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