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