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<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:id="sec-rename-ifs">
<title>Renaming network interfaces</title>
<para>
NixOS uses the udev
<link xlink:href="https://systemd.io/PREDICTABLE_INTERFACE_NAMES/">predictable naming scheme</link>
to assign names to network interfaces. This means that by default
cards are not given the traditional names like
<literal>eth0</literal> or <literal>eth1</literal>, whose order can
change unpredictably across reboots. Instead, relying on physical
locations and firmware information, the scheme produces names like
<literal>ens1</literal>, <literal>enp2s0</literal>, etc.
</para>
<para>
These names are predictable but less memorable and not necessarily
stable: for example installing new hardware or changing firmware
settings can result in a
<link xlink:href="https://github.com/systemd/systemd/issues/3715#issue-165347602">name change</link>.
If this is undesirable, for example if you have a single ethernet
card, you can revert to the traditional scheme by setting
<xref linkend="opt-networking.usePredictableInterfaceNames"/> to
<literal>false</literal>.
</para>
<section xml:id="sec-custom-ifnames">
<title>Assigning custom names</title>
<para>
In case there are multiple interfaces of the same type, it’s better to
assign custom names based on the device hardware address. For
example, we assign the name <literal>wan</literal> to the interface
with MAC address <literal>52:54:00:12:01:01</literal> using a
netword link unit:
</para>
<programlisting>
<link linkend="opt-systemd.network.links">systemd.network.links."10-wan"</link> = {
matchConfig.MACAddress = "52:54:00:12:01:01";
linkConfig.Name = "wan";
};
</programlisting>
<para>
Note that links are directly read by udev, <emphasis>not networkd</emphasis>,
and will work even if networkd is disabled.
</para>
<para>
Alternatively, we can use a plain old udev rule:
</para>
<programlisting>
<link linkend="opt-services.udev.initrdRules">services.udev.initrdRules</link> = ''
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", \
ATTR{address}=="52:54:00:12:01:01", KERNEL=="eth*", NAME="wan"
'';
</programlisting>
<warning><para>
The rule must be installed in the initrd using
<literal>services.udev.initrdRules</literal>, not the usual
<literal>services.udev.extraRules</literal> option. This is to avoid race
conditions with other programs controlling the interface.
</para></warning>
</section>
</section>
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