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| Copyright 2014 Tom Gundersen |
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| <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVED'> |
| |
| <refentryinfo> |
| <title>systemd-resolved.service</title> |
| <productname>systemd</productname> |
| |
| <authorgroup> |
| <author> |
| <contrib>Developer</contrib> |
| <firstname>Tom</firstname> |
| <surname>Gundersen</surname> |
| <email>teg@jklm.no</email> |
| </author> |
| </authorgroup> |
| </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 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, the per-link dynamic settings received over DHCP 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/resolv.conf</filename> (see below).</para> |
| |
| <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> |
| |
| <para>Lookup requests are routed to the available DNS servers |
| and LLMNR 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 are routed to all local |
| interfaces that have a DNS sever configured, plus the globally |
| configured DNS server if there is one. Address lookups from the |
| link-local address range are never routed to |
| DNS.</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. See |
| <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> |
| for details. Lookups for a hostname ending in one of the |
| per-interface domains are exclusively routed to the matching |
| interfaces.</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>Three 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>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 mode of operation is recommended.</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 SIGUSR1 process signal <command>systemd-resolved</command> will dump the |
| contents of all DNS resource record caches it maintains into the system logs.</para></listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><constant>SIGUSR2</constant></term> |
| |
| <listitem><para>Upon reception of the SIGUSR2 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.</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>systemd-resolve</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> |