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man: add footnote explaining why "localhost" query is sent out to the network

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I tried to make the explanation brief, but this isn't so easy. It seems better
to push this out to a footnote instead of the main text.

Fixes #16584.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2021-02-28 17:36:00 +01:00
parent 9273d8e6c9
commit 32f82cccea
1 changed files with 20 additions and 5 deletions
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25
man/systemd-resolved.service.xml
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@ -115,8 +115,8 @@
<refsect1>
<title>Protocols and Routing</title>
<para>Lookup requests are routed to the available DNS servers, LLMNR, and MulticastDNS interfaces
according to the following rules:</para>
<para>The lookup requests that <filename>systemd-resolved.service</filename> receives are routed to the
available DNS servers, LLMNR, and MulticastDNS interfaces according to the following rules:</para>
<itemizedlist>
<listitem><para>Names for which synthetic records are generated (the local hostname,
@ -172,7 +172,8 @@
<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a
description of globally configured DNS settings.</para>
<para>The following query routing logic applies for unicast DNS traffic:</para>
<para>The following query routing logic applies for unicast DNS lookups initiated by
<filename>systemd-resolved.service</filename>:</para>
<itemizedlist>
<listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the
@ -228,8 +229,22 @@
<itemizedlist>
<listitem><para>Some names are always resolved internally (see Synthetic Records above). Traditionally
they would be resolved by <filename>nss-files</filename>, and only if provided in
<filename>/etc/hosts</filename>.</para></listitem>
they would be resolved by <filename>nss-files</filename> if provided in
<filename>/etc/hosts</filename>. But note that the details of how a query is constructed are under the
control of the client library. <filename>nss-dns</filename> will first try to resolve names using
search domains and even if those queries are routed to <filename>systemd-resolved</filename>, it will
send them out over the network using the usual rules for multi-label name routing <footnote><para>For
example, if <filename>/etc/nsswitch.conf</filename> has <programlisting>nameserver 127.0.0.53
search foobar.com barbar.com
</programlisting>and we look up <literal>localhost</literal>, <filename>nss-dns</filename> will send
the following queries to <filename>systemd-resolved</filename> listening on 127.0.0.53:53: first
<literal>localhost.foobar.com</literal>, then <literal>localhost.barbar.com</literal>, and finally
<literal>localhost</literal>. If (hopefully) the first two queries fail,
<filename>systemd-resolved</filename> will synthesize an answer for the third query.</para>
<para>When using <filename>nss-dns</filename> with any search domains, it is thus crucial to always
configure <filename>nss-files</filename> with higher priority and provide mappings for names that
should not be resolved using search domains.</para></footnote>.</para></listitem>
<listitem><para>Single-label names are not resolved for A and AAAA records using unicast DNS (unless
overridden with <varname>ResolveUnicastSingleLabel=</varname>, see