Adding upstream version 6.05.01.upstream/6.05.01

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+'\" t
+.\" SPDX-License-Identifier: Linux-man-pages-1-para
+.\"
+.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
+.\"
+.\" $Id: ip.7,v 1.19 2000/12/20 18:10:31 ak Exp $
+.\"
+.\" FIXME The following socket options are yet to be documented
+.\"
+.\" 	IP_XFRM_POLICY (2.5.48)
+.\"	    Needs CAP_NET_ADMIN
+.\"
+.\" 	IP_IPSEC_POLICY (2.5.47)
+.\"	    Needs CAP_NET_ADMIN
+.\"
+.\"	IP_MINTTL (2.6.34)
+.\"	    commit d218d11133d888f9745802146a50255a4781d37a
+.\"	    Author: Stephen Hemminger <shemminger@vyatta.com>
+.\"
+.\"	MCAST_JOIN_GROUP (2.4.22 / 2.6)
+.\"
+.\"	MCAST_BLOCK_SOURCE (2.4.22 / 2.6)
+.\"
+.\"	MCAST_UNBLOCK_SOURCE (2.4.22 / 2.6)
+.\"
+.\"	MCAST_LEAVE_GROUP (2.4.22 / 2.6)
+.\"
+.\"	MCAST_JOIN_SOURCE_GROUP (2.4.22 / 2.6)
+.\"
+.\"	MCAST_LEAVE_SOURCE_GROUP (2.4.22 / 2.6)
+.\"
+.\"	MCAST_MSFILTER (2.4.22 / 2.6)
+.\"
+.\"	IP_UNICAST_IF (3.4)
+.\"	    commit 76e21053b5bf33a07c76f99d27a74238310e3c71
+.\"	    Author: Erich E. Hoover <ehoover@mines.edu>
+.\"
+.TH ip 7 2023-07-15 "Linux man-pages 6.05.01"
+.SH NAME
+ip \- Linux IPv4 protocol implementation
+.SH SYNOPSIS
+.nf
+.B #include <sys/socket.h>
+.\" .B #include <net/netinet.h> -- does not exist anymore
+.\" .B #include <linux/errqueue.h> -- never include <linux/foo.h>
+.B #include <netinet/in.h>
+.B #include <netinet/ip.h>        \fR/* superset of previous */
+.PP
+.IB tcp_socket " = socket(AF_INET, SOCK_STREAM, 0);"
+.IB udp_socket " = socket(AF_INET, SOCK_DGRAM, 0);"
+.IB raw_socket " = socket(AF_INET, SOCK_RAW, " protocol ");"
+.fi
+.SH DESCRIPTION
+Linux implements the Internet Protocol, version 4,
+described in RFC\ 791 and RFC\ 1122.
+.B ip
+contains a level 2 multicasting implementation conforming to RFC\ 1112.
+It also contains an IP router including a packet filter.
+.PP
+The programming interface is BSD-sockets compatible.
+For more information on sockets, see
+.BR socket (7).
+.PP
+An IP socket is created using
+.BR socket (2):
+.PP
+.in +4n
+.EX
+socket(AF_INET, socket_type, protocol);
+.EE
+.in
+.PP
+Valid socket types include
+.B SOCK_STREAM
+to open a stream socket,
+.B SOCK_DGRAM
+to open a datagram socket, and
+.B SOCK_RAW
+to open a
+.BR raw (7)
+socket to access the IP protocol directly.
+.PP
+.I protocol
+is the IP protocol in the IP header to be received or sent.
+Valid values for
+.I protocol
+include:
+.IP \[bu] 3
+0 and
+.B IPPROTO_TCP
+for
+.BR tcp (7)
+stream sockets;
+.IP \[bu]
+0 and
+.B IPPROTO_UDP
+for
+.BR udp (7)
+datagram sockets;
+.IP \[bu]
+.B IPPROTO_SCTP
+for
+.BR sctp (7)
+stream sockets; and
+.IP \[bu]
+.B IPPROTO_UDPLITE
+for
+.BR udplite (7)
+datagram sockets.
+.PP
+For
+.B SOCK_RAW
+you may specify a valid IANA IP protocol defined in
+RFC\ 1700 assigned numbers.
+.PP
+When a process wants to receive new incoming packets or connections, it
+should bind a socket to a local interface address using
+.BR bind (2).
+In this case, only one IP socket may be bound to any given local
+(address, port) pair.
+When
+.B INADDR_ANY
+is specified in the bind call, the socket will be bound to
+.I all
+local interfaces.
+When
+.BR listen (2)
+is called on an unbound socket, the socket is automatically bound
+to a random free port with the local address set to
+.BR INADDR_ANY .
+When
+.BR connect (2)
+is called on an unbound socket, the socket is automatically bound
+to a random free port or to a usable shared port with the local address
+set to
+.BR INADDR_ANY .
+.PP
+A TCP local socket address that has been bound is unavailable for
+some time after closing, unless the
+.B SO_REUSEADDR
+flag has been set.
+Care should be taken when using this flag as it makes TCP less reliable.
+.SS Address format
+An IP socket address is defined as a combination of an IP interface
+address and a 16-bit port number.
+The basic IP protocol does not supply port numbers, they
+are implemented by higher level protocols like
+.BR udp (7)
+and
+.BR tcp (7).
+On raw sockets
+.I sin_port
+is set to the IP protocol.
+.PP
+.in +4n
+.EX
+struct sockaddr_in {
+    sa_family_t    sin_family; /* address family: AF_INET */
+    in_port_t      sin_port;   /* port in network byte order */
+    struct in_addr sin_addr;   /* internet address */
+};
+\&
+/* Internet address */
+struct in_addr {
+    uint32_t       s_addr;     /* address in network byte order */
+};
+.EE
+.in
+.PP
+.I sin_family
+is always set to
+.BR AF_INET .
+This is required; in Linux 2.2 most networking functions return
+.B EINVAL
+when this setting is missing.
+.I sin_port
+contains the port in network byte order.
+The port numbers below 1024 are called
+.I privileged ports
+(or sometimes:
+.IR "reserved ports" ).
+Only a privileged process
+(on Linux: a process that has the
+.B CAP_NET_BIND_SERVICE
+capability in the user namespace governing its network namespace) may
+.BR bind (2)
+to these sockets.
+Note that the raw IPv4 protocol as such has no concept of a
+port, they are implemented only by higher protocols like
+.BR tcp (7)
+and
+.BR udp (7).
+.PP
+.I sin_addr
+is the IP host address.
+The
+.I s_addr
+member of
+.I struct in_addr
+contains the host interface address in network byte order.
+.I in_addr
+should be assigned one of the
+.B INADDR_*
+values
+(e.g.,
+.BR INADDR_LOOPBACK )
+using
+.BR htonl (3)
+or set using the
+.BR inet_aton (3),
+.BR inet_addr (3),
+.BR inet_makeaddr (3)
+library functions or directly with the name resolver (see
+.BR gethostbyname (3)).
+.PP
+IPv4 addresses are divided into unicast, broadcast,
+and multicast addresses.
+Unicast addresses specify a single interface of a host,
+broadcast addresses specify all hosts on a network, and multicast
+addresses address all hosts in a multicast group.
+Datagrams to broadcast addresses can be sent or received only when the
+.B SO_BROADCAST
+socket flag is set.
+In the current implementation, connection-oriented sockets are allowed
+to use only unicast addresses.
+.\" Leave a loophole for XTP @)
+.PP
+Note that the address and the port are always stored in
+network byte order.
+In particular, this means that you need to call
+.BR htons (3)
+on the number that is assigned to a port.
+All address/port manipulation
+functions in the standard library work in network byte order.
+.SS Special and reserved addresses
+There are several special addresses:
+.TP
+.BR INADDR_LOOPBACK " (127.0.0.1)"
+always refers to the local host via the loopback device;
+.TP
+.BR INADDR_ANY " (0.0.0.0)"
+means any address for socket binding;
+.TP
+.BR INADDR_BROADCAST " (255.255.255.255)"
+has the same effect on
+.BR bind (2)
+as
+.B INADDR_ANY
+for historical reasons.
+A packet addressed to
+.B INADDR_BROADCAST
+through a socket which has
+.B SO_BROADCAST
+set will be broadcast to all hosts on the local network segment,
+as long as the link is broadcast-capable.
+.TP
+Highest-numbered address
+.TQ
+Lowest-numbered address
+On any locally-attached non-point-to-point IP subnet
+with a link type that supports broadcasts,
+the highest-numbered address
+(e.g., the .255 address on a subnet with netmask 255.255.255.0)
+is designated as a broadcast address.
+It cannot usefully be assigned to an individual interface,
+and can only be addressed with a socket on which the
+.B SO_BROADCAST
+option has been set.
+Internet standards have historically
+also reserved the lowest-numbered address
+(e.g., the .0 address on a subnet with netmask 255.255.255.0)
+for broadcast, though they call it "obsolete" for this purpose.
+(Some sources also refer to this as the "network address.")
+Since Linux 5.14,
+.\" commit 58fee5fc83658aaacf60246aeab738946a9ba516
+it is treated as an ordinary unicast address
+and can be assigned to an interface.
+.PP
+Internet standards have traditionally also reserved various addresses
+for particular uses, though Linux no longer treats
+some of these specially.
+.TP
+[0.0.0.1, 0.255.255.255]
+.TQ
+[240.0.0.0, 255.255.255.254]
+Addresses in these ranges (0/8 and 240/4) are reserved globally.
+Since Linux 5.3
+.\" commit 96125bf9985a75db00496dd2bc9249b777d2b19b
+and Linux 2.6.25,
+.\" commit 1e637c74b0f84eaca02b914c0b8c6f67276e9697
+respectively,
+the 0/8 and 240/4 addresses, other than
+.B INADDR_ANY
+and
+.BR INADDR_BROADCAST ,
+are treated as ordinary unicast addresses.
+Systems that follow the traditional behaviors may not
+interoperate with these historically reserved addresses.
+.TP
+[127.0.0.1, 127.255.255.254]
+Addresses in this range (127/8) are treated as loopback addresses
+akin to the standardized local loopback address
+.B INADDR_LOOPBACK
+(127.0.0.1);
+.TP
+[224.0.0.0, 239.255.255.255]
+Addresses in this range (224/4) are dedicated to multicast use.
+.SS Socket options
+IP supports some protocol-specific socket options that can be set with
+.BR setsockopt (2)
+and read with
+.BR getsockopt (2).
+The socket option level for IP is
+.BR IPPROTO_IP .
+.\" or SOL_IP on Linux
+A boolean integer flag is zero when it is false, otherwise true.
+.PP
+When an invalid socket option is specified,
+.BR getsockopt (2)
+and
+.BR setsockopt (2)
+fail with the error
+.BR ENOPROTOOPT .
+.TP
+.BR IP_ADD_MEMBERSHIP " (since Linux 1.2)"
+Join a multicast group.
+Argument is an
+.I ip_mreqn
+structure.
+.IP
+.in +4n
+.EX
+struct ip_mreqn {
+    struct in_addr imr_multiaddr; /* IP multicast group
+                                     address */
+    struct in_addr imr_address;   /* IP address of local
+                                     interface */
+    int            imr_ifindex;   /* interface index */
+};
+.EE
+.in
+.IP
+.I imr_multiaddr
+contains the address of the multicast group the application
+wants to join or leave.
+It must be a valid multicast address
+.\" (i.e., within the 224.0.0.0-239.255.255.255 range)
+(or
+.BR setsockopt (2)
+fails with the error
+.BR EINVAL ).
+.I imr_address
+is the address of the local interface with which the system
+should join the multicast group; if it is equal to
+.BR INADDR_ANY ,
+an appropriate interface is chosen by the system.
+.I imr_ifindex
+is the interface index of the interface that should join/leave the
+.I imr_multiaddr
+group, or 0 to indicate any interface.
+.IP
+The
+.I ip_mreqn
+structure is available only since Linux 2.2.
+For compatibility, the old
+.I ip_mreq
+structure (present since Linux 1.2) is still supported;
+it differs from
+.I ip_mreqn
+only by not including the
+.I imr_ifindex
+field.
+(The kernel determines which structure is being passed based
+on the size passed in
+.IR optlen .)
+.IP
+.B IP_ADD_MEMBERSHIP
+is valid only for
+.BR setsockopt (2).
+.\"
+.TP
+.BR IP_ADD_SOURCE_MEMBERSHIP " (since Linux 2.4.22 / Linux 2.5.68)"
+Join a multicast group and allow receiving data only
+from a specified source.
+Argument is an
+.I ip_mreq_source
+structure.
+.IP
+.in +4n
+.EX
+struct ip_mreq_source {
+    struct in_addr imr_multiaddr;  /* IP multicast group
+                                      address */
+    struct in_addr imr_interface;  /* IP address of local
+                                      interface */
+    struct in_addr imr_sourceaddr; /* IP address of
+                                      multicast source */
+};
+.EE
+.in
+.IP
+The
+.I ip_mreq_source
+structure is similar to
+.I ip_mreqn
+described under
+.BR IP_ADD_MEMBERSHIP .
+The
+.I imr_multiaddr
+field contains the address of the multicast group the application
+wants to join or leave.
+The
+.I imr_interface
+field is the address of the local interface with which
+the system should join the multicast group.
+Finally, the
+.I imr_sourceaddr
+field contains the address of the source the
+application wants to receive data from.
+.IP
+This option can be used multiple times to allow
+receiving data from more than one source.
+.TP
+.BR IP_BIND_ADDRESS_NO_PORT " (since Linux 4.2)"
+.\" commit 90c337da1524863838658078ec34241f45d8394d
+Inform the kernel to not reserve an ephemeral port when using
+.BR bind (2)
+with a port number of 0.
+The port will later be automatically chosen at
+.BR connect (2)
+time,
+in a way that allows sharing a source port as long as the 4-tuple is unique.
+.TP
+.BR IP_BLOCK_SOURCE " (since Linux 2.4.22 / 2.5.68)"
+Stop receiving multicast data from a specific source in a given group.
+This is valid only after the application has subscribed
+to the multicast group using either
+.B IP_ADD_MEMBERSHIP
+or
+.BR IP_ADD_SOURCE_MEMBERSHIP .
+.IP
+Argument is an
+.I ip_mreq_source
+structure as described under
+.BR IP_ADD_SOURCE_MEMBERSHIP .
+.TP
+.BR IP_DROP_MEMBERSHIP " (since Linux 1.2)"
+Leave a multicast group.
+Argument is an
+.I ip_mreqn
+or
+.I ip_mreq
+structure similar to
+.BR IP_ADD_MEMBERSHIP .
+.TP
+.BR IP_DROP_SOURCE_MEMBERSHIP " (since Linux 2.4.22 / 2.5.68)"
+Leave a source-specific group\[em]that is, stop receiving data from
+a given multicast group that come from a given source.
+If the application has subscribed to multiple sources within
+the same group, data from the remaining sources will still be delivered.
+To stop receiving data from all sources at once, use
+.BR IP_DROP_MEMBERSHIP .
+.IP
+Argument is an
+.I ip_mreq_source
+structure as described under
+.BR IP_ADD_SOURCE_MEMBERSHIP .
+.TP
+.BR IP_FREEBIND " (since Linux 2.4)"
+.\" Precisely: since Linux 2.4.0-test10
+If enabled, this boolean option allows binding to an IP address
+that is nonlocal or does not (yet) exist.
+This permits listening on a socket,
+without requiring the underlying network interface or the
+specified dynamic IP address to be up at the time that
+the application is trying to bind to it.
+This option is the per-socket equivalent of the
+.I ip_nonlocal_bind
+.I /proc
+interface described below.
+.TP
+.BR IP_HDRINCL " (since Linux 2.0)"
+If enabled,
+the user supplies an IP header in front of the user data.
+Valid only for
+.B SOCK_RAW
+sockets; see
+.BR raw (7)
+for more information.
+When this flag is enabled, the values set by
+.BR IP_OPTIONS ,
+.BR IP_TTL ,
+and
+.B IP_TOS
+are ignored.
+.TP
+.BR IP_LOCAL_PORT_RANGE " (since Linux 6.3)"
+Set or get the per-socket default local port range.
+This option can be used to clamp down the global local port range,
+defined by the
+.I ip_local_port_range
+.I /proc
+interface described below, for a given socket.
+.IP
+The option takes an
+.I uint32_t
+value with
+the high 16 bits set to the upper range bound,
+and the low 16 bits set to the lower range bound.
+Range bounds are inclusive.
+The 16-bit values should be in host byte order.
+.IP
+The lower bound has to be less than the upper bound
+when both bounds are not zero.
+Otherwise, setting the option fails with EINVAL.
+.IP
+If either bound is outside of the global local port range, or is zero,
+then that bound has no effect.
+.IP
+To reset the setting,
+pass zero as both the upper and the lower bound.
+.TP
+.BR IP_MSFILTER " (since Linux 2.4.22 / 2.5.68)"
+This option provides access to the advanced full-state filtering API.
+Argument is an
+.I ip_msfilter
+structure.
+.IP
+.in +4n
+.EX
+struct ip_msfilter {
+    struct in_addr imsf_multiaddr; /* IP multicast group
+                                      address */
+    struct in_addr imsf_interface; /* IP address of local
+                                      interface */
+    uint32_t       imsf_fmode;     /* Filter\-mode */
+\&
+    uint32_t       imsf_numsrc;    /* Number of sources in
+                                      the following array */
+    struct in_addr imsf_slist[1];  /* Array of source
+                                      addresses */
+};
+.EE
+.in
+.IP
+There are two macros,
+.B MCAST_INCLUDE
+and
+.BR MCAST_EXCLUDE ,
+which can be used to specify the filtering mode.
+Additionally, the
+.BR IP_MSFILTER_SIZE (n)
+macro exists to determine how much memory is needed to store
+.I ip_msfilter
+structure with
+.I n
+sources in the source list.
+.IP
+For the full description of multicast source filtering
+refer to RFC 3376.
+.TP
+.BR IP_MTU " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.124
+Retrieve the current known path MTU of the current socket.
+Returns an integer.
+.IP
+.B IP_MTU
+is valid only for
+.BR getsockopt (2)
+and can be employed only when the socket has been connected.
+.TP
+.BR IP_MTU_DISCOVER " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.124
+Set or receive the Path MTU Discovery setting for a socket.
+When enabled, Linux will perform Path MTU Discovery
+as defined in RFC\ 1191 on
+.B SOCK_STREAM
+sockets.
+For
+.RB non- SOCK_STREAM
+sockets,
+.B IP_PMTUDISC_DO
+forces the don't-fragment flag to be set on all outgoing packets.
+It is the user's responsibility to packetize the data
+in MTU-sized chunks and to do the retransmits if necessary.
+The kernel will reject (with
+.BR EMSGSIZE )
+datagrams that are bigger than the known path MTU.
+.B IP_PMTUDISC_WANT
+will fragment a datagram if needed according to the path MTU,
+or will set the don't-fragment flag otherwise.
+.IP
+The system-wide default can be toggled between
+.B IP_PMTUDISC_WANT
+and
+.B IP_PMTUDISC_DONT
+by writing (respectively, zero and nonzero values) to the
+.I /proc/sys/net/ipv4/ip_no_pmtu_disc
+file.
+.TS
+tab(:);
+c l
+l l.
+Path MTU discovery value:Meaning
+IP_PMTUDISC_WANT:Use per-route settings.
+IP_PMTUDISC_DONT:Never do Path MTU Discovery.
+IP_PMTUDISC_DO:Always do Path MTU Discovery.
+IP_PMTUDISC_PROBE:Set DF but ignore Path MTU.
+.TE
+.sp 1
+When PMTU discovery is enabled, the kernel automatically keeps track of
+the path MTU per destination host.
+When it is connected to a specific peer with
+.BR connect (2),
+the currently known path MTU can be retrieved conveniently using the
+.B IP_MTU
+socket option (e.g., after an
+.B EMSGSIZE
+error occurred).
+The path MTU may change over time.
+For connectionless sockets with many destinations,
+the new MTU for a given destination can also be accessed using the
+error queue (see
+.BR IP_RECVERR ).
+A new error will be queued for every incoming MTU update.
+.IP
+While MTU discovery is in progress, initial packets from datagram sockets
+may be dropped.
+Applications using UDP should be aware of this and not
+take it into account for their packet retransmit strategy.
+.IP
+To bootstrap the path MTU discovery process on unconnected sockets, it
+is possible to start with a big datagram size
+(headers up to 64 kilobytes long) and let it shrink by updates of the path MTU.
+.IP
+To get an initial estimate of the
+path MTU, connect a datagram socket to the destination address using
+.BR connect (2)
+and retrieve the MTU by calling
+.BR getsockopt (2)
+with the
+.B IP_MTU
+option.
+.IP
+It is possible to implement RFC 4821 MTU probing with
+.B SOCK_DGRAM
+or
+.B SOCK_RAW
+sockets by setting a value of
+.B IP_PMTUDISC_PROBE
+(available since Linux 2.6.22).
+This is also particularly useful for diagnostic tools such as
+.BR tracepath (8)
+that wish to deliberately send probe packets larger than
+the observed Path MTU.
+.TP
+.BR IP_MULTICAST_ALL " (since Linux 2.6.31)"
+This option can be used to modify the delivery policy of multicast messages.
+The argument is a boolean integer (defaults to 1).
+If set to 1,
+the socket will receive messages from all the groups that have been joined
+globally on the whole system.
+Otherwise, it will deliver messages only from
+the groups that have been explicitly joined (for example via the
+.B IP_ADD_MEMBERSHIP
+option) on this particular socket.
+.TP
+.BR IP_MULTICAST_IF " (since Linux 1.2)"
+Set the local device for a multicast socket.
+The argument for
+.BR setsockopt (2)
+is an
+.I ip_mreqn
+or
+.\" net: IP_MULTICAST_IF setsockopt now recognizes struct mreq
+.\" Commit: 3a084ddb4bf299a6e898a9a07c89f3917f0713f7
+(since Linux 3.5)
+.I ip_mreq
+structure similar to
+.BR IP_ADD_MEMBERSHIP ,
+or an
+.I in_addr
+structure.
+(The kernel determines which structure is being passed based
+on the size passed in
+.IR optlen .)
+For
+.BR getsockopt (2),
+the argument is an
+.I in_addr
+structure.
+.TP
+.BR IP_MULTICAST_LOOP " (since Linux 1.2)"
+Set or read a boolean integer argument that determines whether
+sent multicast packets should be looped back to the local sockets.
+.TP
+.BR IP_MULTICAST_TTL " (since Linux 1.2)"
+Set or read the time-to-live value of outgoing multicast packets for this
+socket.
+It is very important for multicast packets to set the smallest TTL possible.
+The default is 1 which means that multicast packets don't leave the local
+network unless the user program explicitly requests it.
+Argument is an integer.
+.TP
+.BR IP_NODEFRAG " (since Linux 2.6.36)"
+If enabled (argument is nonzero),
+the reassembly of outgoing packets is disabled in the netfilter layer.
+The argument is an integer.
+.IP
+This option is valid only for
+.B SOCK_RAW
+sockets.
+.TP
+.BR IP_OPTIONS " (since Linux 2.0)"
+.\" Precisely: since Linux 1.3.30
+Set or get the IP options to be sent with every packet from this socket.
+The arguments are a pointer to a memory buffer containing the options
+and the option length.
+The
+.BR setsockopt (2)
+call sets the IP options associated with a socket.
+The maximum option size for IPv4 is 40 bytes.
+See RFC\ 791 for the allowed options.
+When the initial connection request packet for a
+.B SOCK_STREAM
+socket contains IP options, the IP options will be set automatically
+to the options from the initial packet with routing headers reversed.
+Incoming packets are not allowed to change options after the connection
+is established.
+The processing of all incoming source routing options
+is disabled by default and can be enabled by using the
+.I accept_source_route
+.I /proc
+interface.
+Other options like timestamps are still handled.
+For datagram sockets, IP options can be set only by the local user.
+Calling
+.BR getsockopt (2)
+with
+.B IP_OPTIONS
+puts the current IP options used for sending into the supplied buffer.
+.TP
+.BR IP_PASSSEC " (since Linux 2.6.17)"
+.\" commit 2c7946a7bf45ae86736ab3b43d0085e43947945c
+If labeled IPSEC or NetLabel is configured on the sending and receiving
+hosts, this option enables receiving of the security context of the peer
+socket in an ancillary message of type
+.B SCM_SECURITY
+retrieved using
+.BR recvmsg (2).
+This option is supported only for UDP sockets; for TCP or SCTP sockets,
+see the description of the
+.B SO_PEERSEC
+option below.
+.IP
+The value given as an argument to
+.BR setsockopt (2)
+and returned as the result of
+.BR getsockopt (2)
+is an integer boolean flag.
+.IP
+The security context returned in the
+.B SCM_SECURITY
+ancillary message
+is of the same format as the one described under the
+.B SO_PEERSEC
+option below.
+.IP
+Note: the reuse of the
+.B SCM_SECURITY
+message type for the
+.B IP_PASSSEC
+socket option was likely a mistake, since other IP control messages use
+their own numbering scheme in the IP namespace and often use the
+socket option value as the message type.
+There is no conflict currently since the IP option with the same value as
+.B SCM_SECURITY
+is
+.B IP_HDRINCL
+and this is never used for a control message type.
+.TP
+.BR IP_PKTINFO " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.68
+Pass an
+.B IP_PKTINFO
+ancillary message that contains a
+.I pktinfo
+structure that supplies some information about the incoming packet.
+This works only for datagram oriented sockets.
+The argument is a flag that tells the socket whether the
+.B IP_PKTINFO
+message should be passed or not.
+The message itself can be sent/retrieved
+only as a control message with a packet using
+.BR recvmsg (2)
+or
+.BR sendmsg (2).
+.IP
+.in +4n
+.EX
+struct in_pktinfo {
+    unsigned int   ipi_ifindex;  /* Interface index */
+    struct in_addr ipi_spec_dst; /* Local address */
+    struct in_addr ipi_addr;     /* Header Destination
+                                    address */
+};
+.EE
+.in
+.IP
+.I ipi_ifindex
+is the unique index of the interface the packet was received on.
+.I ipi_spec_dst
+is the local address of the packet and
+.I ipi_addr
+is the destination address in the packet header.
+If
+.B IP_PKTINFO
+is passed to
+.BR sendmsg (2)
+and
+.\" This field is grossly misnamed
+.I ipi_spec_dst
+is not zero, then it is used as the local source address for the routing
+table lookup and for setting up IP source route options.
+When
+.I ipi_ifindex
+is not zero, the primary local address of the interface specified by the
+index overwrites
+.I ipi_spec_dst
+for the routing table lookup.
+.TP
+.BR IP_RECVERR " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.15
+Enable extended reliable error message passing.
+When enabled on a datagram socket, all
+generated errors will be queued in a per-socket error queue.
+When the user receives an error from a socket operation,
+the errors can be received by calling
+.BR recvmsg (2)
+with the
+.B MSG_ERRQUEUE
+flag set.
+The
+.I sock_extended_err
+structure describing the error will be passed in an ancillary message with
+the type
+.B IP_RECVERR
+and the level
+.BR IPPROTO_IP .
+.\" or SOL_IP on Linux
+This is useful for reliable error handling on unconnected sockets.
+The received data portion of the error queue contains the error packet.
+.IP
+The
+.B IP_RECVERR
+control message contains a
+.I sock_extended_err
+structure:
+.IP
+.in +4n
+.EX
+#define SO_EE_ORIGIN_NONE    0
+#define SO_EE_ORIGIN_LOCAL   1
+#define SO_EE_ORIGIN_ICMP    2
+#define SO_EE_ORIGIN_ICMP6   3
+\&
+struct sock_extended_err {
+    uint32_t ee_errno;   /* error number */
+    uint8_t  ee_origin;  /* where the error originated */
+    uint8_t  ee_type;    /* type */
+    uint8_t  ee_code;    /* code */
+    uint8_t  ee_pad;
+    uint32_t ee_info;    /* additional information */
+    uint32_t ee_data;    /* other data */
+    /* More data may follow */
+};
+\&
+struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
+.EE
+.in
+.IP
+.I ee_errno
+contains the
+.I errno
+number of the queued error.
+.I ee_origin
+is the origin code of where the error originated.
+The other fields are protocol-specific.
+The macro
+.B SO_EE_OFFENDER
+returns a pointer to the address of the network object
+where the error originated from given a pointer to the ancillary message.
+If this address is not known, the
+.I sa_family
+member of the
+.I sockaddr
+contains
+.B AF_UNSPEC
+and the other fields of the
+.I sockaddr
+are undefined.
+.IP
+IP uses the
+.I sock_extended_err
+structure as follows:
+.I ee_origin
+is set to
+.B SO_EE_ORIGIN_ICMP
+for errors received as an ICMP packet, or
+.B SO_EE_ORIGIN_LOCAL
+for locally generated errors.
+Unknown values should be ignored.
+.I ee_type
+and
+.I ee_code
+are set from the type and code fields of the ICMP header.
+.I ee_info
+contains the discovered MTU for
+.B EMSGSIZE
+errors.
+The message also contains the
+.I sockaddr_in of the node
+caused the error, which can be accessed with the
+.B SO_EE_OFFENDER
+macro.
+The
+.I sin_family
+field of the
+.B SO_EE_OFFENDER
+address is
+.B AF_UNSPEC
+when the source was unknown.
+When the error originated from the network, all IP options
+.RB ( IP_OPTIONS ", " IP_TTL ,
+etc.) enabled on the socket and contained in the
+error packet are passed as control messages.
+The payload of the packet causing the error is returned as normal payload.
+.\" FIXME . Is it a good idea to document that? It is a dubious feature.
+.\" On
+.\" .B SOCK_STREAM
+.\" sockets,
+.\" .B IP_RECVERR
+.\" has slightly different semantics. Instead of
+.\" saving the errors for the next timeout, it passes all incoming
+.\" errors immediately to the user.
+.\" This might be useful for very short-lived TCP connections which
+.\" need fast error handling. Use this option with care:
+.\" it makes TCP unreliable
+.\" by not allowing it to recover properly from routing
+.\" shifts and other normal
+.\" conditions and breaks the protocol specification.
+Note that TCP has no error queue;
+.B MSG_ERRQUEUE
+is not permitted on
+.B SOCK_STREAM
+sockets.
+.B IP_RECVERR
+is valid for TCP, but all errors are returned by socket function return or
+.B SO_ERROR
+only.
+.IP
+For raw sockets,
+.B IP_RECVERR
+enables passing of all received ICMP errors to the
+application, otherwise errors are reported only on connected sockets
+.IP
+It sets or retrieves an integer boolean flag.
+.B IP_RECVERR
+defaults to off.
+.TP
+.BR IP_RECVOPTS " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.15
+Pass all incoming IP options to the user in a
+.B IP_OPTIONS
+control message.
+The routing header and other options are already filled in
+for the local host.
+Not supported for
+.B SOCK_STREAM
+sockets.
+.TP
+.BR IP_RECVORIGDSTADDR " (since Linux 2.6.29)"
+.\" commit e8b2dfe9b4501ed0047459b2756ba26e5a940a69
+This boolean option enables the
+.B IP_ORIGDSTADDR
+ancillary message in
+.BR recvmsg (2),
+in which the kernel returns the original destination address
+of the datagram being received.
+The ancillary message contains a
+.IR "struct sockaddr_in" .
+.TP
+.BR IP_RECVTOS " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.68
+If enabled, the
+.B IP_TOS
+ancillary message is passed with incoming packets.
+It contains a byte which specifies the Type of Service/Precedence
+field of the packet header.
+Expects a boolean integer flag.
+.TP
+.BR IP_RECVTTL " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.68
+When this flag is set, pass a
+.B IP_TTL
+control message with the time-to-live
+field of the received packet as a 32 bit integer.
+Not supported for
+.B SOCK_STREAM
+sockets.
+.TP
+.BR IP_RETOPTS " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.15
+Identical to
+.BR IP_RECVOPTS ,
+but returns raw unprocessed options with timestamp and route record
+options not filled in for this hop.
+.TP
+.BR IP_ROUTER_ALERT " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.68
+Pass all to-be forwarded packets with the
+IP Router Alert option set to this socket.
+Valid only for raw sockets.
+This is useful, for instance, for user-space RSVP daemons.
+The tapped packets are not forwarded by the kernel; it is
+the user's responsibility to send them out again.
+Socket binding is ignored,
+such packets are filtered only by protocol.
+Expects an integer flag.
+.TP
+.BR IP_TOS " (since Linux 1.0)"
+Set or receive the Type-Of-Service (TOS) field that is sent
+with every IP packet originating from this socket.
+It is used to prioritize packets on the network.
+TOS is a byte.
+There are some standard TOS flags defined:
+.B IPTOS_LOWDELAY
+to minimize delays for interactive traffic,
+.B IPTOS_THROUGHPUT
+to optimize throughput,
+.B IPTOS_RELIABILITY
+to optimize for reliability,
+.B IPTOS_MINCOST
+should be used for "filler data" where slow transmission doesn't matter.
+At most one of these TOS values can be specified.
+Other bits are invalid and shall be cleared.
+Linux sends
+.B IPTOS_LOWDELAY
+datagrams first by default,
+but the exact behavior depends on the configured queueing discipline.
+.\" FIXME elaborate on this
+Some high-priority levels may require superuser privileges (the
+.B CAP_NET_ADMIN
+capability).
+.\" The priority can also be set in a protocol-independent way by the
+.\" .RB ( SOL_SOCKET ", " SO_PRIORITY )
+.\" socket option (see
+.\" .BR socket (7)).
+.TP
+.BR IP_TRANSPARENT " (since Linux 2.6.24)"
+.\" commit f5715aea4564f233767ea1d944b2637a5fd7cd2e
+.\"     This patch introduces the IP_TRANSPARENT socket option: enabling that
+.\"     will make the IPv4 routing omit the non-local source address check on
+.\"     output. Setting IP_TRANSPARENT requires NET_ADMIN capability.
+.\" http://lwn.net/Articles/252545/
+Setting this boolean option enables transparent proxying on this socket.
+This socket option allows
+the calling application to bind to a nonlocal IP address and operate
+both as a client and a server with the foreign address as the local endpoint.
+NOTE: this requires that routing be set up in a way that
+packets going to the foreign address are routed through the TProxy box
+(i.e., the system hosting the application that employs the
+.B IP_TRANSPARENT
+socket option).
+Enabling this socket option requires superuser privileges
+(the
+.B CAP_NET_ADMIN
+capability).
+.IP
+TProxy redirection with the iptables TPROXY target also requires that
+this option be set on the redirected socket.
+.TP
+.BR IP_TTL " (since Linux 1.0)"
+Set or retrieve the current time-to-live field that is used in every packet
+sent from this socket.
+.TP
+.BR IP_UNBLOCK_SOURCE " (since Linux 2.4.22 / 2.5.68)"
+Unblock previously blocked multicast source.
+Returns
+.B EADDRNOTAVAIL
+when given source is not being blocked.
+.IP
+Argument is an
+.I ip_mreq_source
+structure as described under
+.BR IP_ADD_SOURCE_MEMBERSHIP .
+.TP
+.BR SO_PEERSEC " (since Linux 2.6.17)"
+If labeled IPSEC or NetLabel is configured on both the sending and
+receiving hosts, this read-only socket option returns the security
+context of the peer socket connected to this socket.
+By default,
+this will be the same as the security context of the process that created
+the peer socket unless overridden by the policy or by a process with
+the required permissions.
+.IP
+The argument to
+.BR getsockopt (2)
+is a pointer to a buffer of the specified length in bytes
+into which the security context string will be copied.
+If the buffer length is less than the length of the security
+context string, then
+.BR getsockopt (2)
+returns \-1, sets
+.I errno
+to
+.BR ERANGE ,
+and returns the required length via
+.IR optlen .
+The caller should allocate at least
+.B NAME_MAX
+bytes for the buffer initially, although this is not guaranteed
+to be sufficient.
+Resizing the buffer to the returned length
+and retrying may be necessary.
+.IP
+The security context string may include a terminating null character
+in the returned length, but is not guaranteed to do so: a security
+context "foo" might be represented as either {'f','o','o'} of length 3
+or {'f','o','o','\\0'} of length 4, which are considered to be
+interchangeable.
+The string is printable, does not contain non-terminating null characters,
+and is in an unspecified encoding (in particular, it
+is not guaranteed to be ASCII or UTF-8).
+.IP
+The use of this option for sockets in the
+.B AF_INET
+address family is supported since Linux 2.6.17
+.\" commit 2c7946a7bf45ae86736ab3b43d0085e43947945c
+for TCP sockets, and since Linux 4.17
+.\" commit d452930fd3b9031e59abfeddb2fa383f1403d61a
+for SCTP sockets.
+.IP
+For SELinux, NetLabel conveys only the MLS portion of the security
+context of the peer across the wire, defaulting the rest of the
+security context to the values defined in the policy for the
+netmsg initial security identifier (SID).
+However, NetLabel can
+be configured to pass full security contexts over loopback.
+Labeled IPSEC always passes full security contexts as part of establishing
+the security association (SA) and looks them up based on the association
+for each packet.
+.\"
+.SS /proc interfaces
+The IP protocol
+supports a set of
+.I /proc
+interfaces to configure some global parameters.
+The parameters can be accessed by reading or writing files in the directory
+.IR /proc/sys/net/ipv4/ .
+.\" FIXME As at 2.6.12, 14 Jun 2005, the following are undocumented:
+.\"  ip_queue_maxlen
+.\"  ip_conntrack_max
+Interfaces described as
+.I Boolean
+take an integer value, with a nonzero value ("true") meaning that
+the corresponding option is enabled, and a zero value ("false")
+meaning that the option is disabled.
+.\"
+.TP
+.IR ip_always_defrag " (Boolean; since Linux 2.2.13)"
+[New with Linux 2.2.13; in earlier kernel versions this feature
+was controlled at compile time by the
+.B CONFIG_IP_ALWAYS_DEFRAG
+option; this option is not present in Linux 2.4.x and later]
+.IP
+When this boolean flag is enabled (not equal 0), incoming fragments
+(parts of IP packets
+that arose when some host between origin and destination decided
+that the packets were too large and cut them into pieces) will be
+reassembled (defragmented) before being processed, even if they are
+about to be forwarded.
+.IP
+Enable only if running either a firewall that is the sole link
+to your network or a transparent proxy; never ever use it for a
+normal router or host.
+Otherwise, fragmented communication can be disturbed
+if the fragments travel over different links.
+Defragmentation also has a large memory and CPU time cost.
+.IP
+This is automagically turned on when masquerading or transparent
+proxying are configured.
+.\"
+.TP
+.IR ip_autoconfig " (since Linux 2.2 to Linux 2.6.17)"
+.\" Precisely: since Linux 2.1.68
+.\" FIXME document ip_autoconfig
+Not documented.
+.\"
+.TP
+.IR ip_default_ttl " (integer; default: 64; since Linux 2.2)"
+.\" Precisely: since Linux 2.1.15
+Set the default time-to-live value of outgoing packets.
+This can be changed per socket with the
+.B IP_TTL
+option.
+.\"
+.TP
+.IR ip_dynaddr " (Boolean; default: disabled; since Linux 2.0.31)"
+Enable dynamic socket address and masquerading entry rewriting on interface
+address change.
+This is useful for dialup interface with changing IP addresses.
+0 means no rewriting, 1 turns it on and 2 enables verbose mode.
+.\"
+.TP
+.IR ip_forward " (Boolean; default: disabled; since Linux 1.2)"
+Enable IP forwarding with a boolean flag.
+IP forwarding can be also set on a per-interface basis.
+.\"
+.TP
+.IR ip_local_port_range " (since Linux 2.2)"
+.\" Precisely: since Linux 2.1.68
+This file contains two integers that define the default local port range
+allocated to sockets that are not explicitly bound to a port number\[em]that
+is, the range used for
+.IR "ephemeral ports" .
+An ephemeral port is allocated to a socket in the following circumstances:
+.RS
+.IP \[bu] 3
+the port number in a socket address is specified as 0 when calling
+.BR bind (2);
+.IP \[bu]
+.BR listen (2)
+is called on a stream socket that was not previously bound;
+.IP \[bu]
+.BR connect (2)
+was called on a socket that was not previously bound;
+.IP \[bu]
+.BR sendto (2)
+is called on a datagram socket that was not previously bound.
+.RE
+.IP
+Allocation of ephemeral ports starts with the first number in
+.I ip_local_port_range
+and ends with the second number.
+If the range of ephemeral ports is exhausted,
+then the relevant system call returns an error (but see BUGS).
+.IP
+Note that the port range in
+.I ip_local_port_range
+should not conflict with the ports used by masquerading
+(although the case is handled).
+Also, arbitrary choices may cause problems with some firewall packet
+filters that make assumptions about the local ports in use.
+The first number should be at least greater than 1024,
+or better, greater than 4096, to avoid clashes
+with well known ports and to minimize firewall problems.
+.\"
+.TP
+.IR ip_no_pmtu_disc " (Boolean; default: disabled; since Linux 2.2)"
+.\" Precisely: 2.1.15
+If enabled, don't do Path MTU Discovery for TCP sockets by default.
+Path MTU discovery may fail if misconfigured firewalls (that drop
+all ICMP packets) or misconfigured interfaces (e.g., a point-to-point
+link where the both ends don't agree on the MTU) are on the path.
+It is better to fix the broken routers on the path than to turn off
+Path MTU Discovery globally, because not doing it incurs a high cost
+to the network.
+.\"
+.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
+.TP
+.IR ip_nonlocal_bind " (Boolean; default: disabled; since Linux 2.4)"
+.\" Precisely: patch-2.4.0-test10
+If set, allows processes to
+.BR bind (2)
+to nonlocal IP addresses,
+which can be quite useful, but may break some applications.
+.\"
+.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
+.TP
+.IR ip6frag_time " (integer; default: 30)"
+Time in seconds to keep an IPv6 fragment in memory.
+.\"
+.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
+.TP
+.IR ip6frag_secret_interval " (integer; default: 600)"
+Regeneration interval (in seconds) of the hash secret (or lifetime
+for the hash secret) for IPv6 fragments.
+.TP
+.IR ipfrag_high_thresh " (integer), " ipfrag_low_thresh " (integer)"
+If the amount of queued IP fragments reaches
+.IR ipfrag_high_thresh ,
+the queue is pruned down to
+.IR ipfrag_low_thresh .
+Contains an integer with the number of bytes.
+.TP
+.I neigh/*
+See
+.BR arp (7).
+.\" FIXME Document the conf/*/* interfaces
+.\"
+.\" FIXME Document the route/* interfaces
+.SS Ioctls
+All ioctls described in
+.BR socket (7)
+apply to
+.BR ip .
+.PP
+Ioctls to configure generic device parameters are described in
+.BR netdevice (7).
+.\" FIXME Add a discussion of multicasting
+.SH ERRORS
+.\" FIXME document all errors.
+.\"     We should really fix the kernels to give more uniform
+.\"     error returns (ENOMEM vs ENOBUFS, EPERM vs EACCES etc.)
+.TP
+.B EACCES
+The user tried to execute an operation without the necessary permissions.
+These include:
+sending a packet to a broadcast address without having the
+.B SO_BROADCAST
+flag set;
+sending a packet via a
+.I prohibit
+route;
+modifying firewall settings without superuser privileges (the
+.B CAP_NET_ADMIN
+capability);
+binding to a privileged port without superuser privileges (the
+.B CAP_NET_BIND_SERVICE
+capability).
+.TP
+.B EADDRINUSE
+Tried to bind to an address already in use.
+.TP
+.B EADDRNOTAVAIL
+A nonexistent interface was requested or the requested source
+address was not local.
+.TP
+.B EAGAIN
+Operation on a nonblocking socket would block.
+.TP
+.B EALREADY
+A connection operation on a nonblocking socket is already in progress.
+.TP
+.B ECONNABORTED
+A connection was closed during an
+.BR accept (2).
+.TP
+.B EHOSTUNREACH
+No valid routing table entry matches the destination address.
+This error can be caused by an ICMP message from a remote router or
+for the local routing table.
+.TP
+.B EINVAL
+Invalid argument passed.
+For send operations this can be caused by sending to a
+.I blackhole
+route.
+.TP
+.B EISCONN
+.BR connect (2)
+was called on an already connected socket.
+.TP
+.B EMSGSIZE
+Datagram is bigger than an MTU on the path and it cannot be fragmented.
+.TP
+.BR ENOBUFS ", " ENOMEM
+Not enough free memory.
+This often means that the memory allocation is limited by the socket
+buffer limits, not by the system memory, but this is not 100% consistent.
+.TP
+.B ENOENT
+.B SIOCGSTAMP
+was called on a socket where no packet arrived.
+.TP
+.B ENOPKG
+A kernel subsystem was not configured.
+.TP
+.BR ENOPROTOOPT " and " EOPNOTSUPP
+Invalid socket option passed.
+.TP
+.B ENOTCONN
+The operation is defined only on a connected socket, but the socket wasn't
+connected.
+.TP
+.B EPERM
+User doesn't have permission to set high priority, change configuration,
+or send signals to the requested process or group.
+.TP
+.B EPIPE
+The connection was unexpectedly closed or shut down by the other end.
+.TP
+.B ESOCKTNOSUPPORT
+The socket is not configured or an unknown socket type was requested.
+.PP
+Other errors may be generated by the overlaying protocols; see
+.BR tcp (7),
+.BR raw (7),
+.BR udp (7),
+and
+.BR socket (7).
+.SH NOTES
+.BR IP_FREEBIND ,
+.BR IP_MSFILTER ,
+.BR IP_MTU ,
+.BR IP_MTU_DISCOVER ,
+.BR IP_RECVORIGDSTADDR ,
+.BR IP_PASSSEC ,
+.BR IP_PKTINFO ,
+.BR IP_RECVERR ,
+.BR IP_ROUTER_ALERT ,
+and
+.B IP_TRANSPARENT
+are Linux-specific.
+.\" IP_XFRM_POLICY is Linux-specific
+.\" IP_IPSEC_POLICY is a nonstandard extension, also present on some BSDs
+.PP
+Be very careful with the
+.B SO_BROADCAST
+option \- it is not privileged in Linux.
+It is easy to overload the network
+with careless broadcasts.
+For new application protocols
+it is better to use a multicast group instead of broadcasting.
+Broadcasting is discouraged.
+See RFC 6762 for an example of a protocol (mDNS)
+using the more modern multicast approach
+to communicating with an open-ended
+group of hosts on the local network.
+.PP
+Some other BSD sockets implementations provide
+.B IP_RCVDSTADDR
+and
+.B IP_RECVIF
+socket options to get the destination address and the interface of
+received datagrams.
+Linux has the more general
+.B IP_PKTINFO
+for the same task.
+.PP
+Some BSD sockets implementations also provide an
+.B IP_RECVTTL
+option, but an ancillary message with type
+.B IP_RECVTTL
+is passed with the incoming packet.
+This is different from the
+.B IP_TTL
+option used in Linux.
+.PP
+Using the
+.B SOL_IP
+socket options level isn't portable; BSD-based stacks use the
+.B IPPROTO_IP
+level.
+.PP
+.B INADDR_ANY
+(0.0.0.0) and
+.B INADDR_BROADCAST
+(255.255.255.255) are byte-order-neutral.
+This means
+.BR htonl (3)
+has no effect on them.
+.SS Compatibility
+For compatibility with Linux 2.0, the obsolete
+.BI "socket(AF_INET, SOCK_PACKET, " protocol )
+syntax is still supported to open a
+.BR packet (7)
+socket.
+This is deprecated and should be replaced by
+.BI "socket(AF_PACKET, SOCK_RAW, " protocol )
+instead.
+The main difference is the new
+.I sockaddr_ll
+address structure for generic link layer information instead of the old
+.BR sockaddr_pkt .
+.SH BUGS
+There are too many inconsistent error values.
+.PP
+The error used to diagnose exhaustion of the ephemeral port range differs
+across the various system calls
+.RB ( connect (2),
+.BR bind (2),
+.BR listen (2),
+.BR sendto (2))
+that can assign ephemeral ports.
+.PP
+The ioctls to configure IP-specific interface options and ARP tables are
+not described.
+.\" .PP
+.\" Some versions of glibc forget to declare
+.\" .IR in_pktinfo .
+.\" Workaround currently is to copy it into your program from this man page.
+.PP
+Receiving the original destination address with
+.B MSG_ERRQUEUE
+in
+.I msg_name
+by
+.BR recvmsg (2)
+does not work in some Linux 2.2 kernels.
+.\" .SH AUTHORS
+.\" This man page was written by Andi Kleen.
+.SH SEE ALSO
+.BR recvmsg (2),
+.BR sendmsg (2),
+.BR byteorder (3),
+.BR capabilities (7),
+.BR icmp (7),
+.BR ipv6 (7),
+.BR netdevice (7),
+.BR netlink (7),
+.BR raw (7),
+.BR socket (7),
+.BR tcp (7),
+.BR udp (7),
+.BR ip (8)
+.PP
+The kernel source file
+.IR Documentation/networking/ip\-sysctl.txt .
+.PP
+RFC\ 791 for the original IP specification.
+RFC\ 1122 for the IPv4 host requirements.
+RFC\ 1812 for the IPv4 router requirements.