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systemd/man/resolved.conf.xml
Kiran Vemula 5ed91481ab resolved: added serve stale feature implementation of RFC 8767 
serve stale feature to keep the DNS resource records beyond TTL to return them as stale records in case of upstream server is not reachable or returns negative response.
SD_RESOLVED_NO_STALE flag has been added to disable serving stale records via dbus.
added serve stale test cases to TEST-75-RESOLVED
Fixes: #21815
2023-06-16 10:20:15 +01:00

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<?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" [
<!ENTITY % entities SYSTEM "custom-entities.ent" >
%entities;
]>
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="resolved.conf" conditional='ENABLE_RESOLVE'
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>resolved.conf</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>resolved.conf</refentrytitle>
<manvolnum>5</manvolnum>
</refmeta>
<refnamediv>
<refname>resolved.conf</refname>
<refname>resolved.conf.d</refname>
<refpurpose>Network Name Resolution configuration files</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>/etc/systemd/resolved.conf</filename></para>
<para><filename>/etc/systemd/resolved.conf.d/*.conf</filename></para>
<para><filename>/run/systemd/resolved.conf.d/*.conf</filename></para>
<para><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>These configuration files control local DNS and LLMNR
name resolution.</para>
</refsect1>
<xi:include href="standard-conf.xml" xpointer="main-conf" />
<refsect1>
<title>Options</title>
<para>The following options are available in the [Resolve] section:</para>
<variablelist class='network-directives'>
<varlistentry>
<term><varname>DNS=</varname></term>
<listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. Each address can
optionally take a port number separated with <literal>:</literal>, a network interface name or index separated with
<literal>%</literal>, and a Server Name Indication (SNI) separated with <literal>#</literal>. When IPv6 address is
specified with a port number, then the address must be in the square brackets. That is, the acceptable full formats
are <literal>111.222.333.444:9953%ifname#example.com</literal> for IPv4 and
<literal>[1111:2222::3333]:9953%ifname#example.com</literal> for IPv6. DNS requests are sent to one of the listed
DNS servers in parallel to suitable per-link DNS servers acquired from
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
set at runtime by external applications. For compatibility reasons, if this setting is not specified, the DNS
servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
are configured in it. This setting defaults to the empty list.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>FallbackDNS=</varname></term>
<listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Please see
<varname>DNS=</varname> for acceptable format of addresses. Any per-link DNS servers obtained from
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
<filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
known. If this option is not given, a compiled-in list of DNS servers is used instead.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>Domains=</varname></term>
<listitem><para>A space-separated list of domains, optionally prefixed with <literal>~</literal>,
used for two distinct purposes described below. Defaults to the empty list.</para>
<para>Any domains <emphasis>not</emphasis> prefixed with <literal>~</literal> are used as search
suffixes when resolving single-label hostnames (domain names which contain no dot), in order to
qualify them into fully-qualified domain names (FQDNs). These "search domains" are strictly processed
in the order they are specified in, until the name with the suffix appended is found. For
compatibility reasons, if this setting is not specified, the search domains listed in
<filename>/etc/resolv.conf</filename> with the <varname>search</varname> keyword are used instead, if
that file exists and any domains are configured in it.</para>
<para>The domains prefixed with <literal>~</literal> are called "route-only domains". All domains
listed here (<emphasis>both search domains and route-only domains</emphasis> after removing the
<literal>~</literal> prefix) define a search path that preferably directs DNS queries to this
interface. This search path has an effect only when suitable per-link DNS servers are known. Such
servers may be defined through the <varname>DNS=</varname> setting (see above) and dynamically at run
time, for example from DHCP leases. If no per-link DNS servers are known, route-only domains have no
effect.</para>
<para>Use the construct <literal>~.</literal> (which is composed from <literal>~</literal> to
indicate a route-only domain and <literal>.</literal> to indicate the DNS root domain that is the
implied suffix of all DNS domains) to use the DNS servers defined for this link preferably for all
domains.</para>
<para>See "Protocols and Routing" in
<citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for details of how search and route-only domains are used.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>LLMNR=</varname></term>
<listitem><para>Takes a boolean argument or
<literal>resolve</literal>. Controls Link-Local Multicast Name
Resolution support (<ulink
url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
the local host. If true, enables full LLMNR responder and
resolver support. If false, disables both. If set to
<literal>resolve</literal>, only resolution support is enabled,
but responding is disabled. Note that
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
also maintains per-link LLMNR settings. LLMNR will be
enabled on a link only if the per-link and the
global setting is on.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>MulticastDNS=</varname></term>
<listitem><para>Takes a boolean argument or
<literal>resolve</literal>. Controls Multicast DNS support (<ulink
url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
the local host. If true, enables full Multicast DNS responder and
resolver support. If false, disables both. If set to
<literal>resolve</literal>, only resolution support is enabled,
but responding is disabled. Note that
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
also maintains per-link Multicast DNS settings. Multicast DNS will be
enabled on a link only if the per-link and the
global setting is on.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>DNSSEC=</varname></term>
<listitem><para>Takes a boolean argument or
<literal>allow-downgrade</literal>. If true all DNS lookups are
DNSSEC-validated locally (excluding LLMNR and Multicast
DNS). If the response to a lookup request is detected to be invalid
a lookup failure is returned to applications. Note that
this mode requires a DNS server that supports DNSSEC. If the
DNS server does not properly support DNSSEC all validations
will fail. If set to <literal>allow-downgrade</literal> DNSSEC
validation is attempted, but if the server does not support
DNSSEC properly, DNSSEC mode is automatically disabled. Note
that this mode makes DNSSEC validation vulnerable to
"downgrade" attacks, where an attacker might be able to
trigger a downgrade to non-DNSSEC mode by synthesizing a DNS
response that suggests DNSSEC was not supported. If set to
false, DNS lookups are not DNSSEC validated.</para>
<para>Note that DNSSEC validation requires retrieval of
additional DNS data, and thus results in a small DNS look-up
time penalty.</para>
<para>DNSSEC requires knowledge of "trust anchors" to prove
data integrity. The trust anchor for the Internet root domain
is built into the resolver, additional trust anchors may be
defined with
<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
Trust anchors may change at regular intervals, and old trust
anchors may be revoked. In such a case DNSSEC validation is
not possible until new trust anchors are configured locally or
the resolver software package is updated with the new root
trust anchor. In effect, when the built-in trust anchor is
revoked and <varname>DNSSEC=</varname> is true, all further
lookups will fail, as it cannot be proved anymore whether
lookups are correctly signed, or validly unsigned. If
<varname>DNSSEC=</varname> is set to
<literal>allow-downgrade</literal> the resolver will
automatically turn off DNSSEC validation in such a case.</para>
<para>Client programs looking up DNS data will be informed
whether lookups could be verified using DNSSEC, or whether the
returned data could not be verified (either because the data
was found unsigned in the DNS, or the DNS server did not
support DNSSEC or no appropriate trust anchors were known). In
the latter case it is assumed that client programs employ a
secondary scheme to validate the returned DNS data, should
this be required.</para>
<para>It is recommended to set <varname>DNSSEC=</varname> to
true on systems where it is known that the DNS server supports
DNSSEC correctly, and where software or trust anchor updates
happen regularly. On other systems it is recommended to set
<varname>DNSSEC=</varname> to
<literal>allow-downgrade</literal>.</para>
<para>In addition to this global DNSSEC setting
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
also maintains per-link DNSSEC settings. For system DNS
servers (see above), only the global DNSSEC setting is in
effect. For per-link DNS servers the per-link
setting is in effect, unless it is unset in which case the
global setting is used instead.</para>
<para>Site-private DNS zones generally conflict with DNSSEC
operation, unless a negative (if the private zone is not
signed) or positive (if the private zone is signed) trust
anchor is configured for them. If
<literal>allow-downgrade</literal> mode is selected, it is
attempted to detect site-private DNS zones using top-level
domains (TLDs) that are not known by the DNS root server. This
logic does not work in all private zone setups.</para>
<para>Defaults to <literal>&DEFAULT_DNSSEC_MODE;</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>DNSOverTLS=</varname></term>
<listitem>
<para>Takes a boolean argument or <literal>opportunistic</literal>. If
true all connections to the server will be encrypted. Note that this
mode requires a DNS server that supports DNS-over-TLS and has a valid
certificate. If the hostname was specified in <varname>DNS=</varname>
by using the format <literal>address#server_name</literal> it
is used to validate its certificate and also to enable Server Name
Indication (SNI) when opening a TLS connection. Otherwise
the certificate is checked against the server's IP.
If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>
<para>When set to <literal>opportunistic</literal>
DNS request are attempted to send encrypted with DNS-over-TLS.
If the DNS server does not support TLS, DNS-over-TLS is disabled.
Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
attacks, where an attacker might be able to trigger a downgrade
to non-encrypted mode by synthesizing a response that suggests
DNS-over-TLS was not supported. If set to false, DNS lookups
are send over UDP.</para>
<para>Note that DNS-over-TLS requires additional data to be
send for setting up an encrypted connection, and thus results
in a small DNS look-up time penalty.</para>
<para>Note that in <literal>opportunistic</literal> mode the
resolver is not capable of authenticating the server, so it is
vulnerable to "man-in-the-middle" attacks.</para>
<para>In addition to this global <varname>DNSOverTLS=</varname> setting
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
also maintains per-link <varname>DNSOverTLS=</varname> settings. For system DNS servers (see above), only the global
<varname>DNSOverTLS=</varname> setting is in effect. For per-link DNS servers the per-link setting is in effect, unless
it is unset in which case the global setting is used instead.</para>
<para>Defaults to <literal>&DEFAULT_DNS_OVER_TLS_MODE;</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Cache=</varname></term>
<listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If
<literal>yes</literal> (the default), resolving a domain name which already got queried earlier will
return the previous result as long as it is still valid, and thus does not result in a new network
request. Be aware that turning off caching comes at a performance penalty, which is particularly high
when DNSSEC is used. If <literal>no-negative</literal>, only positive answers are cached.</para>
<para>Note that caching is turned off by default for host-local DNS servers.
See <varname>CacheFromLocalhost=</varname> for details.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>CacheFromLocalhost=</varname></term>
<listitem><para>Takes a boolean as argument. If <literal>no</literal> (the default), and response cames from
host-local IP address (such as 127.0.0.1 or ::1), the result wouldn't be cached in order to avoid
potential duplicate local caching.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>DNSStubListener=</varname></term>
<listitem><para>Takes a boolean argument or one of <literal>udp</literal> and
<literal>tcp</literal>. If <literal>udp</literal>, a DNS stub resolver will listen for UDP requests
on addresses 127.0.0.53 and 127.0.0.54, port 53. If <literal>tcp</literal>, the stub will listen for
TCP requests on the same addresses and port. If <literal>yes</literal> (the default), the stub listens
for both UDP and TCP requests. If <literal>no</literal>, the stub listener is disabled.</para>
<xi:include href="systemd-resolved.service.xml" xpointer="proxy-stub" />
<para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
in use.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>DNSStubListenerExtra=</varname></term>
<listitem><para>Takes an IPv4 or IPv6 address to listen on. The address may be optionally
prefixed with a protocol name (<literal>udp</literal> or <literal>tcp</literal>) separated with
<literal>:</literal>. If the protocol is not specified, the service will listen on both UDP and
TCP. It may be also optionally suffixed by a numeric port number with separator
<literal>:</literal>. When an IPv6 address is specified with a port number, then the address
must be in the square brackets. If the port is not specified, then the service uses port 53.
Note that this is independent of the primary DNS stub configured with
<varname>DNSStubListener=</varname>, and only configures <emphasis>additional</emphasis>
sockets to listen on. This option can be specified multiple times. If an empty string is
assigned, then the all previous assignments are cleared. Defaults to unset.</para>
<para>Examples:
<programlisting>DNSStubListenerExtra=192.168.10.10
DNSStubListenerExtra=2001:db8:0:f102::10
DNSStubListenerExtra=192.168.10.11:9953
DNSStubListenerExtra=[2001:db8:0:f102::11]:9953
DNSStubListenerExtra=tcp:192.168.10.12
DNSStubListenerExtra=udp:2001:db8:0:f102::12
DNSStubListenerExtra=tcp:192.168.10.13:9953
DNSStubListenerExtra=udp:[2001:db8:0:f102::13]:9953</programlisting>
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>ReadEtcHosts=</varname></term>
<listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default),
<command>systemd-resolved</command> will read <filename>/etc/hosts</filename>, and try to resolve
hosts or address by using the entries in the file before sending query to DNS servers.
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>ResolveUnicastSingleLabel=</varname></term>
<listitem><para>Takes a boolean argument. When false (the default),
<command>systemd-resolved</command> will not resolve A and AAAA queries for single-label names over
classic DNS. Note that such names may still be resolved if search domains are specified (see
<varname>Domains=</varname> above), or using other mechanisms, in particular via LLMNR or from
<filename>/etc/hosts</filename>. When true, queries for single-label names will be forwarded to
global DNS servers even if no search domains are defined.
</para>
<para>This option is provided for compatibility with configurations where <emphasis>public DNS
servers are not used</emphasis>. Forwarding single-label names to servers not under your control is
not standard-conformant, see <ulink
url="https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/">IAB
Statement</ulink>, and may create a privacy and security risk.</para></listitem>
</varlistentry>
<varlistentry>
<term>StaleRetentionSec=<replaceable>SECONDS</replaceable></term>
<listitem><para>Takes a duration value, which determines the length of time DNS resource records can be retained
in the cache beyond their Time To Live (TTL). This allows these records to be returned as stale records.
By default, this value is set to zero, meaning that DNS resource records are not stored in the cache after their TTL expires.</para>
<para>This is useful when a DNS server failure occurs or becomes unreachable.
In such cases, systemd-resolved continues to use the stale records to answer DNS queries, particularly when no valid response
can be obtained from the upstream DNS servers. However, this doesn't apply to NXDOMAIN responses, as those are still perfectly valid responses.
This feature enhances resilience against DNS infrastructure failures and outages.</para>
<para>systemd-resolved always attempts to reach the upstream DNS servers first, before providing the client application with any stale data.
If this feature is enabled, cache will not be flushed when changing servers.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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