<refsect1>
<title>Synthetic Records</title>
- <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following
+ <para><command>systemd-resolved</command> synthetizes DNS resource records (RRs) for the following
cases:</para>
<itemizedlist>
<listitem><para>The local, configured hostname is resolved to all locally configured IP addresses
ordered by their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the local
- loopback) and the IPv6 address ::1 (which is the local host).</para></listitem>
+ loopback interface) and the IPv6 address ::1 (which is the local host).</para></listitem>
<listitem><para>The hostnames <literal>localhost</literal> and <literal>localhost.localdomain</literal>
- (as well as any hostname ending in <literal>.localhost</literal> or
- <literal>.localhost.localdomain</literal>) are resolved to the IP addresses 127.0.0.1 and ::1.
+ as well as any hostname ending in <literal>.localhost</literal> or
+ <literal>.localhost.localdomain</literal> are resolved to the IP addresses 127.0.0.1 and ::1.
</para></listitem>
<listitem><para>The hostname <literal>_gateway</literal> is resolved to all current default routing
according to the following rules:</para>
<itemizedlist>
- <listitem><para>Names for which synthetic records are generated (as listed in the previous section) are
- never routed to the network and a reply is sent immediately. In particular this means that lookups for
- <literal>localhost</literal> are never routed to the network.</para></listitem>
-
- <listitem><para>Single-label names are routed to all local interfaces capable of IP multicasting, where
- LLMNR is not disabled, using the LLMNR protocol. Lookups for IPv4 addresses are only sent via LLMNR on
- IPv4, and lookups for IPv6 addresses are only sent via LLMNR on IPv6. Lookups for the locally
- configured hostname and the <literal>_gateway</literal> hostname are never routed to LLMNR.
- </para></listitem>
-
- <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are routed to all
- local interfaces capable of IP multicasting, where MulticastDNS is not disabled, using the MulticastDNS
- protocol. As with LLMNR, IPv4 address lookups are sent via IPv4 and IPv6 address lookups are sent via
- IPv6.</para></listitem>
-
- <listitem><para>Resolution of address records (A and AAAA) via unicast DNS (i.e. not LLMNR or
- MulticastDNS) for non-synthesized single-label names is allowed for non-top-level domains. This means
- that such records can be resolved when search domains are defined. For any interface which defines
- search domains, such look-ups are routed to that interface, suffixed with each of the search domains
- defined on that interface in turn. When global search domains are defined, such look-ups are routed to
- all interfaces, suffixed by each of the global search domains in turn. Additionally, lookup of
- single-label names via unicast DNS may be enabled with the
- <varname>ResolveUnicastSingleLabel=yes</varname> setting. The details of which servers are queried and
- how the final reply is chosen are described below. Note that this means that address queries for
- single-label names are never sent out to remote DNS servers by default, and if no search domains are
- defined, resolution will fail.</para></listitem>
-
- <listitem><para>Other multi-label names are routed to all local interfaces that have a DNS server
- configured, plus the globally configured DNS servers if there are any. Note that by default, lookups for
- domains with the <literal>.local</literal> suffix are not routed to DNS servers, unless the domain is
- specified explicitly as routing or search domain for the DNS server and interface. This means that on
- networks where the <literal>.local</literal> domain is defined in a site-specific DNS server, explicit
- search or routing domains need to be configured to make lookups within this DNS domain work. Note that
- these days, it's generally recommended to avoid defining <literal>.local</literal> in a DNS server, as
- <ulink url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
+ <listitem><para>Names for which synthetic records are generated (the local hostname,
+ <literal>localhost</literal> and <literal>localdomain</literal>, local gateway, as listed in the
+ previous section) and addresses configured in <filename>/etc/hosts</filename> are never routed to the
+ network and a reply is sent immediately.</para></listitem>
+
+ <listitem><para>Single-label names are resolved using LLMNR on all local interfaces where LLMNR is
+ enabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are
+ only sent via LLMNR on IPv6. Note that lookups for single-label synthetized names are not routed to
+ LLMNR, MulticastDNS or unicast DNS.</para></listitem>
+
+ <listitem><para>Queries for the address records (A and AAAA) of single-label non-synthetized names are
+ resolved via unicast DNS using search domains. For any interface which defines search domains, such
+ look-ups are routed to that interface, suffixed with each of the search domains defined on that
+ interface in turn. When global search domains are defined, such look-ups are routed to all interfaces,
+ suffixed by each of the global search domains in turn. Additionally, lookup of single-label names via
+ unicast DNS may be enabled with the <varname>ResolveUnicastSingleLabel=yes</varname> setting. The
+ details of which servers are queried and how the final reply is chosen are described below. Note that
+ this means that address queries for single-label names are never sent out to remote DNS servers by
+ default, and resoulution is only possible if search domains are defined.</para></listitem>
+
+ <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are resolved using
+ MulticastDNS on all local interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups
+ are sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem>
+
+ <listitem><para>Queries for multi-label names are routed via unicast DNS on local interfaces that have
+ a DNS server configured, plus the globally configured DNS servers if there are any. Which interfaces
+ are used is determined by the routing logic based on search and route-only domains, described below.
+ Note that by default, lookups for domains with the <literal>.local</literal> suffix are not routed to
+ DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server
+ and interface. This means that on networks where the <literal>.local</literal> domain is defined in a
+ site-specific DNS server, explicit search or routing domains need to be configured to make lookups work
+ within this DNS domain. Note that these days, it's generally recommended to avoid defining
+ <literal>.local</literal> in a DNS server, as <ulink
+ url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
MulticastDNS use.</para></listitem>
- <listitem><para>Address lookups are routed similarly to multi-label names, with the exception that
- addresses from the link-local address range are never routed to unicast DNS and are only resolved using
- LLMNR and MulticastDNS (when enabled).</para></listitem>
+ <listitem><para>Address lookups (reverse lookups) are routed similarly to multi-label names, with the
+ exception that addresses from the link-local address range are never routed to unicast DNS and are only
+ resolved using LLMNR and MulticastDNS (when enabled).</para></listitem>
</itemizedlist>
<para>If lookups are routed to multiple interfaces, the first successful response is returned (thus
effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces,
the last failing response is returned.</para>
- <para>Routing of lookups may be influenced by configuring per-interface domain names and other
- settings. See
+ <para>Routing of lookups is determined by the per-interface routing domains (search and route-only) and
+ global search domains. See
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
- <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
- details. The following query routing logic applies for unicast DNS traffic:</para>
+ <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for a
+ description how those settings are set dynamically and the discussion of <varname>Domains=</varname> in
+ <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a
+ description of globally configured DNS settings.</para>
+
+ <para>The following query routing logic applies for unicast DNS traffic:</para>
<itemizedlist>
<listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the
- configured search or route-only domains of any link (see
- <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
- or the globally configured DNS settings (see the discussion of <varname>Domains=</varname> in
- <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
- "best matching" search/route-only domain is determined: the matching one with the most labels. The
- query is then sent to all DNS servers of any links or the globally configured DNS servers associated
- with this "best matching" search/route-only domain. (Note that more than one link might have this same
- "best matching" search/route-only domain configured, in which case the query is sent to all of them in
- parallel).</para>
+ configured routing domains (search or route-only) of any link, or the globally configured DNS settings,
+ "best matching" routing domain is determined: the matching one with the most labels. The query is then
+ sent to all DNS servers of any links or the globally configured DNS servers associated with this "best
+ matching" routing domain. (Note that more than one link might have this same "best matching" routing
+ domain configured, in which case the query is sent to all of them in parallel).</para>
<para>In case of single-label names, when search domains are defined, the same logic applies, except
- that the name is first suffixed by the search domain.</para></listitem>
+ that the name is first suffixed by each of the search domains in turn.</para></listitem>
- <listitem><para>If a query does not match any configured search/route-only domain (neither per-link nor
- global), it is sent to all DNS servers that are configured on links with the "DNS default route" option
- set, as well as the globally configured DNS server.</para></listitem>
+ <listitem><para>If a query does not match any configured routing domain (either per-link or global), it
+ is sent to all DNS servers that are configured on links with the <varname>DefaultRoute=</varname>
+ option set, as well as the globally configured DNS server.</para></listitem>
- <listitem><para>If there is no link configured as "DNS default route" and no global DNS server
- configured, the compiled-in fallback DNS server is used.</para></listitem>
+ <listitem><para>If there is no link configured as <varname>DefaultRoute=</varname> and no global DNS
+ server configured, one of the compiled-in fallback DNS servers is used.</para></listitem>
- <listitem><para>Otherwise the query is failed as no suitable DNS servers could be determined.
+ <listitem><para>Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined.
</para></listitem>
</itemizedlist>
- <para>The "DNS default route" option is a boolean setting configurable with <command>resolvectl</command>
- or in <filename>.network</filename> files. If not set, it is implicitly determined based on the
- configured DNS domains for a link: if there's any route-only domain (not matching <literal>~.</literal>)
- it defaults to false, otherwise to true.</para>
+ <para>The <varname>DefaultRoute=</varname> option is a boolean setting configurable with
+ <command>resolvectl</command> or in <filename>.network</filename> files. If not set, it is implicitly
+ determined based on the configured DNS domains for a link: if there's a route-only domain other than
+ <literal>~.</literal>, it defaults to false, otherwise to true.</para>
<para>Effectively this means: in order to support single-label non-synthetized names, define appropriate
- search domains. In order to preferably route all DNS queries not explicitly matched by search/route-only
- domain configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This
- will ensure that other links will not be considered for these queries (unless they too carry such a
- route-only domain). In order to route all such DNS queries to a specific link only if no other link
- is preferable, set the "DNS default route" option for the link to true and do not configure a
+ search domains. In order to preferably route all DNS queries not explicitly matched by routing domain
+ configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This will
+ ensure that other links will not be considered for these queries (unless they too carry such a routing
+ domain). In order to route all such DNS queries to a specific link only if no other link is preferred,
+ set the <varname>DefaultRoute=</varname> option for the link to true and do not configure a
<literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never
- receives any DNS traffic not matching any of its configured search/route-only domains, set the "DNS
- default route" option for it to false.</para>
+ receives any DNS traffic not matching any of its configured routing domains, set the
+ <varname>DefaultRoute=</varname> option for it to false.</para>
<para>See
<citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
projects / thirdparty / systemd.git / commitdiff
