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man: rework the explanation of Domains= 

Put more emphasis on the routing part. This is the more interesting
thing, and also more complicated and novel.

Explain "search domains" as the special case. Also explain the effect of
~. in more detail.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2016-10-02 14:32:44 +02:00
parent ba9fa3bc48
commit 2df225294f
1 changed files with 22 additions and 15 deletions
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37
man/systemd.network.xml
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@ -467,24 +467,31 @@
<varlistentry>
<term><varname>Domains=</varname></term>
<listitem>
<para>The domains used for DNS host name resolution on this link. Takes a list of DNS domain names which
are used as search suffixes for extending single-label host names (host names containing no dots) to become
fully qualified domain names (FQDNs). If a single-label host name is resolved on this interface, each of
the specified search domains are appended to it in turn, converting it into a fully qualified domain name,
until one of them may be successfully resolved.</para>
<para>A list of domains which should be resolved using the DNS servers on this link. Each item in the list
should be a domain name, optionally prefixed with a tilde (<literal>~</literal>). The domains with the
prefix are called "routing-only domains". The domains without the prefix are called "search domains" and
are first used as search suffixes for extending single-label host names (host names containing no dots) to
become fully qualified domain names (FQDNs). If a single-label host name is resolved on this interface,
each of the specified search domains are appended to it in turn, converting it into a fully qualified
domain name, until one of them may be successfully resolved.</para>
<para>The specified domains are also used for routing of DNS queries: look-ups for host names ending in the
domains specified here are preferably routed to the DNS servers configured for this interface. If a domain
name is prefixed with <literal>~</literal>, the domain name becomes a pure "routing" domain, the DNS server
is used for the given domain names only and is not used in the described domain search logic. By specifying a
routing domain of <literal>~.</literal> (the tilde indicating definition of a routing domain, the dot
referring to the DNS root domain which is the implied suffix of all valid DNS names) it is possible to
route all DNS traffic preferably to the DNS server specified for this interface. The route domain logic is
particularly useful on multi-homed hosts with DNS servers serving particular private DNS zones on each
interface.</para>
<para>Both "search" and "routing-only" domains are used for routing of DNS queries: look-ups for host names
ending in those domains (hence also single label names, if any "search domains" are listed), are routed to
the DNS servers configured for this interface. The domain routing logic is particularly useful on
multi-homed hosts with DNS servers serving particular private DNS zones on each interface.</para>
<para>The "routing-only" domain <literal>~.</literal> (the tilde indicating definition of a routing domain,
the dot referring to the DNS root domain which is the implied suffix of all valid DNS names) has special
effect. It causes all DNS traffic which does not match another configured domain routing entry to be routed
to DNS servers specified for this interface. This setting is useful to prefer a certain set of DNS servers
if a link on which they are connected is available.</para>
<para>This setting is read by
<citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
<citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
"Search domains" correspond to the <varname>domain</varname> and <varname>search</varname> entries in
<citerefentry><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
Domain name routing has no equivalent in the traditional glibc API, which has no concept of domain
name servers limited to a specific link.</para>
</listitem>
</varlistentry>
<varlistentry>