man: document new systemd-resolved.service(8) routing features in more detail

1 files changed, 36 insertions, 7 deletions

diff --git a/man/systemd-resolved.service.xml b/man/systemd-resolved.service.xml
index 3d7bb52d6..d7334e01e 100644
--- a/man/systemd-resolved.service.xml
+++ b/man/systemd-resolved.service.xml
@@ -143,16 +143,45 @@
     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. See
-    <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-    for details. Lookups for a hostname ending in one of the
-    per-interface domains are exclusively routed to the matching
-    interfaces.</para>
+    <para>Routing of lookups may be influenced by configuring per-interface domain names and other settings. 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>
+
+    <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 (or the globally configured DNS settings), the "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></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 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>Otherwise the query is failed as no suitable DNS servers could be determined.</para></listitem>
+    </itemizedlist>
+
+    <para>The "DNS default route" option is a boolean setting configureable 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>Effectively this means: 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 the queries (unless they too carry such a
+    route-only domain). In order to route all such DNS queries to a specific link only in case no other link is
+    preferable, then set the "DNS default route" 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>
 
     <para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
     Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
-
   </refsect1>
 
   <refsect1>