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16eb4da9 | 1 | <?xml version='1.0'?> <!--*-nxml-*--> |
3a54a157 | 2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
12b42c76 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
db9ecf05 | 4 | <!-- SPDX-License-Identifier: LGPL-2.1-or-later --> |
16eb4da9 | 5 | |
08540a95 | 6 | <refentry id="nss-myhostname" conditional='ENABLE_NSS_MYHOSTNAME'> |
16eb4da9 | 7 | |
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8 | <refentryinfo> |
9 | <title>nss-myhostname</title> | |
10 | <productname>systemd</productname> | |
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11 | </refentryinfo> |
12 | ||
13 | <refmeta> | |
14 | <refentrytitle>nss-myhostname</refentrytitle> | |
15 | <manvolnum>8</manvolnum> | |
16 | </refmeta> | |
17 | ||
18 | <refnamediv> | |
19 | <refname>nss-myhostname</refname> | |
20 | <refname>libnss_myhostname.so.2</refname> | |
e9dd6984 | 21 | <refpurpose>Hostname resolution for the locally configured system hostname</refpurpose> |
798d3a52 ZJS |
22 | </refnamediv> |
23 | ||
24 | <refsynopsisdiv> | |
25 | <para><filename>libnss_myhostname.so.2</filename></para> | |
26 | </refsynopsisdiv> | |
27 | ||
28 | <refsect1> | |
29 | <title>Description</title> | |
30 | ||
9053aaad LP |
31 | <para><command>nss-myhostname</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of |
32 | the GNU C Library (<command>glibc</command>), primarily providing hostname resolution for the locally configured | |
33 | system hostname as returned by | |
34 | <citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>. The precise | |
35 | hostnames resolved by this module are:</para> | |
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36 | |
37 | <itemizedlist> | |
38 | <listitem><para>The local, configured hostname is resolved to | |
39 | all locally configured IP addresses ordered by their scope, or | |
40 | — if none are configured — the IPv4 address 127.0.0.2 (which | |
41 | is on the local loopback) and the IPv6 address ::1 (which is the | |
42 | local host).</para></listitem> | |
43 | ||
63003524 DH |
44 | <listitem><para>The hostnames <literal>localhost</literal> and |
45 | <literal>localhost.localdomain</literal> (as well as any hostname | |
46 | ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>) | |
47 | are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem> | |
798d3a52 | 48 | |
1dc92a06 | 49 | <listitem><para>The hostname <literal>_gateway</literal> is |
798d3a52 ZJS |
50 | resolved to all current default routing gateway addresses, |
51 | ordered by their metric. This assigns a stable hostname to the | |
52 | current gateway, useful for referencing it independently of the | |
53 | current network configuration state.</para></listitem> | |
2f166bb7 LP |
54 | |
55 | <listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6 | |
56 | addresses that are most likely used for communication with other hosts. This is determined by | |
57 | requesting a routing decision to the configured default gateways from the kernel and then using the | |
58 | local IP addresses selected by this decision. This hostname is only available if there is at least one | |
59 | local default gateway configured. This assigns a stable hostname to the local outbound IP addresses, | |
60 | useful for referencing them independently of the current network configuration state.</para></listitem> | |
798d3a52 ZJS |
61 | </itemizedlist> |
62 | ||
63 | <para>Various software relies on an always-resolvable local | |
64 | hostname. When using dynamic hostnames, this is traditionally | |
65 | achieved by patching <filename>/etc/hosts</filename> at the same | |
66 | time as changing the hostname. This is problematic since it | |
3b121157 | 67 | requires a writable <filename>/etc/</filename> file system and is |
798d3a52 | 68 | fragile because the file might be edited by the administrator at |
b938cb90 | 69 | the same time. With <command>nss-myhostname</command> enabled, |
ff9b60f3 | 70 | changing <filename>/etc/hosts</filename> is unnecessary, and on |
b938cb90 | 71 | many systems, the file becomes entirely optional.</para> |
798d3a52 | 72 | |
9053aaad LP |
73 | <para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with |
74 | <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para> | |
798d3a52 | 75 | |
ce266330 FK |
76 | <para>It is recommended to place <literal>myhostname</literal> after <literal>file</literal> and before <literal>dns</literal>. |
77 | This resolves well-known hostnames like <literal>localhost</literal> | |
78 | and the machine hostnames locally. It is consistent with the behaviour | |
79 | of <command>nss-resolve</command>, and still allows overriding via | |
946f7ce3 FK |
80 | <filename>/etc/hosts</filename>.</para> |
81 | ||
82 | <para>Please keep in mind that <command>nss-myhostname</command> (and <command>nss-resolve</command>) also resolve | |
bdbb61f6 | 83 | in the other direction — from locally attached IP addresses to |
946f7ce3 FK |
84 | hostnames. If you rely on that lookup being provided by DNS, you might |
85 | want to order things differently. | |
ce266330 | 86 | </para> |
798d3a52 ZJS |
87 | </refsect1> |
88 | ||
89 | <refsect1> | |
90 | <title>Example</title> | |
91 | ||
9053aaad LP |
92 | <para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables |
93 | <command>nss-myhostname</command> correctly:</para> | |
798d3a52 | 94 | |
94f760ec | 95 | <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf --> |
02e93087 LP |
96 | <programlisting>passwd: files systemd |
97 | group: files [SUCCESS=merge] systemd | |
98 | shadow: files systemd | |
f43a19ec LP |
99 | gshadow: files systemd |
100 | ||
d296c20f | 101 | hosts: mymachines resolve [!UNAVAIL=return] files <command>myhostname</command> dns |
16eb4da9 TG |
102 | networks: files |
103 | ||
104 | protocols: db files | |
105 | services: db files | |
0d6868f9 LP |
106 | ethers: db files |
107 | rpc: db files | |
16eb4da9 | 108 | |
9fccdb0f | 109 | netgroup: nis</programlisting> |
16eb4da9 | 110 | |
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111 | <para>To test, use <command>glibc</command>'s |
112 | <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
113 | tool:</para> | |
16eb4da9 | 114 | |
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115 | <programlisting>$ getent ahosts `hostname` |
116 | ::1 STREAM omega | |
117 | ::1 DGRAM | |
118 | ::1 RAW | |
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119 | 127.0.0.2 STREAM |
120 | 127.0.0.2 DGRAM | |
9fccdb0f | 121 | 127.0.0.2 RAW</programlisting> |
16eb4da9 | 122 | |
b938cb90 | 123 | <para>In this case, the local hostname is <varname>omega</varname>.</para> |
16eb4da9 | 124 | |
798d3a52 | 125 | </refsect1> |
16eb4da9 | 126 | |
798d3a52 ZJS |
127 | <refsect1> |
128 | <title>See Also</title> | |
13a69c12 DT |
129 | <para><simplelist type="inline"> |
130 | <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member> | |
131 | <member><citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry></member> | |
132 | <member><citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry></member> | |
133 | <member><citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry></member> | |
134 | <member><citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member> | |
135 | <member><citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry></member> | |
136 | </simplelist></para> | |
798d3a52 | 137 | </refsect1> |
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138 | |
139 | </refentry> |