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man: document restrictions on naming interfaces

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Let's document that "." is a bad choice of character when naming
interfaces. Let's also document the hard restrictions we make when
naming interfaces.

Result of the mess that is #25052.

(cherry picked from commit 8f598a4635)
(cherry picked from commit d1066f33b5)
This commit is contained in:
Lennart Poettering 2022-10-19 11:38:11 +02:00 committed by Zbigniew Jędrzejewski-Szmek
parent bad202a76c
commit e797ec736d
1 changed files with 22 additions and 9 deletions
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31
man/systemd.link.xml
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@ -438,15 +438,22 @@
must either be unset, empty, disabled, or all policies configured there must fail. Also see the
example below with <literal>Name=dmz0</literal>.</para>
<para>Note that specifying a name that the kernel might use for another
interface (for example <literal>eth0</literal>) is dangerous because the
name assignment done by udev will race with the assignment done by the
kernel, and only one interface may use the name. Depending on the order of
operations, either udev or the kernel will win, making the naming
unpredictable. It is best to use some different prefix, for example
<literal>internal0</literal>/<literal>external0</literal> or
<literal>lan0</literal>/<literal>lan1</literal>/<literal>lan3</literal>.
</para>
<para>Note that specifying a name that the kernel might use for another interface (for example
<literal>eth0</literal>) is dangerous because the name assignment done by udev will race with the
assignment done by the kernel, and only one interface may use the name. Depending on the order of
operations, either udev or the kernel will win, making the naming unpredictable. It is best to use
some different prefix, for example <literal>internal0</literal>/<literal>external0</literal> or
<literal>lan0</literal>/<literal>lan1</literal>/<literal>lan3</literal>.</para>
<para>Interface names must have a minimum length of 1 character and a maximum length of 15
characters, and may contain any 7bit ASCII character, with the exception of control characters,
<literal>:</literal>, <literal>/</literal> and <literal>%</literal>. While <literal>.</literal> is
an allowed character, it's recommended to avoid it when naming interfaces as various tools (such as
<citerefentry><refentrytitle>resolvconf</refentrytitle><manvolnum>1</manvolnum></citerefentry>) use
it as separator character. Also, fully numeric interface names are not allowed (in order to avoid
ambiguity with interface specification by numeric indexes), as are the special strings
<literal>.</literal>, <literal>..</literal>, <literal>all</literal> and
<literal>default</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
@ -467,6 +474,12 @@
If the empty string is assigned to this option, the list is reset, and all prior assignments
have no effect. If the kernel does not support the alternative names, then this setting will
be ignored.</para>
<para>Alternative interface names may be used to identify interfaces in various tools. In contrast
to the primary name (as configured with <varname>Name=</varname> above) there may be multiple
alternative names referring to the same interface. Alternative names may have a maximum length of
127 characters, in contrast to the 15 allowed for the primary interface name, but otherwise are
subject to the same naming constraints.</para>
</listitem>
</varlistentry>
<varlistentry>