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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"> |
0307f791 | 4 | <!-- SPDX-License-Identifier: LGPL-2.1+ --> |
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> | |
21 | <refpurpose>Provide hostname resolution for the locally | |
22 | configured system hostname.</refpurpose> | |
23 | </refnamediv> | |
24 | ||
25 | <refsynopsisdiv> | |
26 | <para><filename>libnss_myhostname.so.2</filename></para> | |
27 | </refsynopsisdiv> | |
28 | ||
29 | <refsect1> | |
30 | <title>Description</title> | |
31 | ||
9053aaad LP |
32 | <para><command>nss-myhostname</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of |
33 | the GNU C Library (<command>glibc</command>), primarily providing hostname resolution for the locally configured | |
34 | system hostname as returned by | |
35 | <citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>. The precise | |
36 | hostnames resolved by this module are:</para> | |
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37 | |
38 | <itemizedlist> | |
39 | <listitem><para>The local, configured hostname is resolved to | |
40 | all locally configured IP addresses ordered by their scope, or | |
41 | — if none are configured — the IPv4 address 127.0.0.2 (which | |
42 | is on the local loopback) and the IPv6 address ::1 (which is the | |
43 | local host).</para></listitem> | |
44 | ||
63003524 DH |
45 | <listitem><para>The hostnames <literal>localhost</literal> and |
46 | <literal>localhost.localdomain</literal> (as well as any hostname | |
47 | ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>) | |
48 | are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem> | |
798d3a52 | 49 | |
1dc92a06 | 50 | <listitem><para>The hostname <literal>_gateway</literal> is |
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51 | resolved to all current default routing gateway addresses, |
52 | ordered by their metric. This assigns a stable hostname to the | |
53 | current gateway, useful for referencing it independently of the | |
54 | current network configuration state.</para></listitem> | |
798d3a52 ZJS |
55 | </itemizedlist> |
56 | ||
57 | <para>Various software relies on an always-resolvable local | |
58 | hostname. When using dynamic hostnames, this is traditionally | |
59 | achieved by patching <filename>/etc/hosts</filename> at the same | |
60 | time as changing the hostname. This is problematic since it | |
61 | requires a writable <filename>/etc</filename> file system and is | |
62 | fragile because the file might be edited by the administrator at | |
b938cb90 | 63 | the same time. With <command>nss-myhostname</command> enabled, |
ff9b60f3 | 64 | changing <filename>/etc/hosts</filename> is unnecessary, and on |
b938cb90 | 65 | many systems, the file becomes entirely optional.</para> |
798d3a52 | 66 | |
9053aaad LP |
67 | <para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with |
68 | <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para> | |
798d3a52 | 69 | |
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70 | <para>It is recommended to place <literal>myhostname</literal> either between <literal>resolve</literal> |
71 | and "traditional" modules like <literal>files</literal> and <literal>dns</literal>, or after them. In the | |
72 | first version, well-known names like <literal>localhost</literal> and the machine hostname are given | |
73 | higher priority than the external configuration. This is recommended when the external DNS servers and | |
74 | network are not absolutely trusted. In the second version, external configuration is given higher | |
75 | priority and <command>nss-myhostname</command> only provides a fallback mechanism. This might be suitable | |
76 | in closely controlled networks, for example on a company LAN.</para> | |
798d3a52 ZJS |
77 | </refsect1> |
78 | ||
79 | <refsect1> | |
80 | <title>Example</title> | |
81 | ||
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82 | <para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables |
83 | <command>nss-myhostname</command> correctly:</para> | |
798d3a52 | 84 | |
94f760ec | 85 | <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf --> |
409093fe LP |
86 | <programlisting>passwd: compat mymachines systemd |
87 | group: compat mymachines systemd | |
0d6868f9 | 88 | shadow: compat |
798d3a52 | 89 | |
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90 | # Either (untrusted network): |
91 | hosts: mymachines resolve [!UNAVAIL=return] <command>myhostname</command> files dns | |
92 | # Or (only trusted networks): | |
44b7aedb | 93 | hosts: mymachines resolve [!UNAVAIL=return] files dns <command>myhostname</command> |
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94 | networks: files |
95 | ||
96 | protocols: db files | |
97 | services: db files | |
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98 | ethers: db files |
99 | rpc: db files | |
16eb4da9 | 100 | |
9fccdb0f | 101 | netgroup: nis</programlisting> |
16eb4da9 | 102 | |
798d3a52 | 103 | <para>To test, use <command>glibc</command>'s <command>getent</command> tool:</para> |
16eb4da9 | 104 | |
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105 | <programlisting>$ getent ahosts `hostname` |
106 | ::1 STREAM omega | |
107 | ::1 DGRAM | |
108 | ::1 RAW | |
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109 | 127.0.0.2 STREAM |
110 | 127.0.0.2 DGRAM | |
9fccdb0f | 111 | 127.0.0.2 RAW</programlisting> |
16eb4da9 | 112 | |
b938cb90 | 113 | <para>In this case, the local hostname is <varname>omega</varname>.</para> |
16eb4da9 | 114 | |
798d3a52 | 115 | </refsect1> |
16eb4da9 | 116 | |
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117 | <refsect1> |
118 | <title>See Also</title> | |
119 | <para> | |
120 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
409093fe | 121 | <citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
0d6868f9 | 122 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
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123 | <citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
124 | <citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
125 | <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
126 | </para> | |
127 | </refsect1> | |
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128 | |
129 | </refentry> |