
          By default, Nmap only performs heavy
          probing such as port scans, version detection, or OS
          detection against hosts that are found to be up.  Disabling
          host discovery with <option>-Pn</option> causes Nmap to
          attempt the requested scanning functions against
          <emphasis>every</emphasis> target IP address specified.  So
          if a /16 sized network is specified
          on the command line, all 65,536 IP addresses are scanned.
          Proper host discovery is skipped as with the list scan, but
          instead of stopping and printing the target list, Nmap
          continues to perform requested functions as if each target
          IP is active. Default timing parameters are used, which may result in
          slower scans. To skip host discovery <emphasis>and</emphasis> port
          scan, while still allowing NSE to run, use the two options
          <option>-Pn -sn</option> together.</para>

          <para>For machines on a local ethernet network, ARP
          scanning will still be performed (unless
          <option>--disable-arp-ping</option> or <option>--send-ip</option> is specified) because Nmap needs
          MAC addresses to further scan target hosts. In previous
          versions of Nmap, <option>-Pn</option> was
          <option>-P0</option><indexterm><primary><option>-P0</option></primary><seealso><option>-Pn</option></seealso></indexterm>
          and
          <option>-PN</option>.<indexterm><primary><option>-PN</option></primary><seealso><option>-Pn</option></seealso></indexterm>
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PS <replaceable>port list</replaceable></option> (TCP SYN Ping)
          <indexterm><primary><option>-PS</option></primary></indexterm>
          <indexterm><primary>SYN ping</primary></indexterm>
        </term>
        <listitem>

          <para>This option sends an empty TCP packet with the SYN
          flag set.  The default destination port is 80 (configurable
          at compile time by changing <varname>DEFAULT_TCP_PROBE_PORT_SPEC</varname><indexterm><primary><varname>DEFAULT_TCP_PROBE_PORT_SPEC</varname></primary></indexterm>
          in <filename>nmap.h</filename>).<indexterm><primary><filename>nmap.h</filename></primary></indexterm>
          Alternate ports can be
          specified as a parameter. The syntax is the same as for the
          <option>-p</option> except that port type specifiers like
          <literal>T:</literal> are not allowed. Examples are
          <option>-PS22</option> and
          <option>-PS22-25,80,113,1050,35000</option>. Note that there
          can be no space between <option>-PS</option> and the port
          list. If multiple probes are specified they will be sent in
          parallel.</para>

          <para>The SYN flag suggests to the remote system that you
          are attempting to establish a connection.  Normally the
          destination port will be closed, and a RST (reset) packet
          sent back.  If the port happens to be open, the target will
          take the second step of a TCP
	  three-way-handshake<indexterm><primary>three-way handshake</primary></indexterm>
          by responding
          with a SYN/ACK TCP packet. The machine running Nmap then
          tears down the nascent connection by responding with a RST
          rather than sending an ACK packet which would complete the
          three-way-handshake and establish a full
          connection.  The RST packet is sent by the
          kernel of the machine running Nmap in response to the
          unexpected SYN/ACK, not by Nmap itself.</para>

          <para>Nmap does not care whether the port is open or closed.
          Either the RST or SYN/ACK response discussed previously tell
          Nmap that the host is available and responsive.</para>

          <para>On Unix boxes, only the privileged user
          <literal>root</literal><indexterm><primary>privileged users</primary></indexterm>
	  is generally able to send and receive
	  raw TCP packets.<indexterm><primary>raw packets</primary></indexterm>
          For unprivileged users, a
          workaround is automatically employed<indexterm><primary>unprivileged users</primary><secondary>limitations of</secondary></indexterm>
	  whereby the <function>connect</function> system call is initiated against each
	  target port. This has
          the effect of sending a SYN packet to the target host, in an
          attempt to establish a connection.  If <function>connect</function> returns
          with a quick success or an ECONNREFUSED failure, the
          underlying TCP stack must have received a SYN/ACK or RST and
          the host is marked available.  If the connection attempt
          is left hanging until a timeout is reached, the host is
          marked as down.
          </para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PA <replaceable>port list</replaceable></option> (TCP ACK Ping)
          <indexterm><primary><option>-PA</option></primary></indexterm>
          <indexterm><primary>ACK ping</primary></indexterm>
        </term>
        <listitem>
          <para>The TCP ACK ping is quite similar to the
          just-discussed SYN ping.  The difference, as you could
          likely guess, is that the TCP ACK flag is set instead of the
          SYN flag.  Such an ACK packet purports to be acknowledging
          data over an established TCP connection, but no such
          connection exists.  So remote hosts should always respond
          with a RST packet, disclosing their existence in the
          process.</para>

          <para>The <option>-PA</option> option uses the same default
          port as the SYN probe (80) and can also take a list of
          destination ports in the same format.  If an unprivileged
          user tries this, the
          <function>connect</function> workaround discussed previously is used.  This
          workaround is imperfect because <function>connect</function> is actually
          sending a SYN packet rather than an ACK.</para>

          <para>The reason for offering both SYN and ACK ping probes
          is to maximize the chances of bypassing firewalls.  Many
          administrators configure routers and other simple firewalls
          to block incoming SYN packets except for those destined for
          public services like the company web site or mail server.
          This prevents other incoming connections to the
          organization, while allowing users to make unobstructed
          outgoing connections to the Internet.  This non-stateful
          approach takes up few resources on the firewall/router and
          is widely supported by hardware and software filters.  The
          Linux Netfilter/iptables<indexterm><primary>iptables</primary></indexterm>
          firewall software offers the
          <option>--syn</option> convenience option to implement this
          stateless approach.  When stateless firewall rules such as
          this are in place, SYN ping probes (<option>-PS</option>)
          are likely to be blocked when sent to closed target ports.
          In such cases, the ACK probe shines as it cuts right through
          these rules.</para>

          <para>Another common type of firewall uses stateful rules
          that drop unexpected packets.  This feature was initially
          found mostly on high-end firewalls, though it has become
          much more common over the years.  The Linux
          Netfilter/iptables system supports this through the
          <option>--state</option> option, which categorizes packets
          based on connection state.  A SYN probe is more likely to
          work against such a system, as unexpected ACK packets are
          generally recognized as bogus and dropped.  A solution to this quandary is
          to send both SYN and ACK probes by specifying
          <option>-PS</option> and <option>-PA</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PU <replaceable>port list</replaceable></option> (UDP Ping)
          <indexterm><primary><option>-PU</option></primary></indexterm>
          <indexterm><primary>UDP ping</primary></indexterm>
        </term>
        <listitem>
          <indexterm><primary>payloads, protocol-specific</primary><see>protocol-specific payloads</see></indexterm>

          <para>Another host discovery option is the UDP ping, which
          sends a UDP packet to the given ports. For most ports, the
          packet will be empty, though some use a protocol-specific
          payload that is more likely to elicit a
          response.
          <indexterm><primary>protocol-specific payloads</primary><secondary>UDP</secondary></indexterm>
          The payloads are the same probes used in service and version
          detection and are defined in the
          <filename>nmap-service-probes</filename>
          <indexterm><primary><filename>nmap-service-probes</filename></primary></indexterm>
          file.  Packet content can also be affected with the
          <option>--data</option>, <option>--data-string</option>, and
          <option>--data-length</option> options.</para>

          <para>The port list
          takes the same format as with the previously discussed
          <option>-PS</option> and <option>-PA</option> options.  If
          no ports are specified, the default is
          40125.<indexterm><primary>UDP scan</primary><secondary>default port of</secondary></indexterm>
          This default
          can be configured at compile-time by changing
          <varname>DEFAULT_UDP_PROBE_PORT_SPEC</varname><indexterm><primary><varname>DEFAULT_UDP_PROBE_PORT_SPEC</varname></primary></indexterm>
          in <filename>nmap.h</filename>.<indexterm><primary><filename>nmap.h</filename></primary></indexterm>
          A highly uncommon port is used by default because sending to
          open ports is often undesirable for this particular scan
          type.</para>

          <para>Upon hitting a closed port on the target machine, the
          UDP probe should elicit an ICMP port unreachable packet in
          return.  This signifies to Nmap that the machine is up and
          available.  Many other types of ICMP errors, such as
          host/network unreachables or TTL exceeded are indicative of
          a down or unreachable host.  A lack of response is also
          interpreted this way.  If an open port is reached, most
          services simply ignore the empty packet and fail to return
          any response.  This is why the default probe port is 40125,
          which is highly unlikely to be in use.  A few services, such
          as the Character Generator (chargen) protocol, will respond to an empty UDP packet, and thus
          disclose to Nmap that the machine is available.</para>

          <para>The primary advantage of this scan type is that it
          bypasses firewalls and filters that only screen TCP.  For
          example, I once owned a Linksys BEFW11S4 wireless broadband
          router.  The external interface of this device filtered all
          TCP ports by default, but UDP probes would still elicit port
          unreachable messages and thus give away the device.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PY <replaceable>port list</replaceable></option> (SCTP INIT Ping)
          <indexterm><primary><option>-PY</option></primary></indexterm>
          <indexterm><primary>SCTP INIT ping</primary></indexterm>
        </term>
        <listitem>

          <para>This option sends an SCTP packet containing a minimal
          INIT chunk.  The default destination port is 80 (configurable
          at compile time by changing
          <varname>DEFAULT_SCTP_PROBE_PORT_SPEC</varname><indexterm><primary><varname>DEFAULT_SCTP_PROBE_PORT_SPEC</varname></primary></indexterm>
          in <filename>nmap.h</filename>).
          Alternate ports can be specified as a parameter. The syntax
          is the same as for the
          <option>-p</option> except that port type specifiers like
          <literal>S:</literal> are not allowed. Examples are
          <option>-PY22</option> and
          <option>-PY22,80,179,5060</option>. Note that there
          can be no space between <option>-PY</option> and the port
          list. If multiple probes are specified they will be sent in
          parallel.</para>

          <para>The INIT chunk suggests to the remote system that you
          are attempting to establish an association.  Normally the
          destination port will be closed, and an ABORT chunk will be
          sent back.  If the port happens to be open, the target will
          take the second step of an SCTP
          four-way-handshake<indexterm><primary>four-way handshake</primary></indexterm>
          by responding with an INIT-ACK chunk. If the machine running
          Nmap has a functional SCTP stack, then it tears down the
          nascent association by responding with an ABORT chunk rather
          than sending a COOKIE-ECHO chunk which would be the next step
          in the four-way-handshake.  The ABORT packet is sent by the
          kernel of the machine running Nmap in response to the
          unexpected INIT-ACK, not by Nmap itself.</para>

          <para>Nmap does not care whether the port is open or closed.
          Either the ABORT or INIT-ACK response discussed previously tell
          Nmap that the host is available and responsive.</para>

          <para>On Unix boxes, only the privileged user
          <literal>root</literal><indexterm><primary>privileged users</primary></indexterm>
          is generally able to send and receive raw SCTP
          packets.<indexterm><primary>raw packets</primary></indexterm>
          Using SCTP INIT Pings is currently not possible for unprivileged
          users.<indexterm><primary>unprivileged users</primary><secondary>limitations of</secondary></indexterm>
          </para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PE</option>;
        <option>-PP</option>;
        <option>-PM</option> (ICMP Ping Types)
          <indexterm><primary><option>-PE</option></primary></indexterm>
          <indexterm><primary><option>-PP</option></primary></indexterm>
          <indexterm><primary><option>-PM</option></primary></indexterm>
          <indexterm><primary>ICMP ping</primary></indexterm>
        </term>
        <listitem>

          <para>In addition to the unusual TCP, UDP and SCTP host
          discovery types discussed previously, Nmap can send the
          standard packets sent by the ubiquitous
          <application>ping</application> program.  Nmap sends an ICMP
          type 8 (echo request) packet to the target IP addresses,
          expecting a type 0 (echo reply) in return from available
          hosts.<indexterm><primary>ICMP echo</primary></indexterm>
          Unfortunately for network explorers, many hosts and
          firewalls now block these packets, rather than responding as
          required by <ulink
          role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc1122.txt">RFC
          1122</ulink>.<indexterm><primary>RFC 1122</primary></indexterm>
          For this reason, ICMP-only scans are rarely
          reliable enough against unknown targets over the Internet.
          But for system administrators monitoring an internal
          network, they can be a practical and efficient approach.
          Use the <option>-PE</option> option to enable this echo
          request behavior.</para>

          <para>While echo request is the standard ICMP ping query,
          Nmap does not stop there.  The ICMP standards
          (<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc792.txt">RFC 792</ulink><indexterm><primary>RFC 792</primary></indexterm>
          and
          <ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc950.txt">RFC 950</ulink><indexterm><primary>RFC 950</primary></indexterm>
          )
          also specify timestamp request, information
          request, and address mask request packets as codes 13, 15,
          and 17, respectively.  While the ostensible purpose for
          these queries is to learn information such as address masks
          and current times, they can easily be used for host
          discovery.  A system that replies is up and available.  Nmap
          does not currently implement information request packets, as
          they are not widely supported.  RFC 1122 insists that
          <quote>a host SHOULD NOT implement these messages</quote>.
          Timestamp and address mask queries can be sent with the
          <option>-PP</option> and <option>-PM</option> options,
          respectively.  A timestamp reply (ICMP code 14) or address
          mask reply (code 18) discloses that the host is available.
          These two queries can be valuable when administrators specifically
          block echo request packets while forgetting that other ICMP
          queries can be used for the same purpose.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PO <replaceable>protocol list</replaceable></option> (IP Protocol Ping)
          <indexterm><primary><option>-PO</option></primary></indexterm>
          <indexterm><primary>IP protocol ping</primary></indexterm>
        </term>
        <listitem>

          <para>One of the newer host discovery options is the IP protocol ping,
          which sends IP packets with the specified protocol number
          set in their IP header.  The protocol list
          takes the same format as do port lists in the
          previously discussed TCP, UDP and SCTP host discovery options.
          If no protocols are specified, the default is to send multiple
          IP packets for ICMP (protocol 1), IGMP (protocol 2), and
          IP-in-IP (protocol 4).  The default protocols can be
          configured at compile-time by changing
          <varname>DEFAULT_PROTO_PROBE_PORT_SPEC</varname><indexterm><primary><varname>DEFAULT_PROTO_PROBE_PORT_SPEC</varname></primary></indexterm>
          in <filename>nmap.h</filename>.
          Note that for the ICMP, IGMP, TCP (protocol 6), UDP
          (protocol 17) and SCTP (protocol 132), the packets are sent
          with the proper protocol
          headers<indexterm><primary>protocol-specific payloads</primary><secondary>IP</secondary></indexterm>
          while other protocols are
          sent with no additional data beyond the IP header (unless any of
          <option>--data</option>,
          <option>--data-string</option>, or
          <option>--data-length</option>
          options are specified).</para>

          <para>This host discovery method looks for either responses
          using the same protocol as a probe, or ICMP protocol
          unreachable messages which signify that the given protocol
          isn't supported on the destination host.  Either type of
          response signifies that the target host is alive.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--disable-arp-ping</option> (No ARP or ND Ping)
          <indexterm><primary><option>--disable-arp-ping</option></primary></indexterm>
        </term>
        <listitem>

	  <para>Nmap normally does ARP or IPv6 Neighbor Discovery (ND)
	  discovery of locally connected ethernet hosts, even if other
	  host discovery options such as <option>-Pn</option> or
	  <option>-PE</option> are used. To disable this implicit
	  behavior, use the <option>--disable-arp-ping</option>
	  option.</para>

	  <para>The default behavior is normally faster, but this option
	  is useful on networks using proxy ARP, in which a router
	  speculatively replies to all ARP requests, making every target
	  appear to be up according to ARP scan.</para>

        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>--discovery-ignore-rst</option>
          <indexterm><primary><option>--discovery-ignore-rst</option></primary></indexterm>
        </term>
        <listitem>

          <para>In some cases, firewalls may spoof TCP reset (RST) replies in
            response to probes to unoccupied or disallowed addresses. Since
            Nmap ordinarily considers RST replies to be proof that the target
            is up, this can lead to wasted time scanning targets that aren't
            there. Using the <option>--discovery-ignore-rst</option> will
            prevent Nmap from considering these replies during host discovery.
            You may need to select extra host discovery options to ensure you
            don't miss targets in this case.</para>
        </listitem>
      </varlistentry>

<varlistentry>
 <term>
  <option>--traceroute</option> (Trace path to host)
   <indexterm significance="preferred"><primary><option>--traceroute</option></primary></indexterm>
   <indexterm significance="preferred"><primary>traceroute</primary></indexterm>
 </term>
 <listitem>

<para>
Traceroutes are performed post-scan using information from the scan results to determine the port and protocol most likely to reach the target. It works with all scan types except connect scans (<option>-sT</option>) and idle scans (<option>-sI</option>).  All traces use Nmap's dynamic timing model and are performed in parallel.
</para>

<para>
Traceroute works by sending packets with a low TTL (time-to-live) in an attempt to elicit ICMP Time Exceeded messages from intermediate hops between the scanner and the target host. Standard traceroute implementations start with a TTL of 1 and increment the TTL until the destination host is reached. Nmap's traceroute starts with a high TTL and then decrements the TTL until it reaches zero. Doing it backwards lets Nmap employ clever caching algorithms to speed up traces over multiple hosts. On average Nmap sends 5&ndash;10 fewer packets per host, depending on network conditions. If a single subnet is being scanned (i.e. 192.168.0.0/24) Nmap may only have to send two packets to most hosts.
</para>
</listitem>
</varlistentry>
    </variablelist>
    <indexterm class="endofrange" startref="man-host-discovery-indexterm"/>
  </refsect1>

  <refsect1 id='man-port-scanning-basics'>
    <title>Port Scanning Basics</title>

          <para>While Nmap has grown in functionality over the years,
          it began as an efficient port scanner, and that remains its
          core function.  The simple command <command>nmap
          <replaceable>target</replaceable></command> scans
          1,000 TCP ports on the host
          <replaceable>target</replaceable>.  While many port scanners
          have traditionally lumped all ports into the open or closed
          states, Nmap is much more granular.  It divides ports into
          six states: <literal>open</literal>,
          <literal>closed</literal>, <literal>filtered</literal>,
          <literal>unfiltered</literal>,
          <literal>open|filtered</literal>, or
          <literal>closed|filtered</literal>.</para>

<para>These states are not intrinsic
properties of the port itself, but describe how Nmap sees them.  For
example, an Nmap scan from the same network as the target may show
port <literal>135/tcp</literal> as open, while a scan at the same time with the same
options from across the Internet might show that port as <literal>filtered</literal>.</para>

<variablelist><title>The six port states recognized by Nmap</title>

  <varlistentry><term>open
  <indexterm><primary><literal>open</literal> port state</primary></indexterm>
  </term>
  <listitem><para>An application is actively accepting TCP
  connections, UDP datagrams or SCTP associations on this port.
  Finding these is often the primary goal of port scanning.
  Security-minded people know that each open port is an avenue for attack.
  Attackers and pen-testers want to exploit the open ports, while
  administrators try to close or protect them with firewalls without
  thwarting legitimate users.
  Open ports are also interesting for non-security scans because they show
  services available for use on the network.
  </para></listitem></varlistentry>

  <varlistentry><term>closed
  <indexterm><primary><literal>closed</literal> port state</primary></indexterm>
  </term>

  <listitem><para>A closed port is accessible (it receives and
  responds to Nmap probe packets), but there is no application
  listening on it.  They can be helpful in showing that a host is up
  on an IP address (host discovery, or ping scanning), and as part
  of OS detection.  Because closed ports are reachable, it may be
  worth scanning later in case some open up.  Administrators may want
  to consider blocking such ports with a firewall.  Then they would
  appear in the filtered state, discussed next.
  </para></listitem></varlistentry>

  <varlistentry><term>filtered
  <indexterm><primary><literal>filtered</literal> port state</primary></indexterm>
  </term>

  <listitem><para>Nmap cannot determine whether the port is open
  because packet filtering prevents its probes from reaching the port.
  The filtering could be from a dedicated firewall device, router
  rules, or host-based firewall software.  These ports frustrate
  attackers because they provide so little information.  Sometimes
  they respond with ICMP error messages such as type 3 code 13
  (destination unreachable: communication administratively
  prohibited), but filters that simply drop probes without responding
  are far more common.  This forces Nmap to retry several times just
  in case the probe was dropped due to network congestion rather than
  filtering.  This slows down the scan dramatically.</para></listitem></varlistentry>

  <varlistentry><term>unfiltered
  <indexterm><primary><literal>unfiltered</literal> port state</primary></indexterm>
  </term>
  <listitem><para>The unfiltered state means that a port is accessible,
  but Nmap is unable to determine whether it is open or closed.  Only
  the ACK scan, which is used to map firewall rulesets, classifies
  ports into this state.  Scanning unfiltered ports with other scan
  types such as Window scan, SYN scan, or FIN scan, may help resolve
  whether the port is open.
  </para></listitem></varlistentry>

  <varlistentry><term>open|filtered
  <indexterm><primary><literal>open|filtered</literal> port state</primary></indexterm>
  </term>
  <listitem><para>Nmap places ports in this state when it is unable to
  determine whether a port is open or filtered.  This occurs for scan
  types in which open ports give no response.  The lack of
  response could also mean that a packet filter dropped the probe or
  any response it elicited.  So Nmap does not know for sure whether
  the port is open or being filtered.  The UDP, IP protocol,
  FIN, NULL, and Xmas scans classify ports this
  way.</para></listitem></varlistentry>

  <varlistentry><term>closed|filtered
  <indexterm><primary><literal>closed|filtered</literal> port state</primary></indexterm>
  </term>
  <listitem><para>This state is used when Nmap is unable to determine
  whether a port is closed or filtered.  It is only used for the IP ID
  idle scan.</para></listitem></varlistentry>
  </variablelist>
</refsect1>

<refsect1 id='man-port-scanning-techniques'>
  <title>Port Scanning Techniques</title>

<para>As a novice performing automotive repair, I can struggle
for hours trying to fit my rudimentary tools (hammer, duct tape,
wrench, etc.)  to the task at hand.  When I fail miserably and tow my
jalopy to a real mechanic, he invariably fishes around in a huge tool chest until
pulling out the perfect gizmo which makes the job seem effortless.  The
art of port scanning is similar.  Experts understand the dozens of
scan techniques and choose the appropriate one (or combination) for a
given task.  Inexperienced users and
script kiddies,<indexterm><primary>script kiddies</primary></indexterm>
on the other
hand, try to solve every problem with the default SYN scan.  Since Nmap is
free, the only barrier to port scanning mastery is knowledge.  That
certainly beats the automotive world, where it may take great skill to
determine that you need a strut spring compressor, then you still
have to pay thousands of dollars for it.</para>

<para>Most of the scan types are only available to
privileged users.<indexterm><primary>privileged users</primary></indexterm>
This is because they send and receive
raw packets,<indexterm><primary>raw packets</primary></indexterm>
which requires root
access on Unix systems.  Using an administrator account on Windows is
recommended, though Nmap sometimes works for unprivileged users on that
platform when Npcap has already been loaded into the OS.  Requiring
root privileges was a serious limitation when Nmap was released in
1997, as many users only had access to shared shell accounts.  Now,
the world is different.  Computers are cheaper, far more people have
always-on direct Internet access, and desktop Unix systems (including
Linux and Mac OS X) are prevalent.  A Windows version of Nmap is now
available, allowing it to run on even more desktops.  For all these
reasons, users have less need to run Nmap from limited shared shell accounts.
This is fortunate, as the privileged options make Nmap far more
powerful and flexible.</para>

<para>While Nmap attempts to produce accurate results, keep in mind
that all of its insights are based on packets returned by the target
machines (or firewalls in front of them).  Such hosts may be
untrustworthy and send responses intended to confuse or mislead Nmap.
Much more common are non-RFC-compliant hosts that do not respond as
they should to Nmap probes.  FIN, NULL, and Xmas scans are
particularly susceptible to this problem.  Such issues are specific to
certain scan types and so are
discussed in the individual scan type entries.</para>

<para>This section documents the dozen or so port scan
techniques supported by Nmap.  Only one method may be used at a time,
except that UDP scan (<option>-sU</option>) and any one of the
SCTP scan types (<option>-sY</option>, <option>-sZ</option>)
may be combined with any one of the TCP scan types.
As a memory aid, port scan type options
are of the form <option>-s<replaceable>C</replaceable></option>, where
<replaceable>C</replaceable> is a prominent character in the scan
name, usually the first.  The one exception to this is the deprecated
FTP bounce scan (<option>-b</option>).  By default, Nmap performs a
SYN Scan, though it substitutes a connect scan if the user does not
have proper privileges to send raw packets (requires root access on
Unix).  Of the scans listed in this
section, unprivileged users can only execute connect and FTP bounce
scans.</para>

    <variablelist>
      <varlistentry>
        <term>
        <option>-sS</option> (TCP SYN scan)
        <indexterm><primary><option>-sS</option></primary></indexterm>
        <indexterm><primary>SYN scan</primary></indexterm>
        </term>
        <listitem>

<para>SYN scan is the default and most popular scan option for good
reasons.  It can be performed quickly, scanning thousands of ports per
second on a fast network not hampered by restrictive firewalls. It is also
relatively unobtrusive and stealthy since it never completes TCP
connections.  SYN scan works against any compliant TCP stack rather
than depending on idiosyncrasies of specific platforms as Nmap's
FIN/NULL/Xmas, Maimon and idle scans do.  It also allows clear,
reliable differentiation between the <literal>open</literal>,
<literal>closed</literal>, and <literal>filtered</literal>
states.</para>

<para>This technique is often referred to as half-open scanning,
because you don't open a full TCP connection. You send a SYN packet,
as if you are going to open a real connection and then wait for a
response. A SYN/ACK indicates the port is listening (open), while a
RST (reset) is indicative of a non-listener. If no response is
received after several retransmissions, the port is marked as
filtered.  The port is also marked filtered if an ICMP unreachable
error (type 3, code 0, 1, 2, 3, 9, 10, or 13) is received.  The port is also considered open if a SYN packet (without the ACK flag) is received in response.  This can be due to an extremely rare TCP feature known as a simultaneous open or split handshake connection (see <ulink url="https://nmap.org/misc/split-handshake.pdf"/>).</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sT</option> (TCP connect scan)
        <indexterm><primary><option>-sT</option></primary></indexterm>
        <indexterm><primary>connect scan</primary></indexterm>
        </term>
        <listitem>
<para>TCP connect scan is the default TCP scan type when SYN scan is
not an option.  This is the case when a user does not have raw packet
privileges.  Instead of writing raw
packets as most other scan types do, Nmap asks the underlying
operating system to establish a connection with the target machine and
port by issuing the <function>connect</function> system call.  This is
the same high-level system call that web browsers, P2P clients, and
most other network-enabled applications use to establish a connection.
It is part of a programming interface known as the Berkeley Sockets
API.  Rather than read raw packet responses off the wire, Nmap uses
this API to obtain status information on each connection attempt.
</para>

<para>When SYN scan is available, it is usually a better choice.  Nmap
has less control over the high level <function>connect</function> call
than with raw packets, making it less efficient.  The system call
completes connections to open target ports rather than performing the
half-open reset that SYN scan does.  Not only does this take longer
and require more packets to obtain the same information, but target
machines are more likely to log the connection.  A decent IDS will
catch either, but most machines have no such alarm system.  Many
services on your average Unix system will add a note to syslog, and
sometimes a cryptic error message, when Nmap connects and then closes
the connection without sending data.  Truly pathetic services crash
when this happens, though that is uncommon.  An administrator who sees
a bunch of connection attempts in her logs from a single system should
know that she has been connect scanned.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sU</option> (UDP scans)
<indexterm><primary><option>-sU</option></primary></indexterm>
<indexterm><primary>UDP scan</primary></indexterm>
</term>
        <listitem>

<para>While most popular services on the Internet run over the TCP
protocol, <ulink
role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc768.txt">UDP</ulink> services
are widely deployed.  DNS, SNMP, and DHCP
(registered ports 53, 161/162, and 67/68) are three of the most
common.  Because UDP scanning is generally slower and more difficult
than TCP, some security auditors ignore these ports.  This is a mistake, as
exploitable UDP services are quite common and attackers certainly
don't ignore the whole protocol.  Fortunately, Nmap can help inventory
UDP ports.</para>

<para>UDP scan is activated with the <option>-sU</option> option.  It
can be combined with a TCP scan type such as SYN scan
(<option>-sS</option>) to check both protocols during the same
run.</para>

<para>UDP scan works by sending a UDP packet to every
targeted port. For some common ports such as 53 and 161, a
protocol-specific payload is sent to increase response rate, but for most ports the packet is
empty unless the <option>--data</option>,
<option>--data-string</option>, or <option>--data-length</option>
options are specified.
If an ICMP port unreachable error (type 3, code 3) is
returned, the port is <literal>closed</literal>.  Other ICMP unreachable errors (type 3,
codes 0, 1, 2, 9, 10, or 13) mark the port as <literal>filtered</literal>.  Occasionally, a
service will respond with a UDP packet, proving that it is <literal>open</literal>.  If
no response is received after retransmissions, the port is classified
as <literal>open|filtered</literal>.  This means that the port could be open, or perhaps
packet filters are blocking the communication. Version detection
(<option>-sV</option>) can be used to help differentiate the truly
open ports from the filtered ones.</para>

<para>A big challenge with UDP scanning is doing it quickly.
Open and filtered ports rarely send any response, leaving Nmap to time
out and then conduct retransmissions just in case the probe or
response were lost.  Closed ports are often an even bigger problem.
They usually send back an ICMP port unreachable error.  But unlike the
RST packets sent by closed TCP ports in response to a SYN or connect
scan, many hosts rate limit<indexterm><primary>rate limiting</primary></indexterm>
ICMP port unreachable messages by default.
Linux and Solaris are particularly strict about this.  For example, the
Linux 2.4.20 kernel limits destination unreachable messages to one per
second (in <filename>net/ipv4/icmp.c</filename>).</para>

<para>Nmap detects rate limiting and slows down accordingly to avoid
flooding the network with useless packets that the target machine will
drop.  Unfortunately, a Linux-style limit of one packet per second
makes a 65,536-port scan take more than 18 hours.  Ideas for speeding
your UDP scans up include scanning more hosts in parallel, doing a
quick scan of just the popular ports first, scanning from behind the
firewall, and using <option>--host-timeout</option> to skip slow
hosts.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sY</option> (SCTP INIT scan)
<indexterm><primary><option>-sY</option></primary></indexterm>
<indexterm><primary>SCTP INIT scan</primary></indexterm>
</term>
        <listitem>

<para>
  <ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc4960.txt">SCTP</ulink>
is a relatively new alternative to the TCP and UDP protocols,
combining most characteristics of TCP and UDP, and also adding
new features like multi-homing and multi-streaming.  It is mostly
being used for SS7/SIGTRAN related services but has the potential
to be used for other applications as well.

SCTP INIT scan is the SCTP equivalent of a TCP SYN scan.
It can be performed quickly, scanning thousands of ports per
second on a fast network not hampered by restrictive firewalls.
Like SYN scan, INIT scan is relatively unobtrusive and stealthy,
since it never completes SCTP associations.  It also allows clear,
reliable differentiation between the <literal>open</literal>,
<literal>closed</literal>, and <literal>filtered</literal>
states.</para>

<para>This technique is often referred to as half-open scanning,
because you don't open a full SCTP association. You send an INIT
chunk, as if you are going to open a real association and then wait
for a response. An INIT-ACK chunk indicates the port is listening
(open), while an ABORT chunk is indicative of a non-listener. If no
response is received after several retransmissions, the port is
marked as filtered.  The port is also marked filtered if an ICMP
unreachable error (type 3, code 0, 1, 2, 3, 9, 10, or 13) is
received.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sN</option>; <option>-sF</option>; <option>-sX</option> (TCP NULL, FIN, and Xmas scans)
<indexterm><primary><option>-sN</option></primary></indexterm>
<indexterm><primary><option>-sF</option></primary></indexterm>
<indexterm><primary><option>-sX</option></primary></indexterm>
<indexterm><primary>NULL scan</primary></indexterm>
<indexterm><primary>FIN scan</primary></indexterm>
<indexterm><primary>Xmas scan</primary></indexterm>
</term>
        <listitem>

<para>These three scan types (even more are possible with the
<option>--scanflags</option> option described in the next section)
exploit a subtle loophole in the <ulink
role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc793.txt">TCP RFC</ulink> to
differentiate between <literal>open</literal> and
<literal>closed</literal> ports.  Page 65 of RFC 793 says that <quote>if the
[destination] port state is CLOSED .... an incoming segment not
containing a RST causes a RST to be sent in response.</quote>  Then the next
page discusses packets sent to open ports without the SYN, RST, or ACK
bits set, stating that: <quote>you are unlikely to get here, but if you do, drop the
segment, and return.</quote></para>

<para>When scanning systems compliant with this RFC text, any packet
not containing SYN, RST, or ACK bits will result in a returned RST if
the port is closed and no response at all if the port is open.  As
long as none of those three bits are included, any combination of the
other three (FIN, PSH, and URG) are OK.  Nmap exploits this with three
scan types:</para>

<variablelist>
  <varlistentry><term>Null scan (<option>-sN</option>)</term>
  <listitem><para>Does not set any bits (TCP flag header is 0)</para></listitem></varlistentry> 

  <varlistentry><term>FIN scan (<option>-sF</option>)</term>
  <listitem><para>Sets just the TCP FIN bit.</para></listitem></varlistentry> 

  <varlistentry><term>Xmas scan (<option>-sX</option>)</term>
  <listitem><para>Sets the FIN, PSH, and URG flags, lighting the
  packet up like a Christmas tree.</para></listitem></varlistentry>
</variablelist>

<para>These three scan types are exactly the same in behavior except
for the TCP flags set in probe packets.  If a RST packet is received,
the port is considered <literal>closed</literal>, while no response
means it is <literal>open|filtered</literal>.  The port is marked
<literal>filtered</literal> if an ICMP unreachable error (type 3, code
0, 1, 2, 3, 9, 10, or 13) is received.</para>

<para>The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a little more
stealthy than even a SYN scan.  Don't count on this though&mdash;most
modern IDS products can be configured to detect them.  The big
downside is that not all systems follow RFC 793 to the letter.  A
number of systems send RST responses to the probes regardless of
whether the port is open or not.  This causes all of the ports to be
labeled <literal>closed</literal>.  Major operating systems that do
this are Microsoft Windows, many Cisco devices, BSDI, and IBM OS/400.
This scan does work against most Unix-based systems though. Another
downside of these scans is that they can't distinguish <literal>open</literal> ports from
certain <literal>filtered</literal> ones, leaving you with the response
<literal>open|filtered</literal>.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sA</option> (TCP ACK scan)
        <indexterm><primary><option>-sA</option></primary></indexterm>
        <indexterm><primary>ACK scan</primary></indexterm>
        </term>
        <listitem>

<para>This scan is different than the others discussed so far in that
it never determines <literal>open</literal> (or even
<literal>open|filtered</literal>) ports.  It is used to map out
firewall rulesets, determining whether they are stateful or not and
which ports are filtered.</para>

<para>The ACK scan probe packet has only the ACK flag set (unless you
use <option>--scanflags</option>).  When scanning unfiltered systems,
<literal>open</literal> and <literal>closed</literal> ports will both
return a RST packet.  Nmap then labels them as
<literal>unfiltered</literal>, meaning that they are reachable by the
ACK packet, but whether they are <literal>open</literal> or
<literal>closed</literal> is undetermined.  Ports that don't respond,
or send certain ICMP error messages back (type 3, code 0, 1, 2, 3, 9, 10,
or 13), are labeled <literal>filtered</literal>.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sW</option> (TCP Window scan)
<indexterm><primary><option>-sW</option></primary></indexterm>
<indexterm><primary>window scan</primary></indexterm>
</term>
        <listitem>

<para>Window scan is exactly the same as ACK scan except that it
exploits an implementation detail of certain systems to differentiate
open ports from closed ones, rather than always printing
<literal>unfiltered</literal> when a RST is returned.  It does this by
examining the TCP Window field of the RST packets returned.  On some
systems, open ports use a positive window size (even for RST packets)
while closed ones have a zero window.  So instead of always listing a
port as <literal>unfiltered</literal> when it receives a RST back,
Window scan lists the port as <literal>open</literal> or
<literal>closed</literal> if the TCP Window value in that reset is
positive or zero, respectively.</para>

<para>This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can't always trust it.  Systems
that don't support it will usually return all ports
<literal>closed</literal>.  Of course, it is possible that the machine
really has no open ports.  If most scanned ports are
<literal>closed</literal> but a few common port numbers (such as 22,
25, 53) are <literal>filtered</literal>, the system is most likely
susceptible.  Occasionally, systems will even show the exact opposite
behavior.  If your scan shows 1,000 open ports and three closed or filtered
ports, then those three may very well be the truly open ones.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sM</option> (TCP Maimon scan)
<indexterm><primary><option>-sM</option></primary></indexterm>
<indexterm><primary>Maimon scan</primary></indexterm>
</term>
        <listitem>

<para>The Maimon scan is named after its discoverer,
Uriel Maimon.<indexterm><primary>Maimon, Uriel</primary></indexterm>
He described the technique in
<citetitle>Phrack</citetitle> Magazine issue #49 (November 1996).<indexterm><primary><citetitle>Phrack</citetitle></primary></indexterm>
Nmap, which included this technique, was released two issues later.
This technique is exactly the same as NULL, FIN, and Xmas scans, except
that the probe is FIN/ACK.  According to <ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc793.txt">RFC 793</ulink> (TCP), a RST packet
should be generated in response to such a probe whether the port is
open or closed.  However, Uriel noticed that many BSD-derived systems
simply drop the packet if the port is open.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--scanflags</option> (Custom TCP scan)
        <indexterm><primary><option>--scanflags</option></primary></indexterm></term>
        <listitem>


<para>Truly advanced Nmap users need not limit themselves to the
canned scan types offered.  The <option>--scanflags</option> option allows
you to design your own scan by specifying arbitrary
TCP flags.<indexterm><primary>TCP flags</primary></indexterm>
Let your creative juices flow, while evading
intrusion detection systems<indexterm><primary>intrusion detection systems</primary><secondary>evading</secondary></indexterm>
whose vendors simply paged through the Nmap man page adding specific rules!</para>

<para>The <option>--scanflags</option> argument can be a numerical
flag value such as 9 (PSH and FIN), but using symbolic names is
easier.  Just mash together any combination of  <literal>URG</literal>,
<literal>ACK</literal>, <literal>PSH</literal>,
<literal>RST</literal>, <literal>SYN</literal>, and
<literal>FIN</literal>.  For example, <option>--scanflags
URGACKPSHRSTSYNFIN</option> sets everything, though it's not very
useful for scanning.  The order these are specified in is
irrelevant.</para>

<para>In addition to specifying the desired flags, you can specify a
TCP scan type (such as <option>-sA</option> or <option>-sF</option>).
That base type tells Nmap how to interpret responses.  For
example, a SYN scan considers no-response to indicate a
<literal>filtered</literal> port, while a FIN scan treats the same as
<literal>open|filtered</literal>.  Nmap will behave the same way it
does for the base scan type, except that it will use the TCP flags you
specify instead.  If you don't specify a base type, SYN scan is
used.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sZ</option> (SCTP COOKIE ECHO scan)
<indexterm><primary><option>-sZ</option></primary></indexterm>
<indexterm><primary>SCTP COOKIE ECHO scan</primary></indexterm>
</term>
        <listitem>

<para>
SCTP COOKIE ECHO scan is a more advanced SCTP scan.  It takes
advantage of the fact that SCTP implementations should silently
drop packets containing COOKIE ECHO chunks on open ports, but
send an ABORT if the port is closed.

The advantage of this scan type is that it is not as obvious a
port scan than an INIT scan.  Also, there may be non-stateful
firewall rulesets blocking INIT chunks, but not COOKIE ECHO
chunks.  Don't be fooled into thinking that this will make a
port scan invisible; a good IDS will be able to detect SCTP
COOKIE ECHO scans too.

The downside is that SCTP COOKIE ECHO scans cannot differentiate
between <literal>open</literal> and <literal>filtered</literal>
ports, leaving you with the state <literal>open|filtered</literal>
in both cases.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sI <replaceable>zombie host</replaceable><optional>:<replaceable>probeport</replaceable></optional></option> (idle scan)
          <indexterm><primary><option>-sI</option></primary></indexterm>
          <indexterm><primary>idle scan</primary></indexterm>
        </term>

        <listitem>

          <para>This advanced scan method allows for a truly blind TCP
          port scan of the target (meaning no packets are sent to the
          target from your real IP address). Instead, a unique
          side-channel attack exploits predictable IP fragmentation ID
          sequence generation on the zombie host to glean information
          about the open ports on the target.  IDS systems will
          display the scan as coming from the zombie machine you
          specify (which must be up and meet certain criteria).
          <man>
          This fascinating scan type is too complex to fully describe in this
          reference guide, so I wrote and posted an informal paper with full
          details at <ulink url="https://nmap.org/book/idlescan.html" />.
          </man>
          <notman>
          Full details of this fascinating scan type are in
          <xref linkend="idlescan"/>.
          </notman>
          </para>

          <para>Besides being extraordinarily stealthy (due to its
          blind nature), this scan type permits mapping out
          IP-based trust relationships between machines. The port
          listing shows open ports 
          <emphasis>from the perspective of the zombie
          host.</emphasis> So you can try scanning a target using
          various zombies that you think might be
	  trusted<indexterm><primary>trust relationships</primary></indexterm>
	  (via router/packet filter rules).
          </para>

          <para>You can add a colon followed by a port number to the
          zombie host if you wish to probe a particular port on the
          zombie for IP ID changes. Otherwise Nmap will use the port it
          uses by default for TCP pings (80).</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sO</option> (IP protocol scan)
        <indexterm><primary><option>-sO</option></primary></indexterm>
        <indexterm><primary>IP protocol scan</primary></indexterm>
        </term>
        <listitem>
          
<para>IP protocol scan allows you to determine which IP protocols
(TCP, ICMP, IGMP, etc.) are supported by target machines.  This isn't
technically a port scan, since it cycles through IP protocol numbers
rather than TCP or UDP port numbers. Yet it still uses the
<option>-p</option> option to select scanned protocol numbers, reports
its results within the normal port table format, and even uses the same
underlying scan engine as the true port scanning methods. So it is
close enough to a port scan that it belongs here.</para>

<para>Besides being useful in its own right, protocol scan
demonstrates the power of open-source software.  While the fundamental
idea is pretty simple, I had not thought to add it nor received any
requests for such functionality.  Then in the summer of 2000,
Gerhard Rieger<indexterm><primary>Rieger, Gerhard</primary></indexterm>
conceived the idea, wrote an excellent patch implementing it,
and sent it to the
<citetitle>announce</citetitle> mailing list<indexterm><primary><citetitle>announce</citetitle> mailing list</primary></indexterm>
(then called <citetitle>nmap-hackers</citetitle>).<indexterm><primary><citetitle>nmap-hackers</citetitle> mailing list</primary><see><citetitle>announce</citetitle> mailing list</see></indexterm>
I incorporated that patch into the Nmap tree and released a new
version the next day.  Few pieces of commercial software have users
enthusiastic enough to design and contribute their own
improvements!</para>

<para>Protocol scan works in a similar fashion to UDP scan.  Instead
of iterating through the port number field of a UDP packet, it sends
IP packet headers and iterates through the eight-bit IP protocol field.
The headers are usually empty, containing no data and not even the
proper header for the claimed protocol.  The exceptions are TCP,
UDP, ICMP, SCTP, and IGMP.  A proper protocol header for those is included since
some systems won't send them otherwise and because Nmap already has
functions to create them.  Instead of watching for ICMP port
unreachable messages, protocol scan is on the lookout for ICMP
<emphasis>protocol</emphasis> unreachable messages.  If Nmap receives
any response in any protocol from the target host, Nmap marks that
protocol as <literal>open</literal>.  An ICMP protocol unreachable
error (type 3, code 2) causes the protocol to be marked as
<literal>closed</literal> while port unreachable (type 3, code 3)
marks the protocol <literal>open</literal>. Other ICMP unreachable errors (type 3, code
0, 1, 9, 10, or 13) cause the protocol to be marked
<literal>filtered</literal> (though they prove that ICMP is
<literal>open</literal> at the same time).  If no response is received
after retransmissions, the protocol is marked
<literal>open|filtered</literal></para>

        </listitem>
      </varlistentry>



      <varlistentry>
        <term>
        <option>-b <replaceable>FTP relay host</replaceable></option> (FTP bounce scan)
        <indexterm><primary><option>-b</option></primary></indexterm>
        <indexterm><primary>FTP bounce scan</primary></indexterm>
        </term>
        <listitem>

<para>An interesting feature of the FTP protocol (<ulink
role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc959.txt">RFC 959</ulink>) is
support for so-called proxy FTP connections.  This allows a user to
connect to one FTP server, then ask that files be sent to a
third-party server.  Such a feature is ripe for abuse on many levels,
so most servers have ceased supporting it.  One of the abuses this
feature allows is causing the FTP server to port scan other hosts.
Simply ask the FTP server to send a file to each interesting port of a
target host in turn.  The error message will describe whether the port
is open or not.  This is a good way to bypass firewalls because
organizational FTP servers are often placed where they have
more access to other internal hosts than any old Internet host would.  Nmap supports FTP
bounce scan with the <option>-b</option> option.  It takes an argument
of the form
<replaceable>username</replaceable>:<replaceable>password</replaceable>@<replaceable>server</replaceable>:<replaceable>port</replaceable>.
<replaceable>Server</replaceable> is the name or IP address of a
vulnerable FTP server.  As with a normal URL, you may omit
<replaceable>username</replaceable>:<replaceable>password</replaceable>,
in which case anonymous login credentials (user:
<literal>anonymous</literal> password:<literal>-wwwuser@</literal>)
are used.  The port number (and preceding colon) may be omitted as
well, in which case the default FTP port (21) on
<replaceable>server</replaceable> is used.</para>

<para>This vulnerability was widespread in 1997 when Nmap was
released, but has largely been fixed.  Vulnerable servers are still
around, so it is worth trying when all else fails.  If bypassing a
firewall is your goal, scan the target network for port 21 (or
even for any FTP services if you scan all ports with version
detection) and use the
<filename>ftp-bounce</filename><indexterm><filename>ftp-bounce</filename> script</indexterm>
NSE script.  Nmap will tell you
whether the host is vulnerable or not.  If you are just trying to
cover your tracks, you don't need to (and, in fact, shouldn't) limit
yourself to hosts on the target network.  Before you go scanning
random Internet addresses for vulnerable FTP servers, consider that
sysadmins may not appreciate you abusing their servers in this
way.</para>

        </listitem>
      </varlistentry>
    </variablelist>
   </refsect1>

   <refsect1 id='man-port-specification'>
    <title>Port Specification and Scan Order</title>
    <indexterm><primary>port specification</primary></indexterm>

    <para>In addition to all of the scan methods discussed previously,
    Nmap offers options for specifying which ports are scanned and
    whether the scan order is randomized or sequential.  By default, Nmap scans the most common 1,000 ports for each protocol.
    <indexterm><primary>default ports</primary></indexterm>
    </para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-p <replaceable>port ranges</replaceable></option> (Only scan specified ports)
          <indexterm><primary><option>-p</option></primary></indexterm>
        </term>
        <listitem>


          <para>This option specifies which ports you want to scan and
          overrides the default.  Individual port numbers are OK, as
          are ranges separated by a hyphen (e.g. <literal>1-1023</literal>).  The
          beginning and/or end values of a range may be omitted,
          causing Nmap to use 1 and 65535, respectively.  So you can
          specify <option>-p-</option> to scan ports from 1 through
          65535.  Scanning
	  port zero<indexterm><primary>port zero</primary></indexterm>
          is allowed if you specify it
          explicitly. For IP protocol scanning (<option>-sO</option>), this option
          specifies the protocol numbers you wish to scan for
          (0&ndash;255).</para>

          <para>When scanning a combination of protocols (e.g. TCP and UDP), you can
          specify a particular protocol by preceding the port numbers by
          <literal>T:</literal> for TCP, <literal>U:</literal> for UDP,
          <literal>S:</literal> for SCTP, or <literal>P:</literal> for IP Protocol.
          The qualifier lasts until you specify another
          qualifier. For example, the argument <option>-p
          U:53,111,137,T:21-25,80,139,8080</option> would scan UDP
          ports 53, 111,and 137, as well as the listed TCP ports. Note
          that to scan both UDP and TCP, you have to specify
          <option>-sU</option> and at least one TCP scan type (such as
          <option>-sS</option>, <option>-sF</option>, or
          <option>-sT</option>). If no protocol qualifier is given,
          the port numbers are added to all protocol lists.</para>


          <indexterm><primary>port specification</primary><secondary>wildcards in</secondary></indexterm>
          <indexterm><primary>wildcards</primary><see>port selection, wildcards in</see></indexterm>
          <para>
          Ports can also be specified by name according to what the
          port is referred to in the <filename>nmap-services</filename>. You
          can even use the wildcards <literal>*</literal> and
          <literal>?</literal> with the names. For example, to scan
          FTP and all ports whose names begin with <quote>http</quote>, use <option>-p ftp,http*</option>.
          Be careful about shell expansions and quote the argument to <option>-p</option> if unsure.</para>

          <para>Ranges of ports can be surrounded by square brackets to indicate
          ports inside that range that appear in <filename>nmap-services</filename>.
          For example, the following will scan all ports in <filename>nmap-services</filename>
          equal to or below 1024: <option>-p [-1024]</option>. Be careful with shell
          expansions and quote the argument to <option>-p</option> if unsure.</para> 
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--exclude-ports <replaceable>port ranges</replaceable></option> (Exclude the specified ports from scanning)
          <indexterm><primary><option>--exclude-ports</option></primary></indexterm>
        </term>
        <listitem>

          <para>This option specifies which ports you do want Nmap to exclude
          from scanning. The <replaceable>port ranges</replaceable>
          are specified similar to <option>-p</option>. For IP protocol 
          scanning (<option>-sO</option>), this option specifies the protocol
          numbers you wish to exclude (0&ndash;255).</para>

          <para>When ports are asked to be excluded, they are excluded from all
          types of scans (i.e. they will not be scanned under any circumstances).
          This also includes the discovery phase.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-F</option> (Fast (limited port) scan)
          <indexterm significance="preferred"><primary><option>-F</option></primary></indexterm>
          <indexterm><primary>fast scan</primary><see><option>-F</option></see></indexterm>
        </term>
        <listitem>
          <para>Specifies that you wish to scan fewer ports than the
          default. Normally Nmap scans the most common 1,000 ports for
          each scanned protocol. With <option>-F</option>, this is
          reduced to 100.</para>

          <para>Nmap needs an <filename>nmap-services</filename> file
          with frequency information in order to know which ports are
          the most common<notman> (see <xref linkend="nmap-services"/>
          for more about port frequencies)</notman>. If port frequency
          information isn't available, perhaps because of the use of a
          custom <filename>nmap-services</filename> file, Nmap scans
          all named ports plus ports 1-1024.  In that case,
          <option>-F</option> means to scan only ports that are named
          in the services file.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-r</option> (Don't randomize ports)
          <indexterm><primary><option>-r</option></primary></indexterm>
          <indexterm><primary>randomization of ports</primary></indexterm>
        </term>
        <listitem>
          <para>By default, Nmap randomizes the scanned port order
          (except that certain commonly accessible ports are moved
          near the beginning for efficiency reasons).  This
          randomization is normally desirable, but you can specify
          <option>-r</option> for sequential (sorted from lowest to highest) port scanning
          instead.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--port-ratio <replaceable>ratio</replaceable>&lt;decimal number between 0 and 1&gt;</option>
        </term>
        <listitem>
        <indexterm><primary>--port-ratio</primary></indexterm>
        <para>Scans all ports in <filename>nmap-services</filename> file
        with a ratio greater than the one given.
        <replaceable>ratio</replaceable> must be between 0.0 and 1.0.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--top-ports <replaceable>n</replaceable></option>
        </term>
        <listitem>
        <indexterm><primary>--top-ports</primary></indexterm>
        <para>Scans the <replaceable>n</replaceable> highest-ratio ports
        found in <filename>nmap-services</filename> file after excluding all
        ports specified by <option>--exclude-ports</option>.
        <replaceable>n</replaceable> must be 1 or greater.</para>
        </listitem>
      </varlistentry>

    </variablelist>

</refsect1>


  <refsect1 id='man-version-detection'>
    <title>Service and Version Detection</title>
    <indexterm class="startofrange" id="man-version-detection-indexterm"><primary>version detection</primary></indexterm>


    <para>Point Nmap at a remote machine and it might tell you
    that ports <literal>25/tcp</literal>, <literal>80/tcp</literal>, and <literal>53/udp</literal> are open. Using its
    <filename>nmap-services</filename><indexterm><primary><filename>nmap-services</filename></primary></indexterm>
    database of about 2,200
    well-known services,<indexterm><primary>well-known ports</primary></indexterm>
    Nmap would report that those ports probably correspond to a
    mail server (SMTP), web server (HTTP), and name server (DNS)
    respectively. This lookup is usually accurate&mdash;the vast
    majority of daemons listening on TCP port 25 are, in fact, mail
    servers. However, you should not bet your security on this!
    People can and do run services on
    strange ports.<indexterm><primary>non-standard ports</primary></indexterm>
    </para>

    <para>Even if Nmap is right, and the hypothetical server above is
    running SMTP, HTTP, and DNS servers, that is not a lot of
    information. When doing vulnerability assessments (or even simple
    network inventories) of your companies or clients, you really want
    to know which mail and DNS servers and versions are
    running. Having an accurate version number helps dramatically in
    determining which exploits a server is vulnerable to.  Version
    detection helps you obtain this information.
</para>

    <para>After TCP and/or UDP ports are discovered using one of the
    other scan methods, version detection interrogates those ports to
    determine more about what is actually running. The
    <filename>nmap-service-probes</filename><indexterm><primary><filename>nmap-service-probes</filename></primary></indexterm>
    database contains probes
    for querying various services and match expressions to recognize
    and parse responses. Nmap tries to determine the service protocol
    (e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC
    BIND, Apache httpd, Solaris telnetd), the version number,
    hostname, device type (e.g. printer, router), the OS family
    (e.g. Windows, Linux). When possible, Nmap also gets the
    Common Platform Enumeration (CPE)<indexterm><primary>Common Platform Enumeration</primary><secondary>service</secondary></indexterm>
    representation of this information. Sometimes miscellaneous details like
    whether an X server is open to connections, the SSH protocol
    version, or the KaZaA user name, are available.  Of course, most services don't
    provide all of this information.  If Nmap was compiled with
    OpenSSL support, it will connect to SSL servers to deduce the
    service listening behind that encryption layer.<indexterm><primary>SSL</primary><secondary>in version detection</secondary></indexterm>
    Some UDP ports are left in the
    <literal>open|filtered</literal> state after a UDP port scan is
    unable to determine whether the port is open or filtered. Version
    detection will try to elicit a response from these ports (just as
    it does with open ports), and change the state to open if it
    succeeds. <literal>open|filtered</literal> TCP ports are treated
    the same way.  Note that the Nmap <option>-A</option> option
    enables version detection among other things.
    <man>
    A paper documenting the workings, usage, and customization of version
    detection is available at
    <ulink url="https://nmap.org/book/vscan.html" />.
    </man>
    <notman>
    Version detection is described in detail in <xref linkend="vscan"/>.
    </notman>
    </para>

    <para>When RPC services are discovered, the Nmap RPC
    grinder<indexterm><primary>RPC grinder</primary></indexterm>
    is automatically used to determine the RPC program and version
    numbers. It takes all the TCP/UDP ports detected as RPC and floods
    them with SunRPC program NULL commands in an attempt to determine
    whether they are RPC ports, and if so, what program and version
    number they serve up. Thus you can effectively obtain the same info
    as <command>rpcinfo -p</command> even if the target's portmapper is
    behind a firewall (or protected by TCP wrappers). Decoys do not
    currently work with
    RPC scan.<indexterm><primary>decoys</primary><secondary>which scans use</secondary></indexterm>
    </para>

    <para>When Nmap receives responses from a service but cannot match
    them to its database, it prints out a special fingerprint and
    a URL for you to submit it to if you know for sure what is running
    on the port.  Please take a couple minutes to make the submission
    so that your find can benefit everyone.  Thanks to these
    submissions, Nmap has about 6,500 pattern matches for more than
    650 protocols such as SMTP, FTP, HTTP, etc.<indexterm><primary>submission of service fingerprints</primary></indexterm>
    </para>

    <para>Version detection is enabled and controlled with the
    following options:</para>


    <variablelist>

      <varlistentry>
        <term>
        <option>-sV</option> (Version detection)
          <indexterm><primary><option>-sV</option></primary></indexterm></term>
        <listitem>


          <para>Enables version detection, as discussed above.
          Alternatively, you can use <option>-A</option>, which enables
          version detection among other things.</para>

          <para><option>-sR</option><indexterm><primary><option>-sR</option></primary></indexterm>
          is an alias for <option>-sV</option>. Prior to March 2011, it
          was used to active the RPC grinder separately from version
          detection, but now these options are always combined.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--allports</option> (Don't exclude any ports from
          version detection)
          <indexterm><primary><option>--allports</option></primary></indexterm>
        </term>
        <listitem>

          <para>By default, Nmap version detection skips TCP port 9100
          because some printers simply print anything sent to that
          port, leading to dozens of pages of HTTP GET requests, binary
          SSL session requests, etc.  This behavior can be changed by
          modifying or removing the <literal>Exclude</literal>
          directive in <filename>nmap-service-probes</filename>, or
          you can specify <option>--allports</option> to scan all
          ports regardless of any <literal>Exclude</literal>
          directive.
          <indexterm><primary><literal>Exclude</literal> directive (<filename>nmap-service-probes</filename>)</primary></indexterm>
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--version-intensity <replaceable>intensity</replaceable></option> (Set
          version scan intensity)
          <indexterm><primary><option>--version-intensity</option></primary></indexterm>
        </term>
        <listitem>


          <para>When performing a version scan (<option>-sV</option>), Nmap sends a
          series of probes, each of which is assigned a rarity value
          between one and nine. The lower-numbered probes are effective
          against a wide variety of common services, while the higher-numbered
          ones are rarely useful. The intensity level
          specifies which probes should be applied. The higher the
          number, the more likely it is the service will be correctly
          identified. However, high intensity scans take longer.  The
          intensity must be between 0 and 9.<indexterm><primary>version detection</primary><secondary>intensity</secondary></indexterm>
          The default is 7.<indexterm><primary>version detection</primary><secondary>default intensity</secondary></indexterm>
          When a probe is registered to the target port via the
          <filename>nmap-service-probes</filename> <literal>ports</literal> directive, that probe is tried
          regardless of intensity level.  This ensures that the DNS
          probes will always be attempted against any open port 53,
          the SSL probe will be done against 443, etc.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--version-light</option> (Enable light mode)
          <indexterm><primary><option>--version-light</option></primary></indexterm>
        </term>
        <listitem>

          <para>This is a convenience alias for
          <option>--version-intensity 2</option>.  This light mode
          makes version scanning much faster, but it is slightly less
          likely to identify services.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--version-all</option> (Try every single probe)
          <indexterm><primary><option>--version-all</option></primary></indexterm>
        </term>
        <listitem>

          <para>An alias for <option>--version-intensity 9</option>,
          ensuring that every single probe is attempted against each
          port.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--version-trace</option> (Trace version scan activity)
          <indexterm significance="preferred"><primary><option>--version-trace</option></primary></indexterm>
        </term>
        <listitem>
          <para>This causes Nmap to print out extensive debugging info
          about what version scanning is doing.  It is a subset of
          what you get with <option>--packet-trace</option>.</para>
        </listitem>
      </varlistentry>
  
    </variablelist>
    <indexterm class="endofrange" startref="man-version-detection-indexterm"/>
  </refsect1>

  <refsect1 id='man-os-detection'>

    <title>OS Detection</title>
    <indexterm class="startofrange" id="man-os-detection-indexterm"><primary>OS detection</primary></indexterm>

    <indexterm><primary>CPE</primary><see>Common Platform Enumeration</see></indexterm>
    <para>One of Nmap's best-known features is remote OS detection
    using TCP/IP stack fingerprinting.  Nmap sends a series of TCP and
    UDP packets to the remote host and examines practically every bit
    in the responses.  After performing dozens of tests such as TCP
    ISN sampling, TCP options support and ordering, IP ID sampling, and
    the initial window size check, Nmap compares the results to its
    <filename>nmap-os-db</filename><indexterm><primary><filename>nmap-os-db</filename></primary></indexterm>
    database of more than 2,600 known
    OS fingerprints and prints out the OS details if there is a match.
    Each fingerprint includes a freeform textual description of the
    OS, and a classification which provides the vendor name
    (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10),
    and device type (general purpose, router, switch, game console,
    etc). Most fingerprints also have a Common Platform Enumeration
    (CPE)<indexterm><primary>Common Platform Enumeration</primary><secondary>operating system</secondary></indexterm>
    representation, like
    <literal>cpe:/o:linux:linux_kernel:2.6</literal>.</para>

    <para>If Nmap is unable to guess the OS of a machine, and
    conditions are good (e.g. at least one open port and one closed
    port were found), Nmap will
    provide a URL you can use to submit the fingerprint if you know
    (for sure) the OS running on the machine.  By doing this you
    contribute to the pool of operating systems known to Nmap and thus
    it will be more accurate for everyone.</para>

    <para>OS detection enables some other tests which make use
    of information that is gathered during the process anyway.
    One of these is
    TCP Sequence Predictability Classification.  This measures
    approximately how hard it is to establish a forged
    TCP connection against the remote host.  It is useful for
    exploiting source-IP based trust relationships (rlogin, firewall
    filters, etc) or for hiding the source of an attack.  This sort of
    spoofing is rarely performed any more, but many machines are still
    vulnerable to it.  The actual
    difficulty number is based on statistical sampling and may
    fluctuate.  It is generally better to use the English
    classification such as <quote>worthy challenge</quote> or <quote>trivial joke</quote>.  This
    is only reported in normal output in verbose (<option>-v</option>)
    mode.  When verbose mode is enabled along with <option>-O</option>, IP ID sequence
    generation is also reported.  Most machines are in the
    <quote>incremental</quote> class, which means that they increment the ID
    field in the IP header for each packet they send.  This makes them
    vulnerable to several advanced information gathering and
    spoofing attacks.</para>

    <indexterm><primary>uptime guess</primary></indexterm>
    <para>
    Another bit of extra information enabled by OS detection is a guess
    at a target's uptime. This uses the TCP timestamp option (<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc1323.txt">RFC 1323</ulink>) to guess when a machine was last rebooted.
    The guess can be inaccurate due to the timestamp counter not being
    initialized to zero or the counter overflowing and wrapping around,
    so it is printed only in verbose mode.
    </para>

    <para>
    <man>
    A paper documenting the workings, usage, and customization of OS
    detection is available at
    <ulink url="https://nmap.org/book/osdetect.html" />.
    </man>
    <notman>
    OS detection is covered in <xref linkend="osdetect"/>.
    </notman>
    </para>

    <para>OS detection is enabled and controlled with the following options:</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-O</option> (Enable OS detection)
          <indexterm significance="preferred"><primary><option>-O</option></primary></indexterm>
        </term>
        <listitem>


          <para>Enables OS detection, as discussed above.
          Alternatively, you can use <option>-A</option> to enable
          OS detection along with other things.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--osscan-limit</option> (Limit OS detection to
          promising targets)
          <indexterm significance="preferred"><primary><option>--osscan-limit</option></primary></indexterm>
        </term>
        <listitem>

          <para>OS detection is far more effective if at least one
          open and one closed TCP port are found. Set this option
          and Nmap will not even try OS detection against hosts
          that do not meet this criteria. This can save substantial
          time, particularly on <option>-Pn</option> scans against many hosts. It
          only matters when OS detection is requested with <option>-O</option> or <option>-A</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--osscan-guess</option>; <option>--fuzzy</option> (Guess OS detection results)
          <indexterm significance="preferred"><primary><option>--osscan-guess</option></primary></indexterm>
          <indexterm><primary><option>--fuzzy</option></primary><see><option>--osscan-guess</option></see></indexterm>
        </term>
        <listitem>

          <para>When Nmap is unable to detect a perfect OS match, it
          sometimes offers up near-matches as possibilities.  The
          match has to be very close for Nmap to do this by default.
          Either of these (equivalent) options make Nmap guess more
          aggressively.  Nmap will still tell you when an imperfect
          match is printed and display its confidence level
          (percentage) for each guess.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--max-os-tries</option> (Set the maximum number of OS detection tries against a target)
          <indexterm significance="preferred"><primary><option>--max-os-tries</option></primary></indexterm>
        </term>
        <listitem>


          <para>When Nmap performs OS detection against a target and
          fails to find a perfect match, it usually repeats the
          attempt.  By default, Nmap tries five times if conditions
          are favorable for OS fingerprint submission, and twice when
          conditions aren't so good.  Specifying a lower
          <option>--max-os-tries</option> value (such as 1) speeds
          Nmap up, though you miss out on retries which could
          potentially identify the OS.  Alternatively, a high value
          may be set to allow even more retries when conditions are
          favorable.  This is rarely done, except to generate better
          fingerprints for submission and integration into the Nmap OS
          database.</para>

        </listitem>
      </varlistentry>

    </variablelist>
    <indexterm class="endofrange" startref="man-os-detection-indexterm"/>
  </refsect1>
	<refsect1 id='man-nse'> 
	<title>Nmap Scripting Engine (NSE)</title>
        <indexterm class="startofrange" id="man-nse-indexterm"><primary>Nmap Scripting Engine (NSE)</primary></indexterm>

    <para>The Nmap Scripting Engine (NSE) is one of Nmap's most
    powerful and flexible features.  It allows users to write (and
    share) simple scripts (using the <ulink url="https://lua.org">Lua programming language</ulink>
    <indexterm><primary>Lua programming language</primary></indexterm>
    ) to automate a wide variety of
    networking tasks.  Those scripts are executed in parallel with the
    speed and efficiency you expect from Nmap.  Users can rely on the
    growing and diverse set of scripts distributed with Nmap, or write
    their own to meet custom needs.</para>

    <para>Tasks we had in mind when creating the system include
    network discovery, more sophisticated version detection,
    vulnerability detection.  NSE can even be used for vulnerability
    exploitation.</para>

    <para> 
    To reflect those different uses and to simplify the choice of which
    scripts to run, each script contains a field associating it with one or more categories.  Currently defined categories are
    <literal>auth</literal>,
    <literal>broadcast</literal>,
    <literal>default</literal>.
    <literal>discovery</literal>,
    <literal>dos</literal>,
    <literal>exploit</literal>,
    <literal>external</literal>,
    <literal>fuzzer</literal>,
    <literal>intrusive</literal>,
    <literal>malware</literal>,
    <literal>safe</literal>,
    <literal>version</literal>, and
    <literal>vuln</literal>.
    These are all described
    <man>at <ulink url="https://nmap.org/book/nse-usage.html#nse-categories" />.</man>
    <notman>in <xref linkend="nse-categories"/>.</notman></para>

    <para>
    Scripts are not run in a sandbox and thus could accidentally or
    maliciously damage your system or invade your privacy.  Never run
    scripts from third parties unless you trust the authors or have
    carefully audited the scripts yourself.
    </para>
    
    <para>The Nmap Scripting Engine is described in detail
<man>at <ulink url="https://nmap.org/book/nse.html" /></man>
<notman>in <xref linkend="nse"/></notman> and is controlled by the following options:</para>

	<variablelist>
      <varlistentry>
        <term><option>-sC</option>
          <indexterm significance="preferred">
            <primary><option>-sC</option></primary>
          </indexterm>
         </term>

        <listitem>

          <para>Performs a script scan using the default set of scripts. It is 
		  equivalent to <option>--script=default</option>. Some of the
		  scripts in this category are considered intrusive and should
		  not be run against a target network without permission. </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--script <replaceable>filename</replaceable>|<replaceable>category</replaceable>|<replaceable>directory</replaceable>/|<replaceable>expression</replaceable><optional>,...</optional></option>
          <indexterm><primary><option>--script</option></primary></indexterm>
        </term>

<listitem>

<para>
Runs a script scan using the comma-separated list of filenames, script
categories, and directories. Each element in the list may also be a
Boolean expression describing a more complex set of scripts. Each
element is interpreted first as an expression, then as a category, and
finally as a file or directory name.</para>

<para>There are two special features for advanced users only.
One is to prefix script names and expressions with
<literal>+</literal> to force them to run even if they normally
wouldn't (e.g. the relevant service wasn't detected on the target
port).  The other is that the argument <literal>all</literal> may be
used to specify every script in Nmap's database.  Be cautious with
this because NSE contains dangerous scripts such as exploits, brute
force authentication crackers, and denial of service attacks.
</para>

<para>
File and directory names may be relative or absolute. Absolute names are
used directly. Relative paths are looked for in the
<filename>scripts</filename> of each of the following places until
found:
<indexterm><primary>data files</primary><secondary>directory search order</secondary></indexterm><indexterm><primary>scripts, location of</primary></indexterm>
<simplelist>
<member><option>--datadir</option></member>
<member><envar>$NMAPDIR</envar><indexterm><primary><envar>NMAPDIR</envar> environment variable</primary></indexterm></member>
<member><filename>~/.nmap</filename> (not searched on Windows)<indexterm><primary sortas="nmap directory"><filename>.nmap</filename> directory</primary></indexterm></member>
<member><filename><replaceable>APPDATA</replaceable>\nmap</filename> (only on Windows)<indexterm><primary sortas="nmap directory"><filename>.nmap</filename> directory</primary></indexterm></member>
<member>the directory containing the <filename>nmap</filename>
executable</member>
<member>the directory containing the <filename>nmap</filename>
executable, followed by <filename>../share/nmap</filename> (not searched on Windows)</member>
<member><varname>NMAPDATADIR</varname><indexterm><primary><varname>NMAPDATADIR</varname></primary></indexterm> (not searched on Windows)</member>
<member>the current directory.</member>
</simplelist>
</para>

<para>
When a directory name ending in <literal>/</literal> is given, Nmap loads every file in the directory
whose name ends with <filename>.nse</filename>. All other files are
ignored and directories are not searched recursively. When a filename is
given, it does not have to have the <filename>.nse</filename> extension;
it will be added automatically if necessary.
</para>

<indexterm><primary>script database</primary><see><filename>script.db</filename></see></indexterm>
<indexterm><primary>scripts, location of</primary></indexterm>

<para>Nmap scripts are stored in a <filename>scripts</filename>
subdirectory of the Nmap data directory by default
<man>(see <ulink url="https://nmap.org/book/data-files.html"/>).</man>
<notman>(see <xref linkend="data-files"/>).</notman>
For efficiency, scripts are indexed in
a database stored
in <filename>scripts/script.db</filename>,<indexterm><primary><filename>script.db</filename></primary></indexterm>
which lists the category or categories in which each script belongs.</para>

      <indexterm><primary>wildcards</primary><secondary>in script selection</secondary></indexterm>
      <para>
      When referring to scripts from <filename>script.db</filename> by
      name, you can use a shell-style &lsquo;<literal>*</literal>&rsquo;
      wildcard.
      </para>

      <variablelist>
        <varlistentry>
          <term><command>nmap --script "http-*"</command></term>
          <listitem>
          <para>Loads all scripts whose name starts with
          <filename>http-</filename>, such as
          <filename>http-auth</filename> and
          <filename>http-open-proxy</filename>. The argument to
          <option>--script</option> had to be in quotes to protect the
          wildcard from the shell.</para>
          </listitem>
        </varlistentry>
      </variablelist>

      <indexterm><primary>Boolean expressions in script selection</primary></indexterm>
      <para>
      More complicated script selection can be done using the
      <literal>and</literal>, <literal>or</literal>, and
      <literal>not</literal> operators to build Boolean expressions. The
      operators have the same
      <ulink role="hidepdf" url="http://www.lua.org/manual/5.4/manual.html#3.4.8">precedence</ulink>
      as in Lua: <literal>not</literal> is the highest, followed by
      <literal>and</literal> and then <literal>or</literal>. You can
      alter precedence by using parentheses. Because expressions contain
      space characters it is necessary to quote
      them.
      </para>

      <variablelist>
        <varlistentry>
          <term><command>nmap --script "not intrusive"</command></term>
          <listitem>
          <para>Loads every script except for those in the
          <literal>intrusive</literal> category.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><command>nmap --script "default or safe"</command></term>
          <listitem>
          <para>This is functionally equivalent to
          <command>nmap --script "default,safe"</command>. It loads all
          scripts that are in the <literal>default</literal> category or
          the <literal>safe</literal> category or both.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><command>nmap --script "default and safe"</command></term>
          <listitem>
          <para>Loads those scripts that are in
          <emphasis>both</emphasis> the <literal>default</literal> and
          <literal>safe</literal> categories.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><command>nmap --script "(default or safe or intrusive) and not http-*"</command></term>
          <listitem>
          <para>Loads scripts in the <literal>default</literal>,
          <literal>safe</literal>, or <literal>intrusive</literal>
          categories, except for those whose names start with
          <filename>http-</filename>.</para>
          </listitem>
        </varlistentry>
      </variablelist>

</listitem>
</varlistentry>
      
	  <varlistentry>
        <term><option>--script-args <replaceable>n1</replaceable>=<replaceable>v1</replaceable>,<replaceable>n2</replaceable>={<replaceable>n3</replaceable>=<replaceable>v3</replaceable>},<replaceable>n4</replaceable>={<replaceable>v4</replaceable>,<replaceable>v5</replaceable>}</option>
        <indexterm significance="preferred"><primary><option>--script-args</option></primary></indexterm>
        <indexterm><primary>script arguments</primary><seealso><option>--script-args</option></seealso></indexterm></term>

<listitem>
<para>
Lets you provide arguments to NSE scripts. Arguments are a comma-separated list
of <literal>name=value</literal> pairs. Names and values may be strings not
containing whitespace or the characters
&lsquo;<literal>{</literal>&rsquo;,
&lsquo;<literal>}</literal>&rsquo;,
&lsquo;<literal>=</literal>&rsquo;, or
&lsquo;<literal>,</literal>&rsquo;.
To include one of these characters in a string, enclose the string in single or
double quotes. Within a quoted string, &lsquo;<literal>\</literal>&rsquo;
escapes a quote. A backslash is only used to escape quotation marks in this
special case; in all other cases a backslash is interpreted literally. Values
may also be tables enclosed in <literal>{}</literal>, just as in Lua. A table
may contain simple string values or more name-value pairs, including nested
tables.  Many scripts qualify their arguments with the script name, as in <literal>xmpp-info.server_name</literal>.  You may use that full qualified version to affect just the specified script, or you may pass the unqualified version (<literal>server_name</literal> in this case) to affect all scripts using that argument name.  A script will first check for its fully qualified argument name (the name specified in its documentation) before it accepts an unqualified argument name.  A complex example of script arguments is
<option>--script-args 'user=foo,pass=",{}=bar",whois={whodb=nofollow+ripe},xmpp-info.server_name=localhost'</option>.  The online NSE Documentation Portal at <ulink url="https://nmap.org/nsedoc/"/>
lists the arguments that each script accepts.
</para>
</listitem>
</varlistentry>

	  <varlistentry>
        <term><option>--script-args-file <replaceable>filename</replaceable></option>
        <indexterm significance="preferred"><primary><option>--script-args-file</option></primary></indexterm>
        <indexterm><primary>script arguments from file</primary><seealso><option>--script-args-file</option></seealso></indexterm></term>

<listitem>
<para>
    Lets you load arguments to NSE scripts from a file. Any arguments on the
    command line supersede ones in the file. The file can be an absolute path,
    or a path relative to Nmap's usual search path (NMAPDIR, etc.)
    Arguments can be comma-separated or newline-separated, but otherwise follow
    the same rules as for <option>--script-args</option>, without requiring
    special quoting and escaping, since they are not parsed by the shell.
</para>
</listitem>
</varlistentry>
      <varlistentry>
        <term>
          <option>--script-help <replaceable>filename</replaceable>|<replaceable>category</replaceable>|<replaceable>directory</replaceable>|<replaceable>expression</replaceable>|all<optional>,...</optional></option>
          <indexterm><primary><option>--script-help</option></primary></indexterm>
        </term>

        <listitem>
        <para>
        Shows help about scripts. For each script matching the given
        specification, Nmap prints the script name, its categories, and its
        description. The specifications are the same as those accepted by
        <option>--script</option>; so for example if you want help about
        the <literal>ftp-anon</literal> script, you would run
        <command>nmap --script-help ftp-anon</command>. In addition to getting
        help for individual scripts, you can use this as a preview of what
        scripts will be run for a specification, for example with <command>nmap
        --script-help default</command>.
        </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--script-trace</option>
          <indexterm significance="preferred"><primary><option>--script-trace</option></primary></indexterm></term>

        <listitem>
          <para>
		  This option does what <option>--packet-trace</option> does,
          just one ISO layer higher. If this option is specified all incoming
          and outgoing communication performed by a script is printed. The
          displayed information includes the communication protocol, the
          source, the target and the transmitted data. If more than 5% of all
          transmitted data is not printable, then the trace output is in a hex
          dump format. Specifying <option>--packet-trace</option> enables script
          tracing too.
		  </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--script-updatedb</option>
        <indexterm significance="preferred"><primary><option>--script-updatedb</option></primary></indexterm></term>

        <listitem>

           <para>This option updates the script database found
            in <filename>scripts/script.db</filename> which is used by
            Nmap to determine the available default scripts and
            categories. It is only necessary to update the database if
            you have added or removed NSE scripts from the
            default <filename>scripts</filename> directory or if you
            have changed the categories of any script.  This option is
            generally used by
            itself: <command>nmap --script-updatedb</command>.</para>

        </listitem>
      </varlistentry>
    </variablelist>

    <indexterm class="endofrange" startref="man-nse-indexterm"/>
  </refsect1>

   <refsect1 id='man-performance'>
    <title>Timing and Performance</title>
    <indexterm class="startofrange" id="man-performance-timing-indexterm"><primary>timing</primary></indexterm>
    <indexterm class="startofrange" id="man-performance-indexterm"><primary>performance</primary></indexterm>
    <para>One of my highest Nmap development priorities has always been
performance.  A default scan (<command>nmap
<replaceable>hostname</replaceable></command>) of a host on my local
network takes a fifth of a second.  That is barely enough time to
blink, but adds up when you are scanning hundreds or thousands
of hosts.  Moreover, certain scan options such as UDP scanning and
version detection can increase scan times substantially.  So can
certain firewall configurations, particularly response rate limiting.
While Nmap utilizes parallelism and many advanced algorithms to
accelerate these scans, the user has ultimate control over how Nmap
runs.  Expert users carefully craft Nmap commands to obtain only the
information they care about while meeting their time
constraints.</para>

<para>Techniques for improving scan times include omitting
non-critical tests, and upgrading to the latest version of Nmap
(performance enhancements are made frequently).  Optimizing timing
parameters can also make a substantial difference.  Those options are
listed below.</para>

<para>Some options accept a <literal>time</literal> parameter.  This
is specified in seconds by default, though you can append
&lsquo;ms&rsquo;, &lsquo;s&rsquo;, &lsquo;m&rsquo;, or &lsquo;h&rsquo; to the value to
specify milliseconds, seconds, minutes, or hours. So the
<option>--host-timeout</option> arguments <literal>900000ms</literal>,
<literal>900</literal>, <literal>900s</literal>, and <literal>15m</literal> all do the same thing.</para>


    <variablelist>

      <varlistentry>
        <term>
        <option>--min-hostgroup <replaceable>numhosts</replaceable></option>; 
        <option>--max-hostgroup
        <replaceable>numhosts</replaceable></option> (Adjust parallel scan group sizes)
        <indexterm><primary><option>--min-hostgroup</option></primary></indexterm>
        <indexterm><primary><option>--max-hostgroup</option></primary></indexterm>
        </term>
        <listitem>
<para>Nmap has the ability to port scan or version scan multiple hosts
in parallel.  Nmap does this by dividing the target IP space into
groups and then scanning one group at a time.  In general, larger
groups are more efficient.  The downside is that host results can't be
provided until the whole group is finished.  So if Nmap started out
with a group size of 50, the user would not receive any reports
(except for the updates offered in verbose mode) until the first 50
hosts are completed.</para>

<para>By default, Nmap takes a compromise approach to this conflict.
It starts out with a group size as low as five so the first results
come quickly and then increases the groupsize to as high as 1024.  The
exact default numbers depend on the options given.  For efficiency
reasons, Nmap uses larger group sizes for UDP or few-port TCP
scans.</para>

<para>When a maximum group size is specified with
<option>--max-hostgroup</option>, Nmap will never exceed that size.
Specify a minimum size with <option>--min-hostgroup</option> and Nmap
will try to keep group sizes above that level.  Nmap may have to use
smaller groups than you specify if there are not enough target hosts
left on a given interface to fulfill the specified minimum.  Both may
be set to keep the group size within a specific range, though this is
rarely desired.</para>

<para>These options do not have an effect during the host discovery
phase of a scan. This includes plain ping scans (<option>-sn</option>).
Host discovery always works in large groups of hosts to improve speed
and accuracy.</para>

<para>The primary use of these options is to specify a large minimum
group size so that the full scan runs more quickly.  A common choice
is 256 to scan a network in /24 sized chunks.  For a scan with
many ports, exceeding that number is unlikely to help much. For scans
of just a few port numbers, host group sizes of 2048 or more may be
helpful.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--min-parallelism <replaceable>numprobes</replaceable></option>;
        <option>--max-parallelism
        <replaceable>numprobes</replaceable></option> (Adjust probe parallelization)
        <indexterm><primary><option>--min-parallelism</option></primary></indexterm>
        <indexterm><primary><option>--max-parallelism</option></primary></indexterm>
        </term>
        <listitem>

<para>These options control the total number of probes that may
be outstanding for a host group.  They are used for port scanning and
host discovery.  By default, Nmap calculates an ever-changing ideal
parallelism based on network performance.  If packets are being dropped,
Nmap slows down and allows fewer outstanding probes.  The ideal probe
number slowly rises as the network proves itself worthy.  These
options place minimum or maximum bounds on that variable.  By default,
the ideal parallelism can drop to one if the network proves unreliable
and rise to several hundred in perfect conditions.</para>

<para>The most common usage is to set
<option>--min-parallelism</option> to a number higher than one to
speed up scans of poorly performing hosts or networks.  This is a
risky option to play with, as setting it too high may affect accuracy.
Setting this also reduces Nmap's ability to control parallelism
dynamically based on network conditions.  A value of 10 might be
reasonable, though I only adjust this value as a last resort.</para>

<para>The <option>--max-parallelism</option> option is sometimes set to one
to prevent Nmap from sending more than one probe at a time to hosts.
The <option>--scan-delay</option> option, discussed later, is another
way to do this.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--min-rtt-timeout <replaceable>time</replaceable></option>, 
        <option>--max-rtt-timeout <replaceable>time</replaceable></option>, 
        <option>--initial-rtt-timeout
        <replaceable>time</replaceable></option> (Adjust probe timeouts)
        <indexterm><primary><option>--min-rtt-timeout</option></primary></indexterm>
        <indexterm><primary><option>--max-rtt-timeout</option></primary></indexterm>
        <indexterm><primary><option>--initial-rtt-timeout</option></primary></indexterm>
        </term>
        <listitem>

<para>Nmap maintains a
running timeout value for determining how long it will wait for a
probe response before giving up or retransmitting the probe.  This is
calculated based on the response times of previous probes.
<notman>The exact formula is given in <xref linkend="port-scanning-algorithms" />.</notman>
If the network latency shows itself to be significant and variable,
this timeout can grow to several seconds.  It also starts at a
conservative (high) level and may stay that way for a while when Nmap
scans unresponsive hosts.</para>

<para>
Specifying a lower <option>--max-rtt-timeout</option> and
<option>--initial-rtt-timeout</option> than the defaults can cut scan
times significantly.  This is particularly true for pingless
(<option>-Pn</option>) scans, and those against heavily filtered
networks.  Don't get too aggressive though.  The scan can end up
taking longer if you specify such a low value that many probes are
timing out and retransmitting while the response is in transit.</para>

<para>If all the hosts are on a local network, 100 milliseconds
(<option>--max-rtt-timeout 100ms</option>) is a
reasonable aggressive value.  If
routing is involved, ping a host on the network first with the ICMP
ping utility, or with a custom packet crafter such as
Nping<indexterm><primary>Nping</primary></indexterm>
that is
more likely to get through a firewall.  Look at the maximum round trip
time out of ten packets or so.  You might want to double that for the
<option>--initial-rtt-timeout</option> and triple or quadruple it for
the <option>--max-rtt-timeout</option>.  I generally do not set the
maximum RTT below 100&nbsp;ms, no matter what the ping times are.  Nor do I
exceed 1000&nbsp;ms.</para>

<para><option>--min-rtt-timeout</option> is a rarely used option that
could be useful when a network is so unreliable that even Nmap's
default is too aggressive.  Since Nmap only reduces the timeout down to
the minimum when the network seems to be reliable, this need is
unusual and should be reported as a bug to the
<citetitle>nmap-dev</citetitle> mailing list.<indexterm><primary><citetitle>nmap-dev</citetitle> mailing list</primary></indexterm>
</para>

        </listitem>
      </varlistentry>


      <varlistentry>
        <term>
          <option>--max-retries <replaceable>numtries</replaceable></option> (Specify the
          maximum number of port scan probe retransmissions)
         <indexterm><primary><option>--max-retries</option></primary></indexterm>
        </term>
        <listitem>


<para>When Nmap receives no response to a port scan probe, it could
mean the port is filtered.  Or maybe the probe or response was simply
lost on the network.  It is also possible that the target host has
rate limiting enabled that temporarily blocked the response.  So Nmap
tries again by retransmitting the initial probe.  If Nmap detects poor
network reliability, it may try many more times before giving up on a
port.  While this benefits accuracy, it also lengthens scan times.
When performance is critical, scans may be sped up by limiting the
number of retransmissions allowed.  You can even specify
<option>--max-retries 0</option> to prevent any retransmissions,
though that is only recommended for situations such as informal
surveys where occasional missed ports and hosts are acceptable.
</para>

<para>The default (with no <option>-T</option> template) is to allow
ten retransmissions.  If a network seems reliable and the target hosts
aren't rate limiting, Nmap usually only does one retransmission.  So
most target scans aren't even affected by dropping
<option>--max-retries</option> to a low value such as three.  Such
values can substantially speed scans of slow (rate limited) hosts.
You usually lose some information when Nmap gives up on ports early,
though that may be preferable to letting the
<option>--host-timeout</option> expire and losing all information
about the target.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--host-timeout <replaceable>time</replaceable></option> (Give
          up on slow target hosts)
          <indexterm><primary><option>--host-timeout</option></primary></indexterm>
        </term>
        <listitem>


<para>Some hosts simply take a <emphasis>long</emphasis> time to scan.
This may be due to poorly performing or unreliable networking hardware
or software, packet rate limiting, or a restrictive firewall.  The
slowest few percent of the scanned hosts can eat up a majority of the
scan time.  Sometimes it is best to cut your losses and skip those
hosts initially.  Specify 
<option>--host-timeout</option> with the maximum amount of time you
are willing to wait.  For example,
specify <literal>30m</literal> to ensure that Nmap doesn't waste
more than half an hour on a single host.  Note that Nmap may be
scanning other hosts at the same time during that half an hour, so it isn't a complete loss.  A host that times out is skipped.
No port table, OS detection, or version detection results are printed
for that host.</para>

<para>The special value <literal>0</literal> can be used to mean <quote>no
    timeout</quote>, which can be used to override the <option>T5</option>
  timing template, which sets the host timeout to 15 minutes.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--script-timeout <replaceable>time</replaceable></option>
          <indexterm><primary><option>--script-timeout</option></primary></indexterm>
        </term>
        <listitem>

	  <para>While some scripts complete in fractions of a second,
	  others can take hours or more depending on the nature of the
	  script, arguments passed in, network and application
	  conditions, and more.  The <option>--script-timeout</option>
	  option sets a ceiling on script execution time.  Any script
	  instance which exceeds that time will be terminated and no
	  output will be shown.  If debugging (<option>-d</option>) is
	  enabled, Nmap will report on each timeout. For host and
	  service scripts, a script instance only scans a single
	  target host or port and the timeout period will be reset for
	  the next instance.</para>

<para>The special value <literal>0</literal> can be used to mean <quote>no
    timeout</quote>, which can be used to override the <option>T5</option>
  timing template, which sets the script timeout to 10 minutes.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--scan-delay <replaceable>time</replaceable></option>;
        <option>--max-scan-delay
        <replaceable>time</replaceable></option> (Adjust delay between probes)
        <indexterm><primary><option>--scan-delay</option></primary></indexterm>
        <indexterm><primary><option>--max-scan-delay</option></primary></indexterm>
        </term>
        <listitem>

<para>This option causes Nmap to wait at least the given amount of
time between each probe it sends to a given host.  This is
particularly useful in the case of rate limiting.<indexterm><primary>rate limiting</primary></indexterm>  Solaris machines
(among many others) will usually respond to UDP scan probe packets
with only one ICMP message per second.  Any more than that sent by
Nmap will be wasteful.  A <option>--scan-delay</option> of
<literal>1s</literal> will keep Nmap at that slow rate.  Nmap tries to
detect rate limiting and adjust the scan delay accordingly, but it
doesn't hurt to specify it explicitly if you already know what rate
works best.</para>

<para>When Nmap adjusts the scan delay upward to cope with rate
limiting, the scan slows down dramatically.  The
<option>--max-scan-delay</option> option specifies the largest delay
that Nmap will allow.  A low <option>--max-scan-delay</option>
can speed up Nmap, but it is risky.  Setting this value too low can lead to wasteful
packet retransmissions and possible missed ports when the target
implements strict rate limiting.</para>

<para>Another use of <option>--scan-delay</option> is to evade
threshold based intrusion detection and prevention systems
(IDS/IPS).<indexterm><primary>intrusion detection
systems</primary><secondary>evading</secondary></indexterm> <notman>This
technique is used in <xref linkend="defeating-ids-snort-portscan" />
to defeat the default port scan detector in Snort IDS.  Most other
intrusion detection systems can be defeated in the same way.</notman>
</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--min-rate <replaceable>number</replaceable></option>; <option>--max-rate <replaceable>number</replaceable></option>
        (Directly control the scanning rate)
        <indexterm><primary><option>--min-rate</option></primary></indexterm>
        <indexterm><primary><option>--max-rate</option></primary></indexterm>
        </term>
        <listitem>

<para>Nmap's dynamic timing does a good job of finding an appropriate
speed at which to scan. Sometimes, however, you may happen to know an
appropriate scanning rate for a network, or you may have to guarantee
that a scan will be finished by a certain time. Or perhaps you must keep
Nmap from scanning too quickly. The <option>--min-rate</option> and
<option>--max-rate</option> options are designed for these
situations.</para>

<para>When the
<option>--min-rate</option> option is given Nmap will do its best to
send packets as fast as or faster than the given rate. The argument is a
positive real number representing a packet rate in packets per second.
For example, specifying <option>--min-rate 300</option> means that
Nmap will try to keep the sending rate at or above 300 packets per
second. Specifying a minimum rate does not keep Nmap from going faster
if conditions warrant.</para>

<para>Likewise, <option>--max-rate</option> limits a scan's sending rate to a
given maximum. Use <option>--max-rate 100</option>, for example, to
limit sending to 100 packets per second on a fast network. Use
<option>--max-rate 0.1</option> for a slow scan of one packet every ten
seconds. Use <option>--min-rate</option> and <option>--max-rate</option>
together to keep the rate inside a certain range.</para>

<para>These two options are global, affecting an entire scan, not
individual hosts. They only affect port scans and host discovery scans.
Other features like OS detection implement their own timing.</para>

<para>There are two conditions when the actual scanning rate may fall
below the requested minimum. The first is if the minimum is faster than
the fastest rate at which Nmap can send, which is dependent on hardware.
In this case Nmap will simply send packets as fast as possible, but be
aware that such high rates are likely to cause a loss of accuracy. The
second case is when Nmap has nothing to send, for example at the end of
a scan when the last probes have been sent and Nmap is waiting for them
to time out or be responded to. It's normal to see the scanning rate
drop at the end of a scan or in between hostgroups. The sending rate may
temporarily exceed the maximum to make up for unpredictable delays, but
on average the rate will stay at or below the maximum.</para>

<para>Specifying a minimum rate should be done with care. Scanning
faster than a network can support may lead to a loss of accuracy. In
some cases, using a faster rate can make a scan take
<emphasis>longer</emphasis> than it would with a slower rate. This is
because Nmap's <man>adaptive retransmission</man>
<notman><link linkend="scan-methods-adaptive-retransmission">adaptive
retransmission</link><indexterm><primary>adaptive retransmission</primary><see>retransmission</see></indexterm><indexterm><primary>retransmission</primary></indexterm>
</notman>algorithms will detect the network congestion caused by an excessive scanning rate
and increase the number of retransmissions in order to improve accuracy.
So even though packets are sent at a higher rate, more packets are sent
overall. Cap the number of retransmissions with the
<option>--max-retries</option> option if you need to set an upper limit on total scan
time.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--defeat-rst-ratelimit</option>
        <indexterm><primary><option>--defeat-rst-ratelimit</option></primary></indexterm></term>
        <listitem>

<para>Many hosts have long used
rate limiting<indexterm><primary>rate limiting</primary></indexterm>
to reduce the number
of ICMP error messages (such as port-unreachable errors) they send.
Some systems now apply similar rate limits to the RST (reset)
packets they generate.  This can slow Nmap down dramatically as it
adjusts its timing to reflect those rate limits.  You can tell Nmap to
ignore those rate limits (for port scans such as SYN scan which
<emphasis>don't</emphasis> treat non-responsive ports as
<literal>open</literal>) by specifying
<option>--defeat-rst-ratelimit</option>.</para>

<para>Using this option can reduce accuracy, as some ports will appear
non-responsive because Nmap didn't wait long enough for a rate-limited
RST response.  With a SYN
scan, the non-response results in the port being labeled
<literal>filtered</literal> rather than the <literal>closed</literal>
state we see when RST packets are received.  This option is useful
when you only care about open ports, and distinguishing between
<literal>closed</literal> and <literal>filtered</literal> ports isn't
worth the extra time.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--defeat-icmp-ratelimit</option>
        <indexterm><primary><option>--defeat-icmp-ratelimit</option></primary></indexterm></term>
        <listitem>

<para>Similar to <option>--defeat-rst-ratelimit</option>, the
  <option>--defeat-icmp-ratelimit</option> option trades accuracy for speed,
  increasing UDP scanning speed against hosts that rate-limit ICMP error
  messages.  Because this option causes Nmap to not delay in order to receive
  the port unreachable messages, a non-responsive port will be labeled
  <literal>closed|filtered</literal> instead of the default
  <literal>open|filtered</literal>. This has the effect of only treating ports
  which actually respond via UDP as <literal>open</literal>. Since many UDP
  services do not respond in this way, the chance for inaccuracy is greater
  with this option than with <option>--defeat-rst-ratelimit</option>.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--nsock-engine
        iocp|epoll|kqueue|poll|select</option>
        <indexterm><primary><option>--nsock-engine</option></primary></indexterm>
        <indexterm><primary>Nsock IO engine</primary></indexterm>
        </term>
        <listitem>

<para>Enforce use of a given nsock IO multiplexing engine. Only the
<literal>select(2)</literal>-based fallback engine is guaranteed to be
available on your system.  Engines are named after the name of the IO
management facility they leverage.  Engines currently implemented are
<literal>epoll</literal>, <literal>kqueue</literal>, <literal>poll</literal>,
and <literal>select</literal>, but not all will be present on any platform.
By default, Nmap will use the "best" engine, i.e. the first one in this list that is supported.
Use <command>nmap -V</command> to see which engines are supported on your platform.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-T
          paranoid|sneaky|polite|normal|aggressive|insane</option>
          (Set a timing template)
        <indexterm><primary><option>-T</option></primary></indexterm>
        <indexterm><primary>timing templates</primary><seealso><literal>paranoid</literal>, <literal>sneaky</literal>, <literal>polite</literal>, <literal>normal</literal>, <literal>aggressive</literal>, and <literal>insane</literal></seealso></indexterm>
        </term>
        <listitem>
<indexterm><primary><option>-T0</option></primary><see><literal>paranoid</literal> timing template</see></indexterm>
<indexterm><primary><option>-T1</option></primary><see><literal>sneaky</literal> timing template</see></indexterm>
<indexterm><primary><option>-T2</option></primary><see><literal>polite</literal> timing template</see></indexterm>
<indexterm><primary><option>-T3</option></primary><see><literal>normal</literal> timing template</see></indexterm>
<indexterm><primary><option>-T4</option></primary><see><literal>aggressive</literal> timing template</see></indexterm>
<indexterm><primary><option>-T5</option></primary><see><literal>insane</literal> timing template</see></indexterm>


<para>While the fine-grained timing controls discussed in the previous
section are powerful and effective, some people find them confusing.
Moreover, choosing the appropriate values can sometimes take more time
than the scan you are trying to optimize.  Fortunately, Nmap offers a simpler
approach, with six timing templates.  You can specify them with the
<option>-T</option> option and their number (0&ndash;5) or their name.
The template names are
<option>paranoid</option>&nbsp;(<option>0</option>),
<option>sneaky</option>&nbsp;(<option>1</option>),
<option>polite</option>&nbsp;(<option>2</option>),
<option>normal</option>&nbsp;(<option>3</option>),
<option>aggressive</option>&nbsp;(<option>4</option>),
and <option>insane</option>&nbsp;(<option>5</option>).
The first two are for IDS evasion.
Polite mode slows down the scan to use less bandwidth
and target machine resources.  Normal mode is the default and so
<option>-T3</option> does nothing. Aggressive mode speeds scans up by
making the assumption that you are on a reasonably fast and reliable
network.  Finally
insane mode<indexterm><primary><literal>insane</literal> (<option>-T5</option>) timing template</primary></indexterm>
assumes that you are on an
extraordinarily fast network or are willing to sacrifice some accuracy
for speed.</para>

<para>These templates allow the user to specify how aggressive they
wish to be, while leaving Nmap to pick the exact timing values.  The
templates also make some minor speed adjustments for which
fine-grained control options do not currently exist.  For example,
<option>-T4</option><indexterm><primary><literal>aggressive</literal> (<option>-T4</option>) timing template</primary></indexterm>
prohibits the dynamic scan delay from exceeding
10&nbsp;ms for TCP ports and <option>-T5</option> caps that value at 5&nbsp;ms.
Templates can be used in combination with fine-grained
controls, and the fine-grained controls that you specify will take
precedence over the timing template default for that parameter.  I
recommend using <option>-T4</option> when scanning reasonably modern
and reliable networks.  Keep that option even when you add
fine-grained controls so that you benefit from those extra minor
optimizations that it enables.</para>

<para>If you are on a decent broadband or ethernet connection, I would
recommend always using <option>-T4</option>.  Some people love
<option>-T5</option> though it is too aggressive for my taste.  People
sometimes specify <option>-T2</option> because they think it is less
likely to crash hosts or because they consider themselves to be polite
in general.  They often don't realize just how slow <option>-T
polite</option><indexterm><primary><literal>polite</literal> (<option>-T2</option>) timing template</primary></indexterm>
really is.  Their scan may take ten times longer than a
default scan.
Machine crashes and bandwidth problems are rare with the
default timing options (<option>-T3</option>) and so I normally
recommend that for cautious scanners.  Omitting version detection is
far more effective than playing with timing values at reducing these
problems.</para>

<para>While
<option>-T0</option><indexterm><primary><literal>paranoid</literal> (<option>-T0</option>) timing template</primary></indexterm>
and <option>-T1</option><indexterm><primary><literal>sneaky</literal> (<option>-T1</option>) timing template</primary></indexterm>
may be
useful for avoiding IDS alerts, they will take an extraordinarily long
time to scan thousands of machines or ports.  For such a long scan,
you may prefer to set the exact timing values you need rather than
rely on the canned <option>-T0</option> and <option>-T1</option>
values.</para>

<para>The main effects of <option>T0</option> are serializing the scan
so only one port is scanned at a time, and waiting five minutes
between sending each probe.  <option>T1</option> and
<option>T2</option> are similar but they only wait 15 seconds and 0.4
seconds, respectively, between probes.  <option>T3</option><indexterm><primary><literal>normal</literal> (<option>-T3</option>) timing template</primary></indexterm> is Nmap's
default behavior, which includes parallelization.
<option>-T4</option>
does the equivalent of <option>
  --max-rtt-timeout 1250ms
  --min-rtt-timeout 100ms
  --initial-rtt-timeout 500ms
  --max-retries 6
</option> and sets the maximum TCP and SCTP scan delay
to 10ms.  <option>T5</option>
does the equivalent of
<option>
  --max-rtt-timeout 300ms
  --min-rtt-timeout 50ms
  --initial-rtt-timeout 250ms
  --max-retries 2
  --host-timeout 15m
  --script-timeout 10m
</option> as well as
setting the maximum TCP and SCTP scan delay to 5ms.
Maximum UDP scan delay is not set by <option>T4</option> or <option>T5</option>, but it can be set with the <option>--max-scan-delay</option> option.
</para>

        </listitem>
      </varlistentry>
    </variablelist>
    <indexterm class="endofrange" startref="man-performance-indexterm"/>
    <indexterm class="endofrange" startref="man-performance-timing-indexterm"/>
  </refsect1>

  <refsect1 id='man-bypass-firewalls-ids'>
    <title>Firewall/IDS Evasion and Spoofing</title>
    <indexterm class="startofrange" id="man-bypass-filewalls-indexterm"><primary>firewalls</primary><secondary>bypassing</secondary></indexterm>
    <indexterm class="startofrange" id="man-bypass-ids-indexterm"><primary>intrusion detection systems</primary><secondary>evading</secondary></indexterm>

<para>Many Internet pioneers envisioned a global open network with a
universal IP address space allowing virtual connections between any
two nodes.  This allows hosts to act as true peers, serving and
retrieving information from each other.  People could access all of
their home systems from work, changing the climate control settings or
unlocking the doors for early guests.  This vision of universal
connectivity has been stifled by address space shortages and security
concerns.  In the early 1990s, organizations began deploying
firewalls for the express purpose of reducing connectivity.  Huge
networks were cordoned off from the unfiltered Internet by application
proxies, network address translation, and packet filters.  The
unrestricted flow of information gave way to tight regulation of
approved communication channels and the content that passes over
them.</para>

<para>Network obstructions such as firewalls can make mapping a
network exceedingly difficult.  It will not get any easier, as
stifling casual reconnaissance is often a key goal of implementing the
devices.   Nevertheless, Nmap offers many features to help understand these
complex networks, and to verify that filters are working as intended.
It even supports mechanisms for bypassing poorly implemented
defenses.  One of the best methods of understanding your
network security posture is to try to defeat it.  Place yourself in
the mind-set of an attacker, and deploy techniques from this section
against your networks.  Launch an FTP bounce scan, idle scan,
fragmentation attack, or try to tunnel through one of your own
proxies.</para>

<para>In addition to restricting network activity, companies are
increasingly monitoring traffic with intrusion detection systems
(IDS).  All of the major IDSs ship with rules designed to detect Nmap
scans because scans are sometimes a precursor to attacks.  Many of
these products have recently morphed into intrusion
<emphasis>prevention</emphasis> systems
(IPS)<indexterm><primary>intrusion prevention systems</primary><seealso>intrusion detection systems</seealso></indexterm>
that actively block
traffic deemed malicious.  Unfortunately for network administrators
and IDS vendors, reliably detecting bad intentions by analyzing packet
data is a tough problem.  Attackers with patience, skill, and the help
of certain Nmap options can usually pass by IDSs undetected.
Meanwhile, administrators must cope with large numbers of false
positive results where innocent activity is misdiagnosed and alerted
on or blocked.</para>

<para>Occasionally people suggest that Nmap should not offer features
for evading firewall rules or sneaking past IDSs.  They argue
that these features are just as likely to be misused by attackers as
used by administrators to enhance security.  The problem with this
logic is that these methods would still be used by attackers, who
would just find other tools or patch the functionality into Nmap.
Meanwhile, administrators would find it that much harder to do their
jobs.  Deploying only modern, patched FTP servers is a far more
powerful defense than trying to prevent the distribution of tools
implementing the FTP bounce attack.
</para>

<para>There is no magic bullet (or Nmap option) for detecting and
subverting firewalls and IDS systems.  It takes skill and experience.
A tutorial is beyond the scope of this reference guide, which only
lists the relevant options and describes what they do.</para>

<variablelist>
      <varlistentry>
        <term>
          <option>-f</option> (fragment packets);
          <option>--mtu</option> (using the specified MTU)
          <indexterm significance="preferred"><primary><option>-f</option></primary></indexterm>
          <indexterm significance="preferred"><primary><option>--mtu</option></primary></indexterm>
        </term>

        <listitem>
          <para>The <option>-f</option> option causes the requested scan (including
          host discovery scans) to use tiny fragmented IP packets. The idea
          is to split up the TCP header over several packets to
          make it harder for packet filters, intrusion detection
          systems, and other annoyances to detect what you are
          doing. Be careful with this! Some programs have trouble
          handling these tiny packets. The old-school sniffer named
          Sniffit segmentation faulted immediately upon receiving
          the first fragment. Specify this option once, and Nmap
          splits the packets into eight bytes or less after the IP
          header. So a 20-byte TCP header would be split into three
          packets. Two with eight bytes of the TCP header, and one
          with the final four. Of course each fragment also has an
          IP header. Specify <option>-f</option> again to use 16 bytes per fragment
          (reducing the number of fragments).<indexterm><primary><option>-f</option></primary><secondary>giving twice</secondary></indexterm>
          Or you can specify
          your own offset size with the <option>--mtu</option> option. Don't also
          specify <option>-f</option> if you use <option>--mtu</option>. The offset must be a
          multiple of eight. While fragmented packets won't get by
          packet filters and firewalls that queue all IP fragments,
          such as the <varname>CONFIG_IP_ALWAYS_DEFRAG</varname> option in the Linux
          kernel, some networks can't afford the performance hit
          this causes and thus leave it disabled. Others can't enable
          this because fragments may take different routes into their
          networks.  Some source
          systems defragment outgoing packets in the kernel. Linux
          with the
	  iptables<indexterm><primary>iptables</primary></indexterm>
          connection tracking module is one such
          example. Do a scan while a sniffer such as
	  <application>Wireshark</application><indexterm><primary><application>Wireshark</application></primary></indexterm>
          is running to ensure that sent packets are fragmented.  If your host
          OS is causing problems, try the
	  <option>--send-eth</option><indexterm><primary><option>--send-eth</option></primary></indexterm>
          option to bypass the IP layer and send raw ethernet frames.</para>

<para>Fragmentation is only supported for Nmap's raw packet features,
which includes TCP and UDP port scans (except connect scan and FTP
bounce scan) and OS detection.  Features such as version detection and
the Nmap Scripting Engine generally don't support fragmentation
because they rely on your host's TCP stack to communicate with target
services.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-D <replaceable>decoy1</replaceable><optional>,<replaceable>decoy2</replaceable></optional><optional>,ME</optional><optional>,...</optional></option>
          (Cloak a scan with decoys)
          <indexterm><primary><option>-D</option></primary></indexterm> 
          <indexterm><primary>decoys</primary></indexterm>
        </term>
        <listitem>
          <para>Causes a decoy scan to be performed, which makes it
          appear to the remote host that the host(s) you specify as
          decoys are scanning the target network too. Thus their IDS
          might report 5&ndash;10 port scans from unique IP addresses, but
          they won't know which IP was scanning them and which were
          innocent decoys. While this can be defeated through router
          path tracing, response-dropping, and other active
          mechanisms, it is generally an effective technique for
          hiding your IP address.</para>

          <para>Separate each decoy host with commas, and you can
          optionally use
	  <literal>ME</literal><indexterm><primary><literal>ME</literal> (decoy address)</primary></indexterm>
          as one of the decoys to
          represent the position for your real IP address. If you put
          <literal>ME</literal> in the sixth position or later, some
          common port scan detectors (such as
	  Solar Designer's<indexterm><primary>Solar Designer</primary></indexterm>
          excellent Scanlogd)<indexterm><primary><application>Scanlogd</application></primary></indexterm>
          are unlikely to show your IP address at
          all. If you don't use <literal>ME</literal>, Nmap will put
          you in a random position.  You can also use
	  <literal>RND</literal><indexterm><primary><literal>RND</literal> (decoy address)</primary></indexterm>
          to generate
          a random, non-reserved IP address, or <literal>RND:<replaceable>number</replaceable></literal> to
          generate <replaceable>number</replaceable> addresses.</para> <para>Note that the hosts
          you use as decoys should be up or you might accidentally SYN
          flood your targets. Also it will be pretty easy to determine
          which host is scanning if only one is actually up on the
          network. You might want to use IP addresses instead of names
          (so the decoy networks don't see you in their nameserver
          logs). Right now random IP address generation is only supported with IPv4</para>

          <para>Decoys are used both in the initial host discovery scan (using
          ICMP, SYN, ACK, or whatever) and during the actual port
          scanning phase. Decoys are also used during remote OS
          detection (<option>-O</option>).  Decoys do not work with
          version detection or TCP connect scan.  When a scan delay is
          in effect, the delay is enforced between each batch of
          spoofed probes, not between each individual probe. Because
          decoys are sent as a batch all at once, they may temporarily
          violate congestion control limits.</para>

          <para>It is worth noting that using too many decoys may
          slow your scan and potentially even make it less
          accurate. Also, some ISPs will filter out your spoofed
          packets, but many do not restrict
          spoofed IP packets at all.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-S <replaceable>IP_Address</replaceable></option> (Spoof source address)
          <indexterm><primary><option>-S</option></primary></indexterm>
          <indexterm><primary>spoofing source address</primary></indexterm>
        </term>
        <listitem>

          <para>In some circumstances, 
          Nmap may not be able to determine your
          source address (Nmap will tell you if this is the
          case). In this situation, use <option>-S</option> with the IP address of
          the interface you wish to send packets through.</para>

          <para>Another possible use of this flag is to spoof the scan
          to make the targets think that <emphasis>someone
          else</emphasis> is scanning them. Imagine a company being
          repeatedly port scanned by a competitor!  The
          <option>-e</option> option and <option>-Pn</option> are
          generally required for this sort of usage.  Note that you
          usually won't receive reply packets back (they will be
          addressed to the IP you are spoofing), so Nmap won't produce
          useful reports.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-e <replaceable>interface</replaceable></option> (Use specified interface)
          <indexterm><primary><option>-e</option></primary></indexterm>
          <indexterm><primary>interface</primary><seealso><option>-e</option></seealso></indexterm>
        </term>
        <listitem>

          <para>Tells Nmap what interface to send and receive
          packets on. Nmap should be able to detect this
          automatically, but it
          will tell you if it cannot.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--source-port <replaceable>portnumber</replaceable>;</option>
          <option>-g <replaceable>portnumber</replaceable></option> (Spoof source port number)
          <indexterm significance="preferred"><primary><option>--source-port</option></primary></indexterm>
          <indexterm significance="preferred"><primary><option>-g</option></primary></indexterm>
          <indexterm><primary>source port number</primary></indexterm>
        </term>
        <listitem>

<para>One surprisingly common misconfiguration is to trust traffic
based only on the source port number.  It is easy to understand how
this comes about.  An administrator will set up a shiny new firewall,
only to be flooded with complaints from ungrateful users whose
applications stopped working.  In particular, DNS may be broken
because the UDP DNS replies from external servers can no longer enter
the network.  FTP is another common example.  In active FTP transfers,
the remote server tries to establish a connection back to the client
to transfer the requested file.</para>

<para>Secure solutions to these problems exist, often in the form of
application-level proxies or protocol-parsing firewall modules.
Unfortunately there are also easier, insecure solutions.  Noting that
DNS replies come from port 53 and active FTP from port 20, many administrators
have fallen into the trap of simply allowing incoming traffic from
those ports.  They often assume that no attacker would notice and
exploit such firewall holes.  In other cases, administrators consider this a
short-term stop-gap measure until they can implement a more secure
solution.  Then they forget the security upgrade.
</para>

<para>Overworked network administrators are not the only ones to fall
into this trap.  Numerous products have shipped with these insecure
rules.  Even Microsoft has been guilty.  The IPsec filters that
shipped with Windows 2000 and Windows XP contain an implicit rule that
allows all TCP or UDP traffic from port 88 (Kerberos).  In another well-known
case, versions of the Zone Alarm personal firewall up to 2.1.25
allowed any incoming UDP packets with the source port 53 (DNS) or 67
(DHCP).</para>

<para>Nmap offers the <option>-g</option> and
<option>--source-port</option> options (they are equivalent) to exploit these
weaknesses.  Simply provide a port number and Nmap will send packets
from that port where possible.  Most scanning operations that use raw sockets,
including SYN and UDP scans, support the option completely.  The option notably
doesn't have an effect for any operations that use normal operating system
sockets, including DNS requests, TCP <function>connect</function>
scan,<indexterm><primary>connect scan</primary></indexterm> version detection,
and script scanning. Setting the source port also doesn't work for OS detection,
because Nmap must use different port numbers for certain OS detection tests to
work properly.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
           <option>--data <replaceable>hex string</replaceable></option> (Append custom binary data to sent packets)
          <indexterm><primary><option>--data</option></primary></indexterm>
        </term>
        <listitem>
          <para>
           This option lets you include binary data as payload in sent packets.
           <replaceable>hex string</replaceable> may be specified in any of
           the following formats: <literal>0xAABBCCDDEEFF<replaceable>...</replaceable></literal>,
           <literal>AABBCCDDEEFF<replaceable>...</replaceable></literal> or
           <literal>\xAA\xBB\xCC\xDD\xEE\xFF<replaceable>...</replaceable></literal>.
           Examples of use are <option>--data 0xdeadbeef</option> and
           <option>--data \xCA\xFE\x09</option>. Note that if you specify a
           number like <literal>0x00ff</literal>
           no byte-order conversion is performed. Make sure you specify
           the information in the byte order expected by the receiver.
        </para>
        </listitem>
      </varlistentry>

     
      <varlistentry>
        <term>
          <option>--data-string <replaceable>string</replaceable></option> (Append custom string to sent packets)
          <indexterm><primary><option>--data-string</option></primary></indexterm>
        </term>
        <listitem>
          <para>
            This option lets you include a regular string as payload in
            sent packets. <replaceable>string</replaceable> can
            contain any string. However, note that some characters
            may depend on your system's locale and the receiver may not
            see the same information. Also, make sure you enclose the string
            in double quotes and escape any special characters from the shell.
            Examples: <option>--data-string "Scan conducted by Security Ops, extension 7192"</option>
            or <option>--data-string "Ph34r my l33t skills"</option>.
            Keep in mind that nobody is likely to actually see any comments left by this option
            unless they are carefully monitoring the network with a sniffer or custom IDS rules.
        </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--data-length <replaceable>number</replaceable></option> (Append random
          data to sent packets)
          <indexterm><primary><option>--data-length</option></primary></indexterm>
        </term>
        <listitem>
          <para>Normally Nmap sends minimalist packets containing only
          a header. So its TCP packets are generally 40
          bytes and ICMP echo requests are just 28. Some
          UDP ports<indexterm><primary>protocol-specific payloads</primary><secondary>UDP</secondary></indexterm>
          and IP protocols<indexterm><primary>protocol-specific payloads</primary><secondary>IP</secondary></indexterm>
          get a custom payload by default.
          This option
          tells Nmap to append the given number of random bytes to
          most of the packets it sends, and not to use any
          protocol-specific payloads. (Use <option>--data-length 0</option>
          for no random or protocol-specific
          payloads.<indexterm><primary>protocol-specific payloads</primary><secondary>disabling with <option>--data-length</option></secondary></indexterm>
          OS detection (<option>-O</option>) packets
          are not affected<indexterm><primary><option>--data-length</option></primary><secondary>no effect in OS detection</secondary></indexterm>
          because accuracy there requires probe consistency, but most pinging and portscan packets
          support this. It slows things down a little, but can make a scan slightly less
          conspicuous.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
      <term>
      <option>--ip-options <replaceable>R|S [route]|L [route]|T|U ... </replaceable>;</option>
      <option>--ip-options <replaceable>hex string</replaceable></option>  (Send packets with specified ip options)
      <indexterm><primary><option>--ip-options</option></primary></indexterm>
      <indexterm><primary>IP options</primary></indexterm>
      </term>
      <listitem>
      <para>The <ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc791.txt">IP
      protocol</ulink> offers several options which may be placed in
      packet headers.  Unlike the ubiquitous TCP options, IP options
      are rarely seen due to practicality and security concerns.  In
      fact, many Internet routers block the most dangerous options
      such as source routing.  Yet options can still be useful in some
      cases for determining and manipulating the network route to
      target machines.  For example, you may be able to use the record
      route option to determine a path to a target even when more
      traditional traceroute-style approaches fail.  Or if your
      packets are being dropped by a certain firewall, you may be able
      to specify a different route with the strict or loose source
      routing options.</para>

      <para>The most powerful way to specify IP options is to simply
      pass in values as the argument to
      <option>--ip-options</option>.  Precede each hex number with
      <literal>\x</literal> then the two digits.  You may repeat
      certain characters by following them with an asterisk and then
      the number of times you wish them to repeat.  For example,
      <literal>\x01\x07\x04\x00*36\x01</literal> is a hex string
      containing 36 NUL bytes.</para>

      <para>Nmap also offers a shortcut mechanism for specifying
      options.  Simply pass the letter <literal>R</literal>,
      <literal>T</literal>, or <literal>U</literal> to request
      record-route,<indexterm><primary>record route IP option</primary></indexterm>
      record-timestamp,<indexterm><primary>record timestamp IP option</primary></indexterm>
      or both options together,
      respectively.
      Loose or strict source routing<indexterm><primary>source routing</primary></indexterm>
      may be specified
      with an <literal>L</literal> or <literal>S</literal> followed by
      a space and then a space-separated list of IP addresses.</para>

      <para>If you wish to see the options in packets sent and
      received, specify <option>--packet-trace</option>.  For more
      information and examples of using IP options with Nmap, see
      <ulink url="https://seclists.org/nmap-dev/2006/q3/52"/>.
      </para>

      </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--ttl <replaceable>value</replaceable></option> (Set IP time-to-live field)
          <indexterm><primary><option>--ttl</option></primary></indexterm>
          <indexterm><primary>time to live (TTL)</primary></indexterm>
        </term>
        <listitem>

          <para>Sets the IPv4 time-to-live field in sent packets to
          the given value.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--randomize-hosts</option> (Randomize target host order)
          <indexterm significance="preferred"><primary><option>--randomize-hosts</option></primary></indexterm>
          <indexterm><primary>randomization of hosts</primary></indexterm>
        </term>
        <listitem>

          <para>Tells Nmap to shuffle each group of up to 16384 hosts
          before it scans them. This can make the scans less obvious
          to various network monitoring systems, especially when you
          combine it with slow timing options.  If you
          want to randomize over larger group sizes, increase
          <varname>PING_GROUP_SZ</varname><indexterm><primary><varname>PING_GROUP_SZ</varname></primary></indexterm>
          in <filename>nmap.h</filename><indexterm><primary><filename>nmap.h</filename></primary></indexterm>
          and recompile.
          An alternative solution is to generate the target IP list
          with a list scan (<option>-sL -n -oN
          <replaceable>filename</replaceable></option>), randomize it
          with a Perl script, then provide the whole list to Nmap with
          <option>-iL</option>.<indexterm><primary><option>-iL</option></primary><secondary>randomizing hosts with</secondary></indexterm>
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--spoof-mac <replaceable>MAC address, prefix, or vendor
          name</replaceable></option> (Spoof MAC address)
        <indexterm significance="preferred"><primary><option>--spoof-mac</option></primary></indexterm>
        <indexterm><primary>spoofing MAC address</primary></indexterm>
        </term>
        <listitem>

          <para>Asks Nmap to use the given MAC address
          <indexterm><primary>MAC address</primary></indexterm>
  for all of the raw ethernet frames it sends.  This option implies
  <option>--send-eth</option><indexterm><primary><option>--send-eth</option></primary><secondary>implied by <option>--spoof-mac</option></secondary></indexterm>
  to ensure that Nmap actually sends
  ethernet-level packets.  The MAC given can take several formats.  If
  it is simply the number <literal>0</literal>, Nmap chooses a completely random MAC address
  for the session.  If the given string is an even number of hex
  digits (with the pairs optionally separated by a colon), Nmap will
  use those as the MAC.  If fewer than 12 hex digits are provided, Nmap
  fills in the remainder of the six bytes with random values.  If the
  argument isn't a zero or hex string, Nmap looks through
  <filename>nmap-mac-prefixes</filename> to find a vendor name containing the given string
  (it is case insensitive).  If a match is found, Nmap uses the
  vendor's OUI (three-byte prefix)<indexterm><primary>organizationally unique identifier (OUI)</primary><seealso><filename>nmap-mac-prefixes</filename></seealso></indexterm>
  and fills out the remaining three bytes
  randomly.  Valid <option>--spoof-mac</option> argument examples are <literal>Apple</literal>, <literal>0</literal>,
  <literal>01:02:03:04:05:06</literal>, <literal>deadbeefcafe</literal>, <literal>0020F2</literal>, and <literal>Cisco</literal>.  This option only affects raw packet scans such as SYN scan or OS detection, not connection-oriented features such as version detection or the Nmap Scripting Engine.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--proxies <replaceable>Comma-separated list of proxy
          URLs</replaceable></option> (Relay TCP connections through a chain of
          proxies)

        <indexterm significance="preferred"><primary><option>--proxies</option></primary></indexterm>
        <indexterm><primary>proxy</primary></indexterm>
        <indexterm><primary>proxies</primary></indexterm>
        </term>
        <listitem>

          <para>Asks Nmap to establish TCP connections with a final
          target through supplied chain of one or more HTTP or SOCKS4
          proxies<indexterm><primary>proxies</primary></indexterm>.  Proxies
          can help hide the true source of a scan or evade certain
          firewall restrictions, but they can hamper scan performance
          by increasing latency.  Users may need to adjust Nmap
          timeouts and other scan parameters accordingly. In
          particular, a lower <option>--max-parallelism</option> may
          help because some proxies refuse to handle as many
          concurrent connections as Nmap opens by default.</para>

          <para>This option takes a list of proxies as argument, expressed as
          URLs in the format <literal>proto://host:port</literal>. Use commas to separate
          node URLs in a chain. No authentication is supported yet. Valid
          protocols are <literal>HTTP</literal> and <literal>SOCKS4</literal>.
          </para>

          <para>Warning: this feature is still under development and has
          limitations. It is implemented within the nsock library and thus has
          no effect on the ping, port scanning and OS discovery phases
	  of a scan. Only NSE and version scan benefit from this
	  option so far&mdash;other features may disclose your true address. SSL
          connections are not yet supported, nor is proxy-side DNS
          resolution (hostnames are always resolved by Nmap).</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--badsum</option> (Send packets with bogus TCP/UDP checksums)
        <indexterm><primary><option>--badsum</option></primary></indexterm>
        <indexterm><primary>TCP checksum</primary></indexterm>
        <indexterm><primary>checksums</primary></indexterm>
        </term>
        <listitem>


          <para>Asks Nmap to use an invalid TCP, UDP or SCTP checksum for
  packets sent to target hosts.  Since virtually all
  host IP stacks properly drop these packets, any responses received
  are likely coming from a firewall or IDS that didn't bother to
  verify the checksum.  For more details on this technique, see <ulink
  url="https://nmap.org/p60-12.html"/></para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--adler32</option> (Use deprecated Adler32 instead of CRC32C for SCTP checksums)
        <indexterm><primary><option>--adler32</option></primary></indexterm>
        <indexterm><primary>CRC32C checksum</primary></indexterm>
        <indexterm><primary>Adler32 checksum</primary></indexterm>
        <indexterm><primary>SCTP checksum</primary></indexterm>
        </term>
        <listitem>


          <para>Asks Nmap to use the deprecated Adler32 algorithm
  for calculating the SCTP checksum.  If <option>--adler32</option>
  is not given, CRC-32C (Castagnoli) is used.
  <ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc2960.txt">RFC 2960</ulink>
  originally defined Adler32 as checksum algorithm for SCTP;
  <ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc4960.txt">RFC 4960</ulink>
  later redefined the SCTP checksums to use CRC-32C.  Current SCTP
  implementations should be using CRC-32C, but in order to elicit
  responses from old, legacy SCTP implementations, it may be
  preferable to use Adler32.</para>
        </listitem>
      </varlistentry>

    </variablelist>

    <indexterm class="endofrange" startref="man-bypass-ids-indexterm"/>
    <indexterm class="endofrange" startref="man-bypass-filewalls-indexterm"/>
  </refsect1>

  <refsect1 id='man-output'>
    <title>Output</title>
    <indexterm class="startofrange" id="man-output-indexterm"><primary>output formats</primary></indexterm>

    <para>Any security tool is only as useful as the output it
    generates.  Complex tests and algorithms are of little value if
    they aren't presented in an organized and comprehensible fashion.
    Given the number of ways Nmap is used by people and other
    software, no single format can please everyone.  So Nmap offers
    several formats, including the interactive mode for humans to read
    directly and XML for easy parsing by software.</para>

<para>In addition to offering different output formats, Nmap provides
options for controlling the verbosity of output as well as debugging
messages.  Output types may be sent to standard output or to named
files, which Nmap can append to or clobber.  Output files may also be
used to resume aborted scans.</para>

<para>Nmap makes output available in five different formats.
The default is called
<firstterm>interactive output</firstterm>,<indexterm><primary>interactive output</primary></indexterm>
and it is sent to
standard output (stdout).<indexterm><primary>standard output</primary></indexterm>
There is also
<firstterm>normal output</firstterm>,<indexterm><primary>normal output</primary></indexterm>
which is similar to interactive except that it
displays less runtime information and warnings since it is expected to
be analyzed after the scan completes rather than interactively.</para>

<para><firstterm>XML output</firstterm><indexterm><primary>XML output</primary></indexterm>
is one of the most important output types, as it can
be converted to HTML, easily parsed by programs such as Nmap graphical
user interfaces, or imported into databases.</para>

<para>The two remaining output types are the simple
<firstterm>grepable output</firstterm><indexterm><primary>grepable output</primary></indexterm>
which includes most information for a target host on
a single line, and
<firstterm>sCRiPt KiDDi3 0utPUt</firstterm><indexterm><primary sortas="script kiddie output">scR1pT kIddI3 output</primary></indexterm>
for users
who consider themselves |&lt;-r4d.</para>

<para>While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax.  They take one argument, which is the filename that results
should be stored in.  Multiple formats may be specified, but each
format may only be specified once.  For example, you may wish to save
normal output for your own review while saving XML of the same scan
for programmatic analysis.  You might do this with the options
<option>-oX myscan.xml -oN myscan.nmap</option>.  While this chapter
uses the simple names like <literal>myscan.xml</literal> for brevity,
more descriptive names are generally recommended.  The names chosen
are a matter of personal preference, though I use long ones that
incorporate the scan date and a word or two describing the scan, placed
in a directory named after the company I'm scanning.</para>

<para>While these options save results to files, Nmap still prints
interactive output to stdout as usual.  For example, the command
<command>nmap -oX myscan.xml target</command> prints XML to
<filename>myscan.xml</filename> and fills standard output with the same interactive results it would have printed if <option>-oX</option>
wasn't specified at all.  You can change this by passing a hyphen
character as the argument to one of the format types.  This causes
Nmap to deactivate interactive output, and instead print
results in the format you specified to the standard output stream.  So the
command <command>nmap -oX - target</command> will send only XML output to
stdout.<indexterm><primary>output</primary><secondary>to stdout with <literal>-</literal></secondary></indexterm>
Serious errors may still be printed to the normal error
stream, stderr.<indexterm><primary>standard error</primary></indexterm>
</para>

<para>Unlike some Nmap arguments, the space between the logfile option
flag (such as <option>-oX</option>) and the filename or hyphen is
mandatory.  If you omit the flags and give arguments such as
<option>-oG-</option> or <option>-oXscan.xml</option>, a backwards
compatibility feature of Nmap will cause the creation of
<emphasis>normal format</emphasis> output files named
<filename>G-</filename> and <filename>Xscan.xml</filename>
respectively.</para>

<para>All of these arguments support
<function>strftime</function>-like<indexterm><primary><function>strftime</function> conversions in filenames</primary></indexterm>
conversions in the filename.  <literal>%H</literal>, <literal>%M</literal>,
<literal>%S</literal>, <literal>%m</literal>, <literal>%d</literal>,
<literal>%y</literal>, and <literal>%Y</literal> are all exactly the same
as in <function>strftime</function>.  <literal>%T</literal> is the same
as <literal>%H%M%S</literal>, <literal>%R</literal> is the same as
<literal>%H%M</literal>, and <literal>%D</literal> is the same as
<literal>%m%d%y</literal>.  A <literal>%</literal> followed by any other
character just yields that character (<literal>%%</literal> gives you a
percent symbol).  So <option>-oX 'scan-%T-%D.xml'</option> will use an XML
file with a name in the form of <filename>scan-144840-121307.xml</filename>.</para>

<para>Nmap also offers options to control scan verbosity and to append
to output files rather than clobbering them.  All of these options are
described below.</para>

<variablelist><title>Nmap Output Formats</title>
      <varlistentry>
        <term>
        <option>-oN <replaceable>filespec</replaceable></option> (normal output)
          <indexterm><primary><option>-oN</option></primary></indexterm>
          <indexterm><primary>normal output</primary></indexterm></term>
        <listitem>

          <para>Requests that normal output be
          directed to the given filename.  As discussed above, this
          differs slightly from <literal>interactive output</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-oX <replaceable>filespec</replaceable></option> (XML output)
          <indexterm><primary><option>-oX</option></primary></indexterm>
          <indexterm><primary>XML output</primary></indexterm></term>
        <listitem>


          <para>Requests that XML output be
          directed to the given filename.  Nmap includes a document
          type definition (DTD) which allows XML parsers to validate
          Nmap XML output.  While it is primarily intended for
          programmatic use, it can also help humans interpret Nmap XML
          output.  The DTD defines the legal elements of the format,
          and often enumerates the attributes and values they can take
          on.  The latest version is always available from <ulink
          url="https://svn.nmap.org/nmap/docs/nmap.dtd"/>.</para>

          <para>XML offers a stable format that is easily parsed by
          software. Free XML parsers are available for all major
          computer languages, including C/C++, Perl, Python, and
          Java. People have even written bindings for most of these
          languages to handle Nmap output and execution specifically.
          Examples are
          <ulink url="http://sourceforge.net/projects/nmap-scanner/">Nmap::Scanner</ulink><indexterm><primary>Nmap::Scanner</primary></indexterm>
          and
          <ulink url="http://nmapparser.wordpress.com/">Nmap::Parser</ulink><indexterm><primary>Nmap::Parser</primary></indexterm>
          in Perl
          CPAN.  In almost all cases that a non-trivial application
          interfaces with Nmap, XML is the preferred format.</para>

          <para>The XML output references an XSL stylesheet which can
          be used to format the results as HTML.  The easiest way to
          use this is simply to load the XML output in a web browser
          such as Firefox or IE.  By default, this will only work on
          the machine you ran Nmap on (or a similarly configured one)
          due to the hard-coded <filename>nmap.xsl</filename>
          filesystem path.  Use the <option>--webxml</option> or
          <option>--stylesheet</option> options to create portable XML
          files that render as HTML on any web-connected
          machine.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-oS <replaceable>filespec</replaceable></option> (ScRipT KIdd|3 oUTpuT)
        <indexterm><primary><option>-oS</option></primary></indexterm>
        <indexterm><primary sortas="script kiddie output">scR1pT kIddI3 output</primary></indexterm></term>
        <listitem>

   <para>Script kiddie output is like interactive output, except that
   it is post-processed to better suit the l33t HaXXorZ who
   previously looked down on Nmap due to its consistent capitalization
   and spelling.  Humor impaired people should note that this option
   is making fun of the script kiddies before flaming me for
   supposedly <quote>helping them</quote>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-oG <replaceable>filespec</replaceable></option> (grepable output)
        <indexterm><primary><option>-oG</option></primary></indexterm>
        <indexterm><primary>grepable output</primary></indexterm></term>
        <listitem>


<para>This output format is covered last because it is deprecated.
The XML output format is far more powerful, and is nearly as
convenient for experienced users.  XML is a standard for which dozens
of excellent parsers are available, while grepable output is my own
simple hack.  XML is extensible to support new Nmap features as they
are released, while I often must omit those features from grepable
output for lack of a place to put them.</para>

<para>Nevertheless, grepable output is still quite popular.  It is a
simple format that lists each host on one line and can be trivially
searched and parsed with standard Unix tools such as grep, awk, cut,
sed, diff, and Perl.  Even I usually use it for one-off tests done at the
command line.  Finding all the hosts with the SSH port open or that
are running Solaris takes only a simple grep to identify the hosts,
piped to an awk or cut command to print the desired fields.</para>

<para>Grepable output consists of comments (lines starting with a
pound (#))<indexterm><primary>grepable output</primary><secondary>comments in</secondary></indexterm>
and target lines.  A target line includes a combination
of six labeled fields, separated by tabs and followed with a colon.
The fields are <literal>Host</literal>, <literal>Ports</literal>,
<literal>Protocols</literal>, <literal>Ignored State</literal>,
<literal>OS</literal>, <literal>Seq Index</literal>,
<literal>IP ID</literal>, and <literal>Status</literal>.</para>

<para>The most important of these fields is generally
<literal>Ports</literal>, which gives details on each interesting
port.  It is a comma separated list of port entries.  Each port entry
represents one interesting port, and takes the form of seven slash
(/) separated subfields.  Those subfields are: <literal>Port
number</literal>, <literal>State</literal>, <literal>Protocol</literal>,
<literal>Owner</literal>, <literal>Service</literal>, <literal>SunRPC
info</literal>, and <literal>Version info</literal>.</para>

<para>As with XML output, this man page does not allow for documenting
the entire format.  A more detailed look at the Nmap grepable output
format is available
<man>from <ulink url="https://nmap.org/book/output-formats-grepable-output.html" />.</man>
<notman>in <xref linkend="output-formats-grepable-output"/>.</notman>
</para>
        </listitem>
      </varlistentry>

     <varlistentry>
        <term>
        <option>-oA <replaceable>basename</replaceable></option> (Output to all formats)
           <indexterm significance="preferred"><primary><option>-oA</option></primary></indexterm></term>
       <listitem><para>

           As a convenience, you may specify <option>-oA
           <replaceable>basename</replaceable></option> to store scan
           results in normal, XML, and grepable formats at once.  They
           are stored in
           <filename><replaceable>basename</replaceable>.nmap</filename>,
           <filename><replaceable>basename</replaceable>.xml</filename>, and
           <filename><replaceable>basename</replaceable>.gnmap</filename>,
           respectively. As with most programs, you can prefix the filenames
           with a directory path, such as
           <filename>~/nmaplogs/foocorp/</filename> on Unix or
           <filename>c:\hacking\sco</filename> on Windows.</para>
        </listitem>
      </varlistentry>

   </variablelist>

    <variablelist><title>Verbosity and debugging options</title>
      <varlistentry>
        <term>
          <option>-v</option> (Increase verbosity level)
          <indexterm><primary><option>-v</option></primary></indexterm>
          <indexterm><primary>verbosity</primary></indexterm>
        </term>
        <term>
          <option>-v<replaceable>level</replaceable></option> (Set verbosity level)
        </term>
        <listitem>

          <para>Increases the verbosity level, causing Nmap to
          print more information about the scan in progress. Open
          ports are shown as they are found and completion time
          estimates are provided when Nmap thinks a scan will take
          more than a few minutes. Use it twice or more for even greater
          verbosity: <option>-vv</option>, or give a verbosity level
          directly, for example <option>-v3</option>.<indexterm><primary><option>-v</option></primary><secondary>giving more than once</secondary></indexterm>
          </para>

          <para>Most changes only affect interactive output, and some
          also affect normal and script kiddie output.  The other
          output types are meant to be processed by machines, so Nmap
          can give substantial detail by default in those formats
          without fatiguing a human user.  However, there are a few
          changes in other modes where output size can be reduced
          substantially by omitting some detail.  For example, a
          comment line in the grepable output that provides a list of
          all ports scanned is only printed in verbose mode because it
          can be quite long.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-d</option> (Increase debugging level)
        <indexterm><primary><option>-d</option></primary></indexterm>
        <indexterm><primary>debugging</primary></indexterm>
        </term>
        <term>
          <option>-d<replaceable>level</replaceable></option> (Set debugging level)
        </term>
        <listitem>


<para>When even verbose mode doesn't provide sufficient data for you,
debugging is available to flood you with much more!  As with the
verbosity option (<option>-v</option>), debugging is enabled with a
command-line flag (<option>-d</option>) and the debug level can be
increased by specifying it
multiple times,<indexterm><primary><option>-d</option></primary><secondary>giving more than once</secondary></indexterm>
as in <option>-dd</option>, or by setting a level directly.  For
example, <option>-d9</option> sets level nine.  That is the highest
effective level and will produce thousands of lines unless you run a
very simple scan with very few ports and targets.</para>

<para>Debugging output is useful when a bug is suspected in Nmap,
or if you are simply confused as to what Nmap is doing and why.  As this
feature is mostly intended for developers, debug lines aren't always
self-explanatory.  You may get something like: <computeroutput>Timeout
vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987
rttvar: 14987 to: 100000</computeroutput>.  If you don't understand a line, your only recourses
are to ignore it, look it up in the source code, or request help from
the development list
(<citetitle>nmap-dev</citetitle>).<indexterm><primary><citetitle>nmap-dev</citetitle> mailing list</primary></indexterm>
Some lines are self explanatory, but
the messages become more obscure as the debug level is
increased.</para>

        </listitem>
      </varlistentry>

<varlistentry>
 <term>
  <option>--reason</option> (Host and port state reasons)
   <indexterm><primary><option>--reason</option></primary></indexterm>
   <indexterm><primary>reason reporting</primary><see><option>--reason</option></see></indexterm>
 </term>
 <listitem>

<para>
Shows the reason each port is set to a specific state and the reason
each host is up or down. This option displays the type of the packet
that determined a port or hosts state. For example, A <literal>RST</literal> packet from
a closed port or an echo reply from an alive host. The information
Nmap can provide is determined by the type of scan or ping. The SYN
scan and SYN ping (<option>-sS</option> and <option>-PS</option>) are very detailed, but the
TCP connect scan (<option>-sT</option>) is limited by the
implementation of the <function>connect</function> system call. This feature is automatically enabled by
the debug option
(<option>-d</option>)<indexterm><primary><option>--reason</option></primary><secondary>implied by <option>-d</option></secondary></indexterm>
and the results are stored in XML log files
even if this option is not specified.

</para>
</listitem>
</varlistentry>

      <varlistentry>
        <term>
          <option>--stats-every <replaceable>time</replaceable></option> (Print periodic timing stats)
          <indexterm><primary><option>--stats-every</option></primary></indexterm>
        </term>
        <listitem>
          <para>
          Periodically prints a timing status message after each
          interval of <replaceable>time</replaceable>. The time is a
          specification of the kind described in
          <xref linkend="man-performance"/>; so for example, use
          <option>--stats-every 10s</option> to get a status update
          every 10 seconds. Updates are printed to interactive output
          (the screen) and XML output.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--packet-trace</option> (Trace packets and data sent and received)
          <indexterm><primary><option>--packet-trace</option></primary></indexterm>
        </term>
        <listitem>

          <para>Causes Nmap to print a summary of every packet sent
          or received. This is often used for debugging, but is
          also a valuable way for new users to understand exactly
          what Nmap is doing under the covers. To avoid printing
          thousands of lines, you may want to specify a limited
          number of ports to scan, such as <option>-p20-30</option>.  If you only care
          about the goings on of the version detection subsystem, use
          <option>--version-trace</option> instead.  If you only care about script tracing,
          specify <option>--script-trace</option>.  With <option>--packet-trace</option>, you get
          all of the above.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--open</option> (Show only open (or possibly open) ports)
        <indexterm><primary><option>--open</option></primary></indexterm>
        </term>
        <listitem>

<para>Sometimes you only care about ports you can actually connect to
(<literal>open</literal> ones), and don't want results cluttered with
<literal>closed</literal>, <literal>filtered</literal>, and
<literal>closed|filtered</literal> ports.  Output customization is
normally done after the scan using tools such as
<application>grep</application>, <application>awk</application>, and
<application>Perl</application>, but this feature was added due to
overwhelming requests.  Specify <option>--open</option> to only see
hosts with at least one
<literal>open</literal>, <literal>open|filtered</literal>, or
<literal>unfiltered</literal> port, and only see ports in those states.  These three states are treated just as they normally are, which means that <literal>open|filtered</literal> and <literal>unfiltered</literal> may be condensed into counts if there are an overwhelming number of them.
</para>

<para>Beginning with Nmap 7.40, the <option>--open</option> option implies
  <indexterm><primary><option>--defeat-rst-ratelimit</option></primary></indexterm>
  <option>--defeat-rst-ratelimit</option>, because that option only affects
  <literal>closed</literal> and <literal>filtered</literal> ports, which are
  hidden by <option>--open</option>.
</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--iflist</option> (List interfaces and routes)
        <indexterm><primary><option>--iflist</option></primary></indexterm>
        </term><listitem>
        <para>Prints the interface list and system routes as detected
        by Nmap and quits.  This is useful for debugging routing problems or
        device mischaracterization (such as Nmap treating a PPP
        connection as ethernet).</para> </listitem> </varlistentry>
   
   </variablelist>

   <variablelist><title>Miscellaneous output options</title>

      <varlistentry>
        <term>
          <option>--append-output</option> (Append to rather than clobber output files)
           <indexterm><primary><option>--append-output</option></primary></indexterm>
        </term>
        <listitem>

           <para>When you specify a filename to an output format flag
           such as <option>-oX</option> or <option>-oN</option>, that
           file is overwritten by default.  If you prefer to keep the
           existing content of the file and append the new results,
           specify the <option>--append-output</option> option.  All
           output filenames specified in that Nmap execution will then
           be appended to rather than clobbered.  This doesn't work
           well for XML (<option>-oX</option>) scan data as the
           resultant file generally won't parse properly until you fix
           it up by hand.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--resume <replaceable>filename</replaceable></option> (Resume aborted scan)
          <indexterm><primary><option>--resume</option></primary></indexterm>
          <indexterm><primary>resuming scans</primary></indexterm>
        </term>
        <listitem>


          <para>Some extensive Nmap runs take a very long time&mdash;on
          the order of days.  Such scans don't always run to
          completion.  Restrictions may prevent Nmap from being run
          during working hours, the network could go down, the machine
          Nmap is running on might suffer a planned or unplanned
          reboot, or Nmap itself could crash.  The administrator running Nmap
          could cancel it for any other reason as well, by pressing
          <keycap>ctrl-C</keycap>.  Restarting the whole scan from the
          beginning may be undesirable.  Fortunately, if scan output
          files were kept, the user can ask Nmap to resume scanning
          with the target it was working on when execution ceased.
          Simply specify the <option>--resume</option> option and pass
          the output file as its argument.  No other
          arguments are permitted, as Nmap parses the output file to
          use the same ones specified previously.  Simply call Nmap as
          <command>nmap --resume
          <replaceable>logfilename</replaceable></command>.  Nmap will
          append new results to the data files specified in the
          previous execution. Scans can be resumed from any of the 3 major
          output formats: Normal, Grepable, or XML</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--noninteractive</option> (Disable runtime interactions)
          <indexterm><primary><option>--noninteractive</option></primary></indexterm>
          <indexterm><primary>runtime interaction</primary><secondary>disabling</secondary></indexterm>
        </term>
        <listitem>

          <para>At times, such as when running Nmap in a shell background,
          it might be undesirable for Nmap to monitor and respond to user
          keyboard input when running. (See
          <xref linkend="man-runtime-interaction"/> about how to control
          Nmap during a scan.) Use option <option>--noninteractive</option>
          to prevent Nmap taking control of the terminal.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--stylesheet <replaceable>path or URL</replaceable></option> (Set XSL stylesheet to transform XML output)
          <indexterm significance="preferred"><primary><option>--stylesheet</option></primary></indexterm>
        </term>
        <listitem>

          <para>Nmap ships with an XSL<indexterm><primary>XSL</primary></indexterm>
          stylesheet<indexterm><primary>stylesheet</primary></indexterm>
          named <filename>nmap.xsl</filename><indexterm><primary><filename>nmap.xsl</filename></primary></indexterm>
          for viewing or translating XML output to HTML.<indexterm><primary>HTML from XML output</primary></indexterm>
          The XML output includes an <literal>xml-stylesheet</literal>
          directive which points to <filename>nmap.xml</filename>
          where it was initially installed by Nmap. Run the XML file
          through an XSLT processor such as
          <ulink url="http://xmlsoft.org/XSLT/">xsltproc</ulink><indexterm><primary>xsltproc</primary></indexterm>
          to produce an HTML file. Directly opening the XML file in a
          browser no longer works well because modern browsers limit the
          locations a stylesheet may be loaded from.
          If you wish to use a different
          stylesheet, specify it as the argument to
          <option>--stylesheet</option>.  You must pass the full
          pathname or URL.  One common invocation is
          <option>--stylesheet
          https://nmap.org/svn/docs/nmap.xsl</option>.  This
          tells an XSLT processor to load the latest version of the stylesheet
          from Nmap.Org.  The <option>--webxml</option> option
          does the same thing with less typing and memorization.
          Loading the XSL from Nmap.Org makes it easier to view results on
          a machine that doesn't have Nmap (and thus
          <filename>nmap.xsl</filename>) installed.  So the URL is
          often more useful, but the local filesystem location of
          <filename>nmap.xsl</filename> is used by default for privacy reasons.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--webxml</option> (Load stylesheet from Nmap.Org)
          <indexterm significance="preferred"><primary><option>--webxml</option></primary></indexterm>
        </term>
        <listitem>

          <para>This is a convenience option, nothing more than an alias for
          <option significance="preferred">--stylesheet https://nmap.org/svn/docs/nmap.xsl</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option significance="preferred">--no-stylesheet</option> (Omit XSL stylesheet declaration from XML)
          <indexterm significance="preferred"><primary><option>--no-stylesheet</option></primary></indexterm>
        </term>
        <listitem>

          <para>Specify  this  option  to prevent Nmap from associating any XSL
              stylesheet with its XML output.  The  <literal>xml-stylesheet</literal>  directive
              is omitted.</para>
        </listitem>
      </varlistentry>



    </variablelist>
    <indexterm class="endofrange" startref="man-output-indexterm"/>
  </refsect1>

  <refsect1 id='man-misc-options'>
    <title>Miscellaneous Options</title>
    <para>This section describes some important (and not-so-important)
    options that don't really fit anywhere else.</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-6</option> (Enable IPv6 scanning)
          <indexterm significance="preferred"><primary><option>-6</option></primary></indexterm>
          <indexterm><primary>IPv6</primary></indexterm>
        </term>
        <listitem>

          <para>Nmap has IPv6 support for its most
          popular features.  Ping scanning,
          port scanning, version detection, and the Nmap Scripting
          Engine all support IPv6.  The command syntax is the same as
          usual except that you also add the <option>-6</option>
          option.  Of course, you must use IPv6 syntax if you specify
          an address rather than a hostname.  An address might look
          like
          <literal>3ffe:7501:4819:2000:210:f3ff:fe03:14d0</literal>,
          so hostnames are recommended.  The output looks the same as
          usual, with the IPv6 address on the <quote>interesting
          ports</quote> line being the only IPv6 giveaway.</para>

          <para>While IPv6 hasn't exactly taken the world by storm, it
          gets significant use in some (usually Asian) countries and
          most modern operating systems support it.  To use Nmap with
          IPv6, both the source and target of your scan must be
          configured for IPv6.  If your ISP (like most of them) does
          not allocate IPv6 addresses to you, free tunnel brokers are
          widely available and work fine with Nmap.  I use the free
          IPv6 tunnel broker<indexterm><primary>IPv6 tunnel broker</primary></indexterm>
	  service at <ulink url="http://www.tunnelbroker.net"/>.
          Other tunnel brokers are
          <ulink url="http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers">listed
          at Wikipedia</ulink>.  6to4 tunnels are another popular,
          free approach.</para>

	  <para>On Windows, raw-socket IPv6 scans are supported only on
	  ethernet devices (not tunnels), and only on Windows
	  Vista<indexterm><primary>Windows Vista</primary></indexterm>
	  and later. Use the
	  <option>--unprivileged</option><indexterm><primary><option>--unprivileged</option></primary></indexterm>
	  option in other situations.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-A</option> (Aggressive scan options)
          <indexterm significance="preferred"><primary><option>-A</option></primary></indexterm>
        </term>
        <listitem>
          <para>This option enables additional advanced and aggressive
          options. Presently this enables OS detection
          (<option>-O</option>), version scanning
          (<option>-sV</option>), script scanning
          (<option>-sC</option>) and traceroute
          (<option>--traceroute</option>).<indexterm><primary><option>-A</option></primary><secondary>features
          enabled by</secondary></indexterm> More features may be
          added in the future. The point is to enable a comprehensive
          set of scan options without people having to remember a
          large set of flags. However, because script scanning with
          the default set is considered intrusive, you should not use
          <option>-A</option> against target networks without
          permission. This option only enables features, and not
          timing options (such as <option>-T4</option>) or verbosity
          options (<option>-v</option>) that you might want as
          well. Options which require privileges (e.g. root access)
          such as OS detection and traceroute will only be enabled
          if those privileges are available.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--datadir <replaceable>directoryname</replaceable></option> (Specify custom Nmap data file location)
          <indexterm significance="preferred"><primary><option>--datadir</option></primary></indexterm>
        </term>
        <listitem>

          <para>Nmap obtains some special data at runtime in files
          named <filename>nmap-service-probes</filename>,
          <filename>nmap-services</filename>,
          <filename>nmap-protocols</filename>,
          <filename>nmap-rpc</filename>,
          <filename>nmap-mac-prefixes</filename>, and
          <filename>nmap-os-db</filename>.  If the
          location of any of these files has been specified (using the
          <option>--servicedb</option> or <option>--versiondb</option> options),
          that location is used for that file. After that, Nmap
          searches these files in the directory specified with the
          <option>--datadir</option> option (if any). Any files not
          found there, are searched for in the directory specified by
          the
          <envar>NMAPDIR</envar><indexterm><primary><envar>NMAPDIR</envar> environment variable</primary></indexterm>
          environment variable.
          Next comes
	  <filename>~/.nmap</filename><indexterm><primary sortas="nmap directory"><filename>.nmap</filename> directory</primary></indexterm>
          for real and effective UIDs; or on Windows,
          <filename><replaceable>HOME</replaceable>\AppData\Roaming\nmap</filename>
          (where <replaceable>HOME</replaceable> is the user's home directory,
          like <filename>C:\Users\user</filename>). This is
          followed by the location of the <filename>nmap</filename>
          executable and the same location with
          <code>../share/nmap</code> appended. Then a compiled-in
          location such as <filename>/usr/local/share/nmap</filename> or <filename>/usr/share/nmap</filename>.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--servicedb <replaceable>services file</replaceable></option> (Specify custom services file)
          <indexterm significance="preferred"><primary><option>--servicedb</option></primary></indexterm>
        </term>
        <listitem>

          <para>Asks Nmap to use the specified services file rather than
          the <filename>nmap-services</filename> data file that
          comes with Nmap.  Using this option also causes a fast scan
          (<option>-F</option>) to be used. See the description for
          <option>--datadir</option> for more information on Nmap's data
          files.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--versiondb <replaceable>service probes file</replaceable></option> (Specify custom service probes file)
          <indexterm significance="preferred"><primary><option>--versiondb</option></primary></indexterm>
        </term>
        <listitem>

	  <para>Asks Nmap to use the specified service probes file rather than
	  the <filename>nmap-service-probes</filename> data file
	  that comes with Nmap.  See the description for
	  <option>--datadir</option> for more information on Nmap's data
	  files.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--send-eth</option> (Use raw ethernet sending)
          <indexterm significance="preferred"><primary><option>--send-eth</option></primary></indexterm>
        </term>
        <listitem>

          <para>Asks Nmap to send packets at the raw ethernet (data
          link) layer rather than the higher IP (network) layer. By
          default, Nmap chooses the one which is generally best for
          the platform it is running on.
	  Raw sockets (IP layer)<indexterm><primary>raw sockets</primary></indexterm>
          are
          generally most efficient for Unix machines, while ethernet
          frames are required for Windows operation since Microsoft
          disabled raw socket support. Nmap still uses raw IP packets
          on Unix despite this option when there is no other choice
          (such as non-ethernet connections).</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--send-ip</option> (Send at raw IP level)
          <indexterm significance="preferred"><primary><option>--send-ip</option></primary></indexterm>
        </term>
        <listitem>

          <para>Asks Nmap to send packets via raw IP sockets rather
          than sending lower level ethernet frames. It is the
          complement to the <option>--send-eth</option> option discussed
          previously.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--privileged</option> (Assume that the user is fully privileged)
          <indexterm significance="preferred"><primary><option>--privileged</option></primary></indexterm>
        </term>
        <listitem>

          <para>Tells Nmap to simply assume that it is privileged
          enough to perform raw socket sends, packet sniffing, and
          similar operations that usually require
	  root privileges<indexterm><primary>privileged users</primary></indexterm><indexterm><primary>authorized users</primary><see>privileged users</see></indexterm>
          on Unix systems. By default Nmap quits if such operations are
          requested but <function>geteuid</function> is not
          zero. <option>--privileged</option> is useful with Linux
          kernel capabilities and similar systems that may be
          configured to allow unprivileged users to perform raw-packet
          scans. Be sure to provide this option flag before any flags
          for options that require privileges (SYN scan, OS detection,
          etc.). The
	  <envar>NMAP_PRIVILEGED</envar><indexterm><primary><envar>NMAP_PRIVILEGED</envar> environment variable</primary></indexterm>
	  environment variable
          may be set as an equivalent alternative to
          <option>--privileged</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--unprivileged</option> (Assume that the user lacks raw socket privileges)
          <indexterm significance="preferred"><primary><option>--unprivileged</option></primary></indexterm>
          <indexterm><primary>unprivileged users</primary></indexterm>
        </term>
        <listitem>


          <para>This option is the opposite of
          <option>--privileged</option>.  It tells Nmap to treat the
          user as lacking network raw socket and sniffing privileges.
          This is useful for testing, debugging, or when the raw
          network functionality of your operating system is somehow
          broken. The
	  <envar>NMAP_UNPRIVILEGED</envar><indexterm><primary><envar>NMAP_UNPRIVILEGED</envar> environment variable</primary></indexterm>
	  environment variable
          may be set as an equivalent alternative to
          <option>--unprivileged</option>.</para>


        </listitem>
      </varlistentry>

      <varlistentry>
	 <term>
	 <option>--release-memory</option> (Release memory before quitting)
	 <indexterm><primary><option>--release-memory</option></primary></indexterm>
         </term>
         <listitem>

          <para>This option is only useful for memory-leak debugging.
          It causes Nmap to release allocated memory just before it
          quits so that actual memory leaks are easier to spot.
          Normally Nmap skips this as the OS does this anyway upon
          process termination.</para>
        </listitem>
      </varlistentry>


      <varlistentry>
        <term>
          <option>-V</option>; <option>--version</option> (Print version number)
          <indexterm><primary><option>-V</option></primary></indexterm>
          <indexterm><primary><option>--version</option></primary></indexterm>
        </term>
        <listitem>
          <para>Prints the Nmap version number and exits.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-h</option>; <option>--help</option> (Print help summary page)
          <indexterm><primary><option>-h</option></primary></indexterm>
          <indexterm><primary><option>--help</option></primary></indexterm>
        </term>
        <listitem>
          <para>Prints a short help screen with the most common
          command flags.  Running Nmap without any arguments does the
          same thing.</para>
        </listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1 id='man-runtime-interaction'>
    <title>Runtime Interaction<indexterm><primary>runtime interaction</primary></indexterm></title>

    <para>During the execution of Nmap, all key presses are
    captured. This allows you to interact with the program
    without aborting and restarting it. Certain special
    keys will change options, while any other keys will print out a
    status message telling you about the scan. The convention is
    that 
    <emphasis>lowercase letters increase</emphasis> the
    amount of printing, and 
    <emphasis>uppercase letters decrease</emphasis> the
    printing.  You may also press &lsquo;<emphasis>?</emphasis>&rsquo; for help.</para>
    <variablelist>
      <varlistentry>
        <term>
          <option>v</option> / <option>V</option>
        </term>
        <listitem>
          <para>Increase / decrease the verbosity level</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>d</option> / <option>D</option>
        </term>
        <listitem>
          <para>Increase / decrease the debugging Level</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>p</option> / <option>P</option>
        </term>
        <listitem>
          <para>Turn on / off packet tracing</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>?</option>
        </term>
        <listitem>
          <para>Print a runtime interaction help screen</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          Anything else
        </term>
        <listitem>
          <para>Print out a status message like this:</para>
<screen>
Stats: 0:00:07 elapsed; 20 hosts completed (1 up), 1 undergoing Service Scan
Service scan Timing: About 33.33% done; ETC: 20:57 (0:00:12 remaining)
</screen>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id='man-examples'>
    <title>Examples</title>
    <para>Here are some Nmap usage examples, from the simple and
    routine to a little more complex and esoteric. Some actual
    IP addresses and domain names are used to make things
    more concrete. In their place you should substitute
    addresses/names from 
    <emphasis>your own network</emphasis>.  While I don't think
    port scanning other networks is or should be illegal, some network
    administrators don't appreciate unsolicited scanning of their networks and may
    complain.  Getting permission first is the best approach.</para>

    <para>For testing purposes, you have permission to scan the host
    scanme.nmap.org.<indexterm><primary>scanme.nmap.org</primary></indexterm>
    This permission only includes
    scanning via Nmap and not testing exploits or denial of service
    attacks.  To conserve bandwidth, please do not initiate more than
    a dozen scans against that host per day.  If this free scanning
    target service is abused, it will be taken down and Nmap will
    report <computeroutput>Failed to resolve given hostname/IP:
    scanme.nmap.org</computeroutput>.  These permissions also apply to
    the hosts scanme2.nmap.org,
    scanme3.nmap.org, and so on, though those hosts
    do not currently exist.</para>

    <para>
      <command>nmap -v scanme.nmap.org</command>
      <indexterm><primary><option>-v</option></primary><secondary>example of</secondary></indexterm>
    </para>
    <para>This option scans all reserved TCP ports on the machine
    <literal>scanme.nmap.org</literal> . The <option>-v</option>
    option enables verbose mode.</para>
    <para>
      <command>nmap -sS -O scanme.nmap.org/24</command>
      <indexterm><primary><option>-sS</option></primary><secondary>example of</secondary></indexterm>
      <indexterm><primary><option>-O</option></primary><secondary>example of</secondary></indexterm>
    </para>
    <para>Launches a stealth SYN scan against each machine that is
    up out of the 256 IPs on the /24 sized network where
    Scanme resides. It also tries to determine what
    operating system is running on each host that is up and
    running. This requires root privileges because of the SYN scan
    and OS detection.</para>
    <para>
      <command>nmap -sV -p 22,53,110,143,4564
      198.116.0-255.1-127</command>
      <indexterm><primary><option>-p</option></primary><secondary>example of</secondary></indexterm>
    </para>

    <para>Launches host enumeration and a TCP scan at the first half
    of each of the 255 possible eight-bit subnets in the 198.116.0.0/16
    address space. This tests whether the systems run SSH, DNS, POP3,
    or IMAP on their standard ports, or anything on port 4564. For any
    of these ports found open, version detection is used to determine
    what application is running.</para>

    <para>
      <command>nmap -v -iR 100000 -Pn -p 80</command>
      <indexterm><primary><option>-iR</option></primary><secondary>example of</secondary></indexterm>
      <indexterm><primary><option>-Pn</option></primary><secondary>example of</secondary></indexterm>
    </para>

    <para>Asks Nmap to choose 100,000 hosts at random and scan them
    for web servers (port 80).  Host enumeration is disabled with
    <option>-Pn</option> since first sending a couple probes to
    determine whether a host is up is wasteful when you are only
    probing one port on each target host anyway.</para>

    <para>
      <command>nmap -Pn -p80 -oX logs/pb-port80scan.xml -oG
      logs/pb-port80scan.gnmap 216.163.128.20/20</command>
      <indexterm><primary><option>-oX</option></primary><secondary>example of</secondary></indexterm>
      <indexterm><primary><option>-oG</option></primary><secondary>example of</secondary></indexterm>
    </para>
    <para>This scans 4096 IPs for any web servers (without pinging
    them) and saves the output in grepable and XML formats.</para>

  </refsect1>

<notbook>
<refsect1 id='man-book'>
    <title>Nmap Book</title>
    <!-- This paragraph is a bit jumbled together for man page rendering purposes, where extra newlines were causing problems -->
    <para>While this reference guide details all material Nmap
    options, it can't fully demonstrate how to apply those features to
    quickly solve real-world tasks.  For that, we
    released <notweb><citetitle>Nmap Network Scanning: The Official
    Nmap Project Guide to Network Discovery and Security
    Scanning</citetitle>.</notweb><web><ulink url="https://nmap.org/book/"><citetitle>Nmap
    Network Scanning: The Official Nmap Project Guide to Network
    Discovery and Security Scanning</citetitle></ulink>.</web> Topics
    include subverting firewalls and intrusion detection systems,
    optimizing Nmap performance, and automating common networking
    tasks with the Nmap Scripting Engine.  Hints and instructions are
    provided for common Nmap tasks such as taking network inventory,
    penetration testing, detecting rogue wireless access points, and
    quashing network worm outbreaks.  Examples and diagrams show
    actual communication on the wire.  More than half of the book is
    available free online.  See <ulink url="https://nmap.org/book"/>
    for more information.</para>

</refsect1>
</notbook>

  <refsect1 id='man-bugs'>
    <title>Bugs</title>
    <indexterm><primary>bugs, reporting</primary></indexterm>

    <para>Like its author, Nmap isn't perfect.  But you can help make
    it better by sending bug reports or even writing patches.  If Nmap
    doesn't behave the way you expect, first upgrade to the latest
    version available from <ulink
    url="https://nmap.org"/>.  If the problem persists,
    do some research to determine whether it has already been
    discovered and addressed.  Try searching for the problem or error message on
    Google since that aggregates so many forums.  If nothing comes of this, create an Issue on our tracker
    (<ulink url="http://issues.nmap.org"/>) and/or mail a bug report to
    <email>dev@nmap.org</email>.  If you subscribe to the nmap-dev
    list before posting, your message will bypass moderation and get
    through more quickly.  Subscribe at <ulink
    url="https://nmap.org/mailman/listinfo/dev"/>. Please include everything
    you have learned about the problem, as well as what version of
    Nmap you are using and what operating system version it is
    running on.  Other suggestions for improving Nmap may be sent to
    the Nmap dev mailing list as well.</para>

    <para>If you are able to write a patch improving Nmap or fixing a
    bug, that is even better!  Instructions for submitting patches or
    git pull requests are available from <ulink
    url="https://github.com/nmap/nmap/blob/master/CONTRIBUTING.md"/></para>

    <para>Particularly sensitive issues such as a security reports may
    be sent directly to Nmap's author Fyodor directly at
    <email>fyodor@nmap.org</email>.  All other reports and comments
    should use the dev list or issue tracker instead because more
    people read, follow, and respond to those.</para>

  </refsect1>

  <refsect1 id='man-author'>
    <title>Authors</title>

    <para>Gordon <quote>Fyodor</quote> Lyon
    <email>fyodor@nmap.org</email> wrote and released Nmap in 1997.  Since then, hundreds
    of people have made valuable contributions, as detailed in the
    <filename>CHANGELOG</filename><indexterm><primary>changelog</primary></indexterm>
    file distributed with Nmap and also available from <ulink
    url="https://nmap.org/changelog.html" />.  David Fifield and
    Daniel Miller deserve special recognition for their enormous
    multi-year contributions!
    </para>
  </refsect1>

  <refsect1 id='man-legal'>
    &legal-notices;
  </refsect1>
</refentry>
<indexterm class="endofrange" startref="man-nmap1-indexterm"/>