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In the troff output, this doesn't seem to make any difference. But in the html output, the whitespace is sometimes preserved, creating an additional gap before the following content. Drop it everywhere to avoid this.
524 lines
34 KiB
XML
524 lines
34 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/4.5/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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xmlns:xi="http://www.w3.org/2001/XInclude">
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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 via D-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 native API <command>systemd-resolved</command> exposes via Varlink on the
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<filename>/run/systemd/resolve/io.systemd.Resolve</filename> AF_UNIX socket. This provides similar
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functionality as the D-Bus interface, but is available during the entire runtime, without requiring a
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running D-Bus system bus broker service.</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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the IP addresses 127.0.0.53 and 127.0.0.54 on the local loopback interface. Programs issuing DNS
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requests directly, 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>
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<para id="proxy-stub">The DNS stub resolver on 127.0.0.53 provides the full feature set of the local
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resolver, which includes offering LLMNR/MulticastDNS resolution. The DNS stub resolver on 127.0.0.54
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provides a more limited resolver, that operates in "proxy" mode only, i.e. it will pass most DNS
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messages relatively unmodified to the current upstream DNS servers and back, but not try to process the
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messages locally, and hence does not validate DNSSEC, or offer up LLMNR/MulticastDNS. (It will
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translate to DNS-over-TLS communication if needed however.)</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 the preferred source
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addresses of default gateways if specified, or determined by requesting a routing decision to the
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configured default gateways from the kernel and then using the local IP addresses selected by this
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decision. This hostname is only available if there is at least one local default gateway configured.
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This assigns a stable hostname to the local outbound IP addresses, useful for referencing them
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independently of the current network configuration state.</para></listitem>
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<listitem><para>The hostname <literal>_localdnsstub</literal> is resolved to the IP address 127.0.0.53,
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i.e. the address the local DNS stub (see above) is listening on.</para></listitem>
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<listitem><para>The hostname <literal>_localdnsproxy</literal> is resolved to the IP address 127.0.0.54,
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i.e. the address the local DNS proxy (see above) is listening on.</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 configured as the default route, as well as the
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globally configured DNS server.</para></listitem>
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<listitem><para>If there are no DNS servers configured on any link also configured as the default route
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and no global DNS server configured, one of the compiled-in fallback DNS servers is used.</para>
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</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>Whether a link is the default route or not can be configured with
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<command>resolvectl default-route</command> command or <varname>DNSDefaultRoute=</varname> setting in
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<filename>.network</filename> files. If not configured explicitly, it is implicitly determined based on
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the configured DNS domains for a link: if there's a route-only domain other than <literal>~.</literal>,
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it defaults to false, otherwise to true.</para>
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<para>Effectively this means: in order to 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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configure the link as the default route and do not configure a <literal>~.</literal> route-only domain on
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it. Finally, in order to ensure that a specific link never receives any DNS traffic not matching any of
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its configured routing domains, make it not the default route.</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 <command>systemd-resolved</command> 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 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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<listitem><para>The <filename>nss-dns</filename> resolver maintains little state between subsequent DNS
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queries, and for each query always talks to the first listed DNS server from
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<filename>/etc/resolv.conf</filename> first, and on failure continues with the next until reaching the
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end of the list which is when the query fails. The resolver in
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<filename>systemd-resolved.service</filename> however maintains state, and will continuously talk to
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the same server for all queries on a particular lookup scope until some form of error is seen at which
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point it switches to the next, and then continuously stays with it for all queries on the scope until
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the next failure, and so on, eventually returning to the first configured server. This is done to
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optimize lookup times, in particular given that the resolver typically must first probe server feature
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sets when talking to a server, which is time consuming. This different behaviour implies that listed
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DNS servers per lookup scope must be equivalent in the zones they serve, so that sending a query to one
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of them will yield the same results as sending it to another configured DNS server.</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
|
||
<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 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>
|
||
|
||
<xi:include href="version-info.xml" xpointer="v231"/></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>
|
||
|
||
<xi:include href="version-info.xml" xpointer="v231"/></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>
|
||
|
||
<xi:include href="version-info.xml" xpointer="v235"/></listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term><constant>SIGHUP</constant></term>
|
||
|
||
<listitem><para>Upon reception of the <constant>SIGHUP</constant> process signal
|
||
<command>systemd-resolved</command> will flush all caches it maintains, drop all open TCP
|
||
connections (if any), and reload its configuration files.</para>
|
||
|
||
<xi:include href="version-info.xml" xpointer="v256"/></listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
</refsect1>
|
||
|
||
<refsect1>
|
||
<title>Credentials</title>
|
||
|
||
<para><command>systemd-resolved</command> supports the service credentials logic as implemented by
|
||
<varname>ImportCredential=</varname>/<varname>LoadCredential=</varname>/<varname>SetCredential=</varname>
|
||
(see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
|
||
details). The following credentials are used when passed in:</para>
|
||
|
||
<variablelist class='system-credentials'>
|
||
<varlistentry>
|
||
<term><varname>network.dns</varname></term>
|
||
<term><varname>network.search_domains</varname></term>
|
||
|
||
<listitem><para>May contain a space separated list of DNS server IP addresses and DNS search
|
||
domains. This information is only used when no explicit configuration via
|
||
<filename>/etc/systemd/resolved.conf</filename>, <filename>/etc/resolv.conf</filename> or the kernel
|
||
command line has been provided.</para>
|
||
|
||
<xi:include href="version-info.xml" xpointer="v253"/></listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
</refsect1>
|
||
|
||
<refsect1>
|
||
<title>Kernel Command Line</title>
|
||
|
||
<para><command>systemd-resolved</command> also honours two kernel command line options:</para>
|
||
|
||
<variablelist class='kernel-commandline-options'>
|
||
<varlistentry>
|
||
<term><varname>nameserver=</varname></term>
|
||
<term><varname>domain=</varname></term>
|
||
|
||
<listitem><para>Takes the IP address of a DNS server (in case of <varname>nameserver=</varname>), and
|
||
a DNS search domain (in case of <varname>domain=</varname>). May be used multiple times, to define
|
||
multiple DNS servers/search domains. If either of these options are specified
|
||
<filename>/etc/resolv.conf</filename> will not be read and the <varname>DNS=</varname> and
|
||
<varname>Domains=</varname> settings of
|
||
<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
||
will be ignored. These two kernel command line options hence override system
|
||
configuration.</para>
|
||
|
||
<xi:include href="version-info.xml" xpointer="v253"/></listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
</refsect1>
|
||
|
||
<refsect1>
|
||
<title>IP Ports</title>
|
||
|
||
<para>The <command>systemd-resolved</command> service listens on the following IP ports:</para>
|
||
|
||
<itemizedlist>
|
||
<listitem><para>Port 53 on IPv4 addresses 127.0.0.53 and 127.0.0.54 (both are on the local loopback
|
||
interface <literal>lo</literal>). This is the local DNS stub, as discussed above. Both UDP and TCP are
|
||
covered.</para></listitem>
|
||
|
||
<listitem><para>Port 5353 on all local addresses, both IPv4 and IPv6 (0.0.0.0 and ::0), for
|
||
MulticastDNS on UDP. Note that even though the socket is bound to all local interfaces via the selected
|
||
"wildcard" IP addresses, the incoming datagrams are filtered by the network interface they are coming
|
||
in on, and separate MulticastDNS link-local scopes are maintained for each, taking into consideration
|
||
whether MulticastDNS is enabled for the interface or not.</para></listitem>
|
||
|
||
<listitem><para>Port 5355 on all local addresses, both IPv4 and IP6 (0.0.0.0 and ::0), for LLMNR, on
|
||
both TCP and UDP. As with MulticastDNS filtering by incoming network interface is applied.</para></listitem>
|
||
</itemizedlist>
|
||
</refsect1>
|
||
|
||
<refsect1>
|
||
<title>See Also</title>
|
||
<para><simplelist type="inline">
|
||
<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
|
||
<member><citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
||
<member><citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
||
<member><citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
|
||
<member><citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
|
||
<member><citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
||
<member><citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
||
<member><citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
||
<member><citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
|
||
<member><citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
||
</simplelist></para>
|
||
</refsect1>
|
||
|
||
</refentry>
|