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man: document the order in which we talk to DNS servers

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Lennart Poettering 2023-10-18 18:14:00 +02:00 committed by Luca Boccassi
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commit 612a91c11a
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15
man/systemd-resolved.service.xml
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@ -244,7 +244,7 @@
<refsect1> <refsect1>
<title>Compatibility with the traditional glibc stub resolver</title> <title>Compatibility with the traditional glibc stub resolver</title>
<para>This section provides a short summary of differences in the stub resolver implemented by <para>This section provides a short summary of differences in the resolver implemented by
<citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together
with <command>systemd-resolved</command> and the traditional stub resolver implemented in with <command>systemd-resolved</command> and the traditional stub resolver implemented in
<filename>nss-dns</filename>.</para> <filename>nss-dns</filename>.</para>
@ -309,6 +309,19 @@ search foobar.com barbar.com
<varname>$RES_OPTIONS</varname> described in <varname>$RES_OPTIONS</varname> described in
<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
are not supported currently.</para></listitem> are not supported currently.</para></listitem>
<listitem><para>The <filename>nss-dns</filename> resolver maintains little state between subsequent DNS
queries, and for each query always talks to the first listed DNS server from
<filename>/etc/resolv.conf</filename> first, and on failure continues with the next until reaching the
end of the list which is when the query fails. The resolver in
<filename>systemd-resolved.service</filename> however maintains state, and will continuously talk to
the same server for all queries on a particular lookup scope until some form of error is seen at which
point it switches to the next, and then continuously stays with it for all queries on the scope until
the next failure, and so on, eventually returning to the first configured server. This is done to
optimize lookup times, in particular given that the resolver typically must first probe server feature
sets when talking to a server, which is time consuming. This different behaviour implies that listed
DNS servers per lookup scope must be equivalent in the zones they serve, so that sending a query to one
of them will yield the same results as sending it to another configured DNS server.</para></listitem>
</itemizedlist> </itemizedlist>
</refsect1> </refsect1>