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diff --git a/man/systemd-resolved.service.xml b/man/systemd-resolved.service.xml new file mode 100644 index 0000000..d7334e0 --- /dev/null +++ b/man/systemd-resolved.service.xml @@ -0,0 +1,288 @@ +<?xml version='1.0'?> +<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" + "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> + +<!-- + SPDX-License-Identifier: LGPL-2.1+ +--> + +<refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'> + + <refentryinfo> + <title>systemd-resolved.service</title> + <productname>systemd</productname> + </refentryinfo> + + <refmeta> + <refentrytitle>systemd-resolved.service</refentrytitle> + <manvolnum>8</manvolnum> + </refmeta> + + <refnamediv> + <refname>systemd-resolved.service</refname> + <refname>systemd-resolved</refname> + <refpurpose>Network Name Resolution manager</refpurpose> + </refnamediv> + + <refsynopsisdiv> + <para><filename>systemd-resolved.service</filename></para> + <para><filename>/usr/lib/systemd/systemd-resolved</filename></para> + </refsynopsisdiv> + + <refsect1> + <title>Description</title> + + <para><command>systemd-resolved</command> is a system service that provides network name resolution to local + applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR and MulticastDNS + resolver and responder. Local applications may submit network name resolution requests via three interfaces:</para> + + <itemizedlist> + <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus. See the + <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved">API Documentation</ulink> for + details. Usage of this API is generally recommended to clients as it is asynchronous and fully featured (for + example, properly returns DNSSEC validation status and interface scope for addresses as necessary for supporting + link-local networking).</para></listitem> + + <listitem><para>The glibc + <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> API as defined + by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related resolver functions, + including <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This + API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC + validation status information however, and is synchronous only. This API is backed by the glibc Name Service + Switch (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Usage of the + glibc NSS module <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> + is required in order to allow glibc's NSS resolver functions to resolve host names via + <command>systemd-resolved</command>.</para></listitem> + + <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on IP + address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local + API may be directed to this stub, in order to connect them to <command>systemd-resolved</command>. Note however + that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above), + as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped + to the unicast DNS protocol.</para></listitem> + </itemizedlist> + + <para>The DNS servers contacted are determined from the global settings in + <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in + <filename>/etc/systemd/network/*.network</filename> files (in case + <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> is + used), the per-link dynamic settings received over DHCP, user request made via + <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, and any DNS server + information made available by other system services. See + <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and + <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details + about systemd's own configuration files for DNS servers. To improve compatibility, + <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but only if it is + not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>, + <filename>/usr/lib/systemd/resolv.conf</filename> or <filename>/run/systemd/resolve/resolv.conf</filename> (see + below).</para> + + </refsect1> + + <refsect1> + <title>Synthetic Records</title> + + <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para> + + <itemizedlist> + <listitem><para>The local, configured hostname is resolved to + all locally configured IP addresses ordered by their scope, or + — if none are configured — the IPv4 address 127.0.0.2 (which + is on the local loopback) and the IPv6 address ::1 (which is the + local host).</para></listitem> + + <listitem><para>The hostnames <literal>localhost</literal> and + <literal>localhost.localdomain</literal> (as well as any hostname + ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>) + are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem> + + <listitem><para>The hostname <literal>_gateway</literal> is + resolved to all current default routing gateway addresses, + ordered by their metric. This assigns a stable hostname to the + current gateway, useful for referencing it independently of the + current network configuration state.</para></listitem> + + <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved + to their configured addresses and back, but they will not affect lookups for + non-address types (like MX).</para></listitem> + </itemizedlist> + </refsect1> + + <refsect1> + <title>Protocols and Routing</title> + + <para>Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces according to the + following rules:</para> + + <itemizedlist> + <listitem><para>Lookups for the special hostname <literal>localhost</literal> are never routed to the network. (A + few other, special domains are handled the same way.)</para></listitem> + + <listitem><para>Single-label names are routed to all local interfaces capable of IP multicasting, using the LLMNR + protocol. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are only + sent via LLMNR on IPv6. Lookups for the locally configured host name and the <literal>_gateway</literal> host + name are never routed to LLMNR.</para></listitem> + + <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are routed to all local + interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4 address lookups are + sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem> + + <listitem><para>Other multi-label names are routed to all local interfaces that have a DNS server configured, + plus the globally configured DNS server if there is one. Address lookups from the link-local address range are + never routed to DNS. Note that by default lookups for domains with the <literal>.local</literal> suffix are not + routed to DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server + and interface. This means that on networks where the <literal>.local</literal> domain is defined in a + site-specific DNS server, explicit search or routing domains need to be configured to make lookups within this + DNS domain work. Note that today it's generally recommended to avoid defining <literal>.local</literal> in a DNS + server, as <ulink url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive + MulticastDNS use.</para></listitem> + </itemizedlist> + + <para>If lookups are routed to multiple interfaces, the first + successful response is returned (thus effectively merging the + lookup zones on all matching interfaces). If the lookup failed on + all interfaces, the last failing response is returned.</para> + + <para>Routing of lookups may be influenced by configuring per-interface domain names and other settings. See + <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and + <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for details. The + following query routing logic applies for unicast DNS traffic:</para> + + <itemizedlist> + <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the configured search + or route-only domains of any link (or the globally configured DNS settings), the "best matching" + search/route-only domain is determined: the matching one with the most labels. The query is then sent to all DNS + servers of any links or the globally configured DNS servers associated with this "best matching" + search/route-only domain. (Note that more than one link might have this same "best matching" search/route-only + domain configured, in which case the query is sent to all of them in parallel).</para></listitem> + + <listitem><para>If a query does not match any configured search/route-only domain (neither per-link nor global), + it is sent to all DNS servers that are configured on links with the "DNS default route" option set, as well as + the globally configured DNS server.</para></listitem> + + <listitem><para>If there is no link configured as "DNS default route" and no global DNS server configured, the + compiled-in fallback DNS server is used.</para></listitem> + + <listitem><para>Otherwise the query is failed as no suitable DNS servers could be determined.</para></listitem> + </itemizedlist> + + <para>The "DNS default route" option is a boolean setting configureable with <command>resolvectl</command> or in + <filename>.network</filename> files. If not set, it is implicitly determined based on the configured DNS domains + for a link: if there's any route-only domain (not matching <literal>~.</literal>) it defaults to false, otherwise + to true.</para> + + <para>Effectively this means: in order to preferably route all DNS queries not explicitly matched by + search/route-only domain configuration to a specific link, configure a <literal>~.</literal> route-only domain on + it. This will ensure that other links will not be considered for the queries (unless they too carry such a + route-only domain). In order to route all such DNS queries to a specific link only in case no other link is + preferable, then set the "DNS default route" option for the link to true, and do not configure a + <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never receives any + DNS traffic not matching any of its configured search/route-only domains, set the "DNS default route" option for it + to false.</para> + + <para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API + Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para> + </refsect1> + + <refsect1> + <title><filename>/etc/resolv.conf</filename></title> + + <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see + <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are + supported:</para> + + <itemizedlist> + <listitem><para><command>systemd-resolved</command> maintains the + <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional Linux + programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists the 127.0.0.53 + DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by + systemd-resolved. The list of search domains is always kept up-to-date. Note that + <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications, but only + through a symlink from <filename>/etc/resolv.conf</filename>. This file may be symlinked from + <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to + <command>systemd-resolved</command> with correct search domains settings. This mode of operation is + recommended.</para></listitem> + + <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists + the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from + <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to + <command>systemd-resolved</command>. This file does not contain any search domains.</para></listitem> + + <listitem><para><command>systemd-resolved</command> maintains the + <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux + programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept up-to-date, + containing information about all known DNS servers. Note the file format's limitations: it does not know a + concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that + <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications, but only + through a symlink from <filename>/etc/resolv.conf</filename>. If this mode of operation is used local clients + that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the + known DNS servers.</para> </listitem> + + <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which + case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation + <command>systemd-resolved</command> is consumer rather than provider of this configuration + file. </para></listitem> + </itemizedlist> + + <para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether + <filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or + lists 127.0.0.53 as DNS server.</para> + </refsect1> + + <refsect1> + <title>Signals</title> + + <variablelist> + <varlistentry> + <term><constant>SIGUSR1</constant></term> + + <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal + <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it maintains, as + well as all feature level information it learnt about configured DNS servers into the system + logs.</para></listitem> + </varlistentry> + + <varlistentry> + <term><constant>SIGUSR2</constant></term> + + <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal + <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally not be + necessary to request this explicitly – except for debugging purposes – as <command>systemd-resolved</command> + flushes the caches automatically anyway any time the host's network configuration changes. Sending this signal + to <command>systemd-resolved</command> is equivalent to the <command>resolvectl --flush-caches</command> + command, however the latter is recommended since it operates in a synchronous way.</para></listitem> + </varlistentry> + + <varlistentry> + <term><constant>SIGRTMIN+1</constant></term> + + <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal + <command>systemd-resolved</command> will forget everything it learnt about the configured DNS + servers. Specifically any information about server feature support is flushed out, and the server feature + probing logic is restarted on the next request, starting with the most fully featured level. Note that it + should normally not be necessary to request this explicitly – except for debugging purposes – as + <command>systemd-resolved</command> automatically forgets learnt information any time the DNS server + configuration changes. Sending this signal to <command>systemd-resolved</command> is equivalent to the + <command>resolvectl --reset-server-features</command> command, however the latter is recommended since it + operates in a synchronous way.</para></listitem> + </varlistentry> + </variablelist> + + </refsect1> + + <refsect1> + <title>See Also</title> + <para> + <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, + <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, + <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>, + <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>, + <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, + <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, + <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>, + <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>, + <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> + </para> + </refsect1> + +</refentry> |