networkd: optinally use DHCP lease domain info for routing only

This changes the UseDomains= setting of .network files to take an optional third value "route", in addition to the
boolean values. If set, the passed domain information is used for routing rules only, but not for the search path
logic.

1 files changed, 14 insertions, 9 deletions

diff --git a/man/systemd.network.xml b/man/systemd.network.xml
index be88d66072..f88751b672 100644
--- a/man/systemd.network.xml
+++ b/man/systemd.network.xml
@@ -717,15 +717,20 @@
         <varlistentry>
           <term><varname>UseDomains=</varname></term>
           <listitem>
-            <para>When true (not the default), the domain name
-            received from the DHCP server will be used for DNS
-            resolution over this link. When a name cannot be resolved
-            as specified, the domain name will be used a suffix and
-            name resolution of that will be attempted.</para>
-
-            <para>This corresponds to the <option>domain</option>
-            option in <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-            and should not be enabled on untrusted networks.</para>
+            <para>Takes a boolean argument, or a the special value <literal>route</literal>. When true, the domain name
+            received from the DHCP server will be used as DNS search domain over this link, similar to the effect of
+            the <option>Domains=</option> setting. If set to <literal>route</literal>, the domain name received from
+            the DHCP server will be used for routing DNS queries only, but not for searching, similar to the effect of
+            the <option>Domains=</option> setting when the argument is prefixed with <literal>~</literal>. Defaults to
+            false.</para>
+
+            <para>It is recommended to enable this option only on trusted networks, as setting this affects resolution
+            of all host names, in particular to single-label names. It is generally safer to use the supplied domain
+            only as routing domain, rather than as search domain, in order to not have it affect local resolution of
+            single-label names.</para>
+
+            <para>When set to true, this setting corresponds to the <option>domain</option> option in <citerefentry
+            project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
           </listitem>
         </varlistentry>
         <varlistentry>