resolved: add support for explicitly forgetting everything we learnt about DNS server feature levels

This adds "systemd-resolve --reset-server-features" for explicitly
forgetting what we learnt. This might be useful for debugging
purposes, and to force systemd-resolved to restart its learning logic
for all DNS servers.

1 files changed, 24 insertions, 6 deletions

diff --git a/man/systemd-resolved.service.xml b/man/systemd-resolved.service.xml
index fd717e4963..1ad9500d78 100644
--- a/man/systemd-resolved.service.xml
+++ b/man/systemd-resolved.service.xml
@@ -202,19 +202,37 @@
       <varlistentry>
         <term><constant>SIGUSR1</constant></term>
 
-        <listitem><para>Upon reception of the SIGUSR1 process signal <command>systemd-resolved</command> will dump the
-        contents of all DNS resource record caches it maintains into the system logs.</para></listitem>
+        <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 into
+        the system logs.</para></listitem>
       </varlistentry>
 
       <varlistentry>
         <term><constant>SIGUSR2</constant></term>
 
-        <listitem><para>Upon reception of the SIGUSR2 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.</para></listitem>
+        <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>systemd-resolve --flush-caches</command>
+        command, however the latter is recommended since it operates in a synchronous way.</para></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>systemd-resolve --reset-server-features</command> command, however the latter is recommended since it
+        operates in a synchronous way.</para></listitem>
       </varlistentry>
     </variablelist>
+
   </refsect1>
 
   <refsect1>