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diff --git a/man/systemd-resolved.service.xml b/man/systemd-resolved.service.xml new file mode 100644 index 0000000..12aefd5 --- /dev/null +++ b/man/systemd-resolved.service.xml @@ -0,0 +1,381 @@ +<?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-or-later --> + +<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> synthetizes 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 interface) 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 (the local hostname, + <literal>localhost</literal> and <literal>localdomain</literal>, local gateway, as listed in the + previous section) and addresses configured in <filename>/etc/hosts</filename> are never routed to the + network and a reply is sent immediately.</para></listitem> + + <listitem><para>Single-label names are resolved using LLMNR on all local interfaces where LLMNR is + enabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are + only sent via LLMNR on IPv6. Note that lookups for single-label synthetized names are not routed to + LLMNR, MulticastDNS or unicast DNS.</para></listitem> + + <listitem><para>Queries for the address records (A and AAAA) of single-label non-synthetized names are + resolved via unicast DNS using search domains. 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 resoulution is only possible if search domains are defined.</para></listitem> + + <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are resolved using + MulticastDNS on all local interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups + are sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem> + + <listitem><para>Queries for multi-label names are routed via unicast DNS on local interfaces that have + a DNS server configured, plus the globally configured DNS servers if there are any. Which interfaces + are used is determined by the routing logic based on search and route-only domains, described below. + 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 work + within this DNS domain. 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 (reverse 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 is determined by the per-interface routing domains (search and route-only) and + global search domains. See + <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and + <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for a + description how those settings are set dynamically and the discussion of <varname>Domains=</varname> in + <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a + description of globally configured DNS settings.</para> + + <para>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 routing domains (search or route-only) of any link, or the globally configured DNS settings, + "best matching" routing 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" routing domain. (Note that more than one link might have this same "best matching" routing + 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 each of the search domains in turn. Note that this search logic + doesn't apply to any names with at least one dot. Also see the discussion about compatibility with + the traditional glibc resolver below.</para></listitem> + + <listitem><para>If a query does not match any configured routing domain (either per-link or global), it + is sent to all DNS servers that are configured on links with the <varname>DefaultRoute=</varname> + option set, as well as the globally configured DNS server.</para></listitem> + + <listitem><para>If there is no link configured as <varname>DefaultRoute=</varname> and no global DNS + server configured, one of the compiled-in fallback DNS servers is used.</para></listitem> + + <listitem><para>Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined. + </para></listitem> + </itemizedlist> + + <para>The <varname>DefaultRoute=</varname> 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 a route-only domain other than + <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 routing 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 routing + domain). In order to route all such DNS queries to a specific link only if no other link is preferred, + set the <varname>DefaultRoute=</varname> 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 routing domains, set the + <varname>DefaultRoute=</varname> option for it to false.</para> + + <para>See + <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry> + for information about the D-Bus APIs <filename>systemd-resolved</filename> provides.</para> + </refsect1> + + <refsect1> + <title>Compatibility with the traditional glibc stub resolver</title> + + <para>This section provides a short summary of differences in the stub resolver implemented by + <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together + with <command>systemd-resolved</command> and the tranditional stub resolver implemented in + <citerefentry><refentrytitle>nss-dns</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para> + + <itemizedlist> + <listitem><para>Some names are always resolved internally (see Synthetic Records above). Traditionally + they would be resolved by <filename>nss-files</filename>, and only if provided in + <filename>/etc/hosts</filename>.</para></listitem> + + <listitem><para>Single-label names are not resolved for A and AAAA records using unicast DNS (unless + overridden with <varname>ResolveUnicastSingleLabel=</varname>, see + <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>). + This is similar to the <option>no-tld-query</option> option being set in + <citerefentry><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. + </para></listitem> + + <listitem><para>Search domains are not used for <emphasis>suffixing</emphasis> of multi-label names. + (Search domains are nevertheless used for lookup <emphasis>routing</emphasis>, for names that were + originally specified as single-label or multi-label.) Any name with at least one dot is always + interpreted as a FQDN. <filename>nss-dns</filename> would resolve names both as relative (using search + domains) and absolute FQDN names. Some names would be resolved as relative first, and after that query + has failed, as absolute, while other names would be resolved in opposite order. The + <varname>ndots</varname> option in <filename>/etc/resolv.conf</filename> was used to control how many + dots the name needs to have to be resolved as relative first. This stub resolver does not implement + this at all: multi-label names are only resolved as FQDNs. (There are currently more than 1500 + top-level domain names defined, and new ones are added regularly, often using "attractive" names that + are also likely to be used locally. Not looking up multi-label names in this fashion avoids fragility + in both directions: a valid global name could be obscured by a local name, and resolution of a relative + local name could suddenly break when a new top-level domain is created, or when a new subdomain of a + top-level domain in registered. Resolving any given name as either relative or absolute avoids this + ambiguity.)</para></listitem> + + <listitem><para>This resolver has a notion of the special <literal>.local</literal> domain used for + MulticastDNS, and will not route queries with that suffix to unicast DNS servers unless explicitly + configured, see above. Also, reverse lookups for link-local addresses are not sent to unicast DNS + servers.</para></listitem> + + <listitem><para>This resolver reads and caches <filename>/etc/hosts</filename> internally. (In other + words, <filename>nss-resolve</filename> replaces <filename>nss-files</filename> in addition to + <filename>nss-dns</filename>). Entries in <filename>/etc/hosts</filename> have highest priority.</para> + </listitem> + + <listitem><para>This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast + DNS protocol, and will resolve single-label names using LLMNR (when enabled) and names ending in + <literal>.local</literal> using MulticastDNS (when enabled).</para></listitem> + + <listitem><para>Environment variables <varname>$LOCALDOMAIN</varname> and + <varname>$RES_OPTIONS</varname> described in + <citerefentry><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> are not + supported currently.</para></listitem> + </itemizedlist> + </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> |