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diff --git a/man/nss-myhostname.xml b/man/nss-myhostname.xml new file mode 100644 index 0000000..360e6d7 --- /dev/null +++ b/man/nss-myhostname.xml @@ -0,0 +1,139 @@ +<?xml version='1.0'?> <!--*-nxml-*--> +<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" + "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> +<!-- SPDX-License-Identifier: LGPL-2.1-or-later --> + +<refentry id="nss-myhostname" conditional='ENABLE_NSS_MYHOSTNAME'> + + <refentryinfo> + <title>nss-myhostname</title> + <productname>systemd</productname> + </refentryinfo> + + <refmeta> + <refentrytitle>nss-myhostname</refentrytitle> + <manvolnum>8</manvolnum> + </refmeta> + + <refnamediv> + <refname>nss-myhostname</refname> + <refname>libnss_myhostname.so.2</refname> + <refpurpose>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> + + <listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6 + addresses that are most likely used for communication with other hosts. This is determined by + requesting a routing decision to the configured default gateways from the kernel and then using the + local IP addresses selected by this decision. This hostname is only available if there is at least one + local default gateway configured. This assigns a stable hostname to the local outbound IP addresses, + useful for referencing them 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> after <literal>file</literal> and before <literal>dns</literal>. + This resolves well-known hostnames like <literal>localhost</literal> + and the machine hostnames locally. It is consistent with the behaviour + of <command>nss-resolve</command>, and still allows overriding via + <filename>/etc/hosts</filename>.</para> + + <para>Please keep in mind that <command>nss-myhostname</command> (and <command>nss-resolve</command>) also resolve + in the other direction — from locally attached IP addresses to + hostnames. If you rely on that lookup being provided by DNS, you might + want to order things differently. + </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> + + <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf --> +<programlisting>passwd: files systemd +group: files [SUCCESS=merge] systemd +shadow: files systemd +gshadow: files systemd + +hosts: mymachines resolve [!UNAVAIL=return] files <command>myhostname</command> dns +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 + <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry> + 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> |