| <?xml version='1.0'?> |
| <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//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 |
| <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry> |
| and |
| <citerefentry><refentrytitle>org.freedesktop.LogControl1</refentrytitle><manvolnum>5</manvolnum></citerefentry> |
| 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 hostnames 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, information provided 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). |
| Support for <filename>/etc/hosts</filename> may be disabled with <varname>ReadEtcHosts=no</varname>, |
| see <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. |
| </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>Names for which synthetic records are generated (as listed in the previous section) are |
| never routed to the network and a reply is sent immediately. In particular this means that lookups for |
| <literal>localhost</literal> are never routed to the network.</para></listitem> |
| |
| <listitem><para>Single-label names are routed to all local interfaces capable of IP multicasting, where |
| LLMNR is not disabled, 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 hostname and the <literal>_gateway</literal> hostname 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, where MulticastDNS is not disabled, 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>Resolution of address records (A and AAAA) via unicast DNS (i.e. not LLMNR or |
| MulticastDNS) for non-synthesized single-label names is allowed for non-top-level domains. This means |
| that such records can be resolved when search domains are defined. For any interface which defines |
| search domains, such look-ups are routed to that interface, suffixed with each of the search domains |
| defined on that interface in turn. When global search domains are defined, such look-ups are routed to |
| all interfaces, suffixed by each of the global search domains in turn. Additionally, lookup of |
| single-label names via unicast DNS may be enabled with the |
| <varname>ResolveUnicastSingleLabel=yes</varname> setting. The details of which servers are queried and |
| how the final reply is chosen are described below. Note that this means that address queries for |
| single-label names are never sent out to remote DNS servers by default, and if no search domains are |
| defined, resolution will fail.</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 servers if there are any. 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 |
| these days, 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> |
| |
| <listitem><para>Address lookups are routed similarly to multi-label names, with the exception that |
| addresses from the link-local address range are never routed to unicast DNS and are only resolved using |
| LLMNR and MulticastDNS (when enabled).</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 (see |
| <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>), |
| or the globally configured DNS settings (see the discussion of <varname>Domains=</varname> in |
| <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>), |
| "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> |
| |
| <para>In case of single-label names, when search domains are defined, the same logic applies, except |
| that the name is first suffixed by the search domain.</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 configurable 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 support single-label non-synthetized names, define appropriate |
| search domains. 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 these queries (unless they too carry such a |
| route-only domain). In order to route all such DNS queries to a specific link only if no other link |
| is preferable, 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> |