296 lines
18 KiB
XML
296 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
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local applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR
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and MulticastDNS resolver and responder. Local applications may submit network name resolution requests
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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,
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see
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<citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for details. Usage of this API is generally recommended to clients as it is asynchronous and fully
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featured (for example, properly returns DNSSEC validation status and interface scope for addresses as
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necessary for supporting 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>
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API as defined by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related
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resolver functions, including
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<citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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This API is widely supported, including beyond the Linux platform. In its current form it does not
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expose DNSSEC validation status information however, and is synchronous only. This API is backed by the
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glibc Name Service Switch
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(<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
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Usage of the glibc NSS module
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<citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
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required in order to allow glibc's NSS resolver functions to resolve hostnames 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
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IP address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly,
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bypassing any local API may be directed to this stub, in order to connect them to
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<command>systemd-resolved</command>. Note however that it is strongly recommended that local programs
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use the glibc NSS or bus APIs instead (as described above), as various network resolution concepts
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(such as link-local addressing, or LLMNR Unicode domains) cannot be mapped to the unicast DNS
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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>
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is 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
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DNS server 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
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details 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
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only if it is not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>,
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<filename>/usr/lib/systemd/resolv.conf</filename> or
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<filename>/run/systemd/resolve/resolv.conf</filename> (see 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
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cases:</para>
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<itemizedlist>
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<listitem><para>The local, configured hostname is resolved to all locally configured IP addresses
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ordered by their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the local
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loopback) and the IPv6 address ::1 (which is the local host).</para></listitem>
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<listitem><para>The hostnames <literal>localhost</literal> and <literal>localhost.localdomain</literal>
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(as well as any hostname ending in <literal>.localhost</literal> or
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<literal>.localhost.localdomain</literal>) are resolved to the IP addresses 127.0.0.1 and ::1.
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</para></listitem>
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<listitem><para>The hostname <literal>_gateway</literal> is resolved to all current default routing
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gateway addresses, ordered by their metric. This assigns a stable hostname to the current gateway,
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useful for referencing it independently of the current network configuration state.</para></listitem>
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<listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their
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configured addresses and back, but they will not affect lookups for non-address types (like MX).
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</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
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according to the 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
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network. (A 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
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the LLMNR protocol. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6
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addresses are only sent via LLMNR on IPv6. Lookups for the locally configured hostname and the
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<literal>_gateway</literal> hostname 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
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local interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4
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address lookups are 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
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configured, plus the globally configured DNS server if there is one. Address lookups from the
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link-local address range are never routed to DNS. Note that by default lookups for domains with the
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<literal>.local</literal> suffix are not routed to DNS servers, unless the domain is specified
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explicitly as routing or search domain for the DNS server and interface. This means that on networks
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where the <literal>.local</literal> domain is defined in a site-specific DNS server, explicit search or
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routing domains need to be configured to make lookups within this DNS domain work. Note that today it's
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generally recommended to avoid defining <literal>.local</literal> in a DNS server, as <ulink
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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 successful response is returned (thus
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effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces,
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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
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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
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details. The 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
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configured search or route-only domains of any link (or the globally configured DNS settings), the
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"best matching" search/route-only domain is determined: the matching one with the most labels. The
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query is then sent to all DNS servers of any links or the globally configured DNS servers associated
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with this "best matching" search/route-only domain. (Note that more than one link might have this same
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"best matching" search/route-only domain configured, in which case the query is sent to all of them in
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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
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global), it is sent to all DNS servers that are configured on links with the "DNS default route" option
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set, as well as 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
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configured, the 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.
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</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>
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or in <filename>.network</filename> files. If not set, it is implicitly determined based on the
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configured DNS domains for a link: if there's any route-only domain (not matching <literal>~.</literal>)
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it defaults to false, otherwise 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
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domain on it. This will ensure that other links will not be considered for the queries (unless they too
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carry such a route-only domain). In order to route all such DNS queries to a specific link only in case
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no other link is preferable, then set the "DNS default route" option for the link to true, and do not
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configure a <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific
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link never receives any DNS traffic not matching any of its configured search/route-only domains, set the
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"DNS default route" option for it 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.
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</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
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Linux programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists
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the 127.0.0.53 DNS stub (see above) as the only DNS server. It also contains a list of search domains
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that are in use by 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,
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but only 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
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to <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
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to <command>systemd-resolved</command>. This file does not contain any search domains.
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</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
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up-to-date, containing information about all known DNS servers. Note the file format's limitations: it
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does not know a concept of per-interface DNS servers and hence only contains system-wide DNS server
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definitions. Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used
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directly by applications, but only through a symlink from <filename>/etc/resolv.conf</filename>. If
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this mode of operation is used local clients that bypass any local DNS API will also bypass
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<command>systemd-resolved</command> and will talk directly to the known DNS servers.</para></listitem>
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<listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages,
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in which case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode
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of operation <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
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on whether <filename>/etc/resolv.conf</filename> is a symlink to
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<filename>/run/systemd/resolve/resolv.conf</filename> or 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
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maintains, as well as all feature level information it learnt about configured DNS servers into the
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system 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
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not be necessary to request this explicitly – except for debugging purposes – as
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<command>systemd-resolved</command> flushes the caches automatically anyway any time the host's
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network configuration changes. Sending this signal to <command>systemd-resolved</command> is
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equivalent to the <command>resolvectl flush-caches</command> command, however the latter is
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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
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feature probing logic is restarted on the next request, starting with the most fully featured
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level. Note that it should normally not be necessary to request this explicitly – except for
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debugging purposes – as <command>systemd-resolved</command> automatically forgets learnt information
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any time the DNS server configuration changes. Sending this signal to
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<command>systemd-resolved</command> is equivalent to the <command>resolvectl
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reset-server-features</command> command, however the latter is recommended since it operates in a
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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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