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be0d27ee0c
Fixes #20297.
405 lines
27 KiB
XML
405 lines
27 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-or-later -->
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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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and
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<citerefentry><refentrytitle>org.freedesktop.LogControl1</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, information provided 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 interface) 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 hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6
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addresses that are most likely used for communication with other hosts. This is determined by
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requesting a routing decision to the configured default gateways from the kernel and then using the
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local IP addresses selected by this decision. This hostname is only available if there is at least one
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local default gateway configured. This assigns a stable hostname to the local outbound IP addresses,
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useful for referencing them 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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Support for <filename>/etc/hosts</filename> may be disabled with <varname>ReadEtcHosts=no</varname>,
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see <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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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>The lookup requests that <filename>systemd-resolved.service</filename> receives are routed to the
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available DNS servers, LLMNR, and MulticastDNS interfaces according to the following rules:</para>
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<itemizedlist>
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<listitem><para>Names for which synthetic records are generated (the local hostname,
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<literal>localhost</literal> and <literal>localdomain</literal>, local gateway, as listed in the
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previous section) and addresses configured in <filename>/etc/hosts</filename> are never routed to the
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network and a reply is sent immediately.</para></listitem>
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<listitem><para>Single-label names are resolved using LLMNR on all local interfaces where LLMNR is
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enabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are
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only sent via LLMNR on IPv6. Note that lookups for single-label synthesized names are not routed to
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LLMNR, MulticastDNS or unicast DNS.</para></listitem>
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<listitem><para>Queries for the address records (A and AAAA) of single-label non-synthesized names are
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resolved via unicast DNS using search domains. For any interface which defines search domains, such
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look-ups are routed to the servers defined for that interface, suffixed with each of those search
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domains. When global search domains are defined, such look-ups are routed to the global servers. For
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each search domain, queries are performed by suffixing the name with each of the search domains in
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turn. Additionally, lookup of single-label names via unicast DNS may be enabled with the
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<varname>ResolveUnicastSingleLabel=yes</varname> setting. The details of which servers are queried and
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how the final reply is chosen are described below. Note that this means that address queries for
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single-label names are never sent out to remote DNS servers by default, and resolution is only
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possible if search domains are defined.</para></listitem>
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<listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are resolved using
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MulticastDNS on all local interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups
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are sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem>
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<listitem><para>Queries for multi-label names are routed via unicast DNS on local interfaces that have
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a DNS server configured, plus the globally configured DNS servers if there are any. Which interfaces
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are used is determined by the routing logic based on search and route-only domains, described below.
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Note that by default, lookups for domains with the <literal>.local</literal> suffix are not routed to
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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 work
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within this DNS domain. Note that these days, it's generally recommended to avoid defining
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<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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<listitem><para>Address lookups (reverse lookups) are routed similarly to multi-label names, with the
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exception that addresses from the link-local address range are never routed to unicast DNS and are only
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resolved using LLMNR and MulticastDNS (when enabled).</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 is determined by the per-interface routing domains (search and route-only) and
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global search domains. 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 a
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description how those settings are set dynamically and the discussion of <varname>Domains=</varname> in
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<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a
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description of globally configured DNS settings.</para>
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<para>The following query routing logic applies for unicast DNS lookups initiated by
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<filename>systemd-resolved.service</filename>:</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 routing domains (search or route-only) of any link, or the globally configured DNS settings,
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"best matching" routing domain is determined: the matching one with the most labels. The query is then
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sent to all DNS servers of any links or the globally configured DNS servers associated with this "best
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matching" routing domain. (Note that more than one link might have this same "best matching" routing
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domain configured, in which case the query is sent to all of them in parallel).</para>
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<para>In case of single-label names, when search domains are defined, the same logic applies, except
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that the name is first suffixed by each of the search domains in turn. Note that this search logic
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doesn't apply to any names with at least one dot. Also see the discussion about compatibility with
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the traditional glibc resolver below.</para></listitem>
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<listitem><para>If a query does not match any configured routing domain (either per-link or global), it
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is sent to all DNS servers that are configured on links with the <varname>DefaultRoute=</varname>
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option set, as well as the globally configured DNS server.</para></listitem>
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<listitem><para>If there is no link configured as <varname>DefaultRoute=</varname> and no global DNS
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server configured, one of the compiled-in fallback DNS servers is used.</para></listitem>
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<listitem><para>Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined.
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</para></listitem>
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</itemizedlist>
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<para>The <varname>DefaultRoute=</varname> option is a boolean setting configurable with
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<command>resolvectl</command> or in <filename>.network</filename> files. If not set, it is implicitly
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determined based on the configured DNS domains for a link: if there's a route-only domain other than
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<literal>~.</literal>, it defaults to false, otherwise to true.</para>
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<para>Effectively this means: in order to support single-label non-synthesized names, define appropriate
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search domains. In order to preferably route all DNS queries not explicitly matched by routing domain
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configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This will
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ensure that other links will not be considered for these queries (unless they too carry such a routing
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domain). In order to route all such DNS queries to a specific link only if no other link is preferred,
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set the <varname>DefaultRoute=</varname> 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
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receives any DNS traffic not matching any of its configured routing domains, set the
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<varname>DefaultRoute=</varname> option for it to false.</para>
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<para>See
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<citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for information about the D-Bus APIs <filename>systemd-resolved</filename> provides.</para>
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</refsect1>
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<refsect1>
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<title>Compatibility with the traditional glibc stub resolver</title>
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<para>This section provides a short summary of differences in the stub resolver implemented by
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<citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together
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with <command>systemd-resolved</command> and the traditional stub resolver implemented in
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<filename>nss-dns</filename>.</para>
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<itemizedlist>
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<listitem><para>Some names are always resolved internally (see Synthetic Records above). Traditionally
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they would be resolved by <filename>nss-files</filename> if provided in
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<filename>/etc/hosts</filename>. But note that the details of how a query is constructed are under the
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control of the client library. <filename>nss-dns</filename> will first try to resolve names using
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search domains and even if those queries are routed to <filename>systemd-resolved</filename>, it will
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send them out over the network using the usual rules for multi-label name routing <footnote><para>For
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example, if <filename>/etc/resolv.conf</filename> has <programlisting>nameserver 127.0.0.53
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search foobar.com barbar.com
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</programlisting>and we look up <literal>localhost</literal>, <filename>nss-dns</filename> will send
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the following queries to <filename>systemd-resolved</filename> listening on 127.0.0.53:53: first
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<literal>localhost.foobar.com</literal>, then <literal>localhost.barbar.com</literal>, and finally
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<literal>localhost</literal>. If (hopefully) the first two queries fail,
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<filename>systemd-resolved</filename> will synthesize an answer for the third query.</para>
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<para>When using <filename>nss-dns</filename> with any search domains, it is thus crucial to always
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configure <filename>nss-files</filename> with higher priority and provide mappings for names that
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should not be resolved using search domains.</para></footnote>.</para></listitem>
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<listitem><para>Single-label names are not resolved for A and AAAA records using unicast DNS (unless
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overridden with <varname>ResolveUnicastSingleLabel=</varname>, see
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<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
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This is similar to the <option>no-tld-query</option> option being set in
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<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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</para></listitem>
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<listitem><para>Search domains are not used for <emphasis>suffixing</emphasis> of multi-label names.
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(Search domains are nevertheless used for lookup <emphasis>routing</emphasis>, for names that were
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originally specified as single-label or multi-label.) Any name with at least one dot is always
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interpreted as a FQDN. <filename>nss-dns</filename> would resolve names both as relative (using search
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domains) and absolute FQDN names. Some names would be resolved as relative first, and after that query
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has failed, as absolute, while other names would be resolved in opposite order. The
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<varname>ndots</varname> option in <filename>/etc/resolv.conf</filename> was used to control how many
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dots the name needs to have to be resolved as relative first. This stub resolver does not implement
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this at all: multi-label names are only resolved as FQDNs.<footnote><para>There are currently more than
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1500 top-level domain names defined, and new ones are added regularly, often using "attractive" names
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that are also likely to be used locally. Not looking up multi-label names in this fashion avoids
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fragility in both directions: a valid global name could be obscured by a local name, and resolution of
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a relative local name could suddenly break when a new top-level domain is created, or when a new
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subdomain of a top-level domain in registered. Resolving any given name as either relative or absolute
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avoids this ambiguity.</para></footnote></para></listitem>
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<listitem><para>This resolver has a notion of the special <literal>.local</literal> domain used for
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MulticastDNS, and will not route queries with that suffix to unicast DNS servers unless explicitly
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configured, see above. Also, reverse lookups for link-local addresses are not sent to unicast DNS
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servers.</para></listitem>
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<listitem><para>This resolver reads and caches <filename>/etc/hosts</filename> internally. (In other
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words, <filename>nss-resolve</filename> replaces <filename>nss-files</filename> in addition to
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<filename>nss-dns</filename>). Entries in <filename>/etc/hosts</filename> have highest priority.</para>
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</listitem>
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<listitem><para>This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast
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DNS protocol, and will resolve single-label names using LLMNR (when enabled) and names ending in
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<literal>.local</literal> using MulticastDNS (when enabled).</para></listitem>
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<listitem><para>Environment variables <varname>$LOCALDOMAIN</varname> and
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<varname>$RES_OPTIONS</varname> described in
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<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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are not supported currently.</para></listitem>
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</itemizedlist>
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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
|
||
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>
|