systemd/man/nss-myhostname.xml
Lennart Poettering 75555c2824 man: sync up the suggested nsswitch.conf configuration for our four NSS modules
This unifies the suggested nsswitch.conf configuration for our four NSS modules to this:

    hosts: files mymachines resolve [!UNAVAIL=return] dns myhostname

Note that this restores "myhostname" to the suggested configuration of
nss-resolve for the time being, undoing 4484e1792b.

"myhostname" should probably be dropped eventually, but when we do this we
should do it in full, and not only drop it from the suggested nsswitch.conf
for one of the modules, but also drop it in source and stop referring to it
altogether.

Note that nss-resolve doesn't replace nss-myhostname in full: the former only
works if D-Bus/resolved is available for resolving the local hostname, the
latter works in all cases even if D-Bus or resolved are not in operation, hence
there's some value in keeping the line as it is right now. Note that neither
dns nor myhostname are considered at all with the above configuration unless
the resolve module actually returns UNAVAIL. Thus, even though handling of
local hostname resolving is implemented twice this way it is only executed once
for each lookup.
2016-10-24 19:04:43 +02:00

150 lines
5.8 KiB
XML

<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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Copyright 2011 Lennart Poettering
Copyright 2013 Tom Gundersen
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<refentry id="nss-myhostname" conditional='HAVE_MYHOSTNAME'>
<refentryinfo>
<title>nss-myhostname</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>nss-myhostname</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>nss-myhostname</refname>
<refname>libnss_myhostname.so.2</refname>
<refpurpose>Provide hostname resolution for the locally
configured system hostname.</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>libnss_myhostname.so.2</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><command>nss-myhostname</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of
the GNU C Library (<command>glibc</command>), primarily providing hostname resolution for the locally configured
system hostname as returned by
<citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>. The precise
hostnames resolved by this module are:</para>
<itemizedlist>
<listitem><para>The local, configured hostname is resolved to
all locally configured IP addresses ordered by their scope, or
— if none are configured — the IPv4 address 127.0.0.2 (which
is on the local loopback) and the IPv6 address ::1 (which is the
local host).</para></listitem>
<listitem><para>The hostnames <literal>localhost</literal> and
<literal>localhost.localdomain</literal> (as well as any hostname
ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
<listitem><para>The hostname <literal>gateway</literal> is
resolved to all current default routing gateway addresses,
ordered by their metric. This assigns a stable hostname to the
current gateway, useful for referencing it independently of the
current network configuration state.</para></listitem>
</itemizedlist>
<para>Various software relies on an always-resolvable local
hostname. When using dynamic hostnames, this is traditionally
achieved by patching <filename>/etc/hosts</filename> at the same
time as changing the hostname. This is problematic since it
requires a writable <filename>/etc</filename> file system and is
fragile because the file might be edited by the administrator at
the same time. With <command>nss-myhostname</command> enabled,
changing <filename>/etc/hosts</filename> is unnecessary, and on
many systems, the file becomes entirely optional.</para>
<para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with
<literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>
<para>It is recommended to place <literal>myhostname</literal> last in the <filename>nsswitch.conf</filename>'
<literal>hosts:</literal> line to make sure that this mapping is only used as fallback, and that any DNS or
<filename>/etc/hosts</filename> based mapping takes precedence.</para>
</refsect1>
<refsect1>
<title>Example</title>
<para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables
<command>nss-myhostname</command> correctly:</para>
<programlisting>passwd: compat mymachines systemd
group: compat mymachines systemd
shadow: compat
hosts: files mymachines resolve [!UNAVAIL=return] dns <command>myhostname</command>
networks: files
protocols: db files
services: db files
ethers: db files
rpc: db files
netgroup: nis</programlisting>
<para>To test, use <command>glibc</command>'s <command>getent</command> tool:</para>
<programlisting>$ getent ahosts `hostname`
::1 STREAM omega
::1 DGRAM
::1 RAW
127.0.0.2 STREAM
127.0.0.2 DGRAM
127.0.0.2 RAW</programlisting>
<para>In this case, the local hostname is <varname>omega</varname>.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>