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Linux From Scratch - Version 7.0

197

With Udev and modular network drivers, the network interface numbering is not persistent across reboots by default,

because the drivers are loaded in parallel and, thus, in random order. For example, on a computer having two network

cards made by Intel and Realtek, the network card manufactured by Intel may become eth0 and the Realtek card

becomes eth1. In some cases, after a reboot the cards get renumbered the other way around. To avoid this, Udev

comes with a script and some rules to assign stable names to network cards based on their MAC address.

Pre-generate the rules to ensure the same names get assigned to the same devices at every boot, including the first:

for NIC in /sys/class/net/* ; do

INTERFACE=${NIC##*/} udevadm test --action=add $NIC

done

Now, inspect the /etc/udev/rules.d/70-persistent-net.rules file, to find out which name was

assigned to which network device:

cat /etc/udev/rules.d/70-persistent-net.rules

The file begins with a comment block followed by two lines for each NIC. The first line for each NIC is a commented

description showing its hardware IDs (e.g. its PCI vendor and device IDs, if it's a PCI card), along with its driver

in parentheses, if the driver can be found. Neither the hardware ID nor the driver is used to determine which name

to give an interface; this information is only for reference. The second line is the Udev rule that matches this NIC

and actually assigns it a name.

All Udev rules are made up of several keys, separated by commas and optional whitespace. This rule's keys and an

explanation of each of them are as follows:

• SUBSYSTEM=="net" - This tells Udev to ignore devices that are not network cards.

• ACTION=="add" - This tells Udev to ignore this rule for a uevent that isn't an add ("remove" and "change"

uevents also happen, but don't need to rename network interfaces).

• DRIVERS=="?*" - This exists so that Udev will ignore VLAN or bridge sub-interfaces (because these sub-

interfaces do not have drivers). These sub-interfaces are skipped because the name that would be assigned would

collide with their parent devices.

• ATTR{address} - The value of this key is the NIC's MAC address.

• ATTR{type}=="1" - This ensures the rule only matches the primary interface in the case of certain wireless

drivers, which create multiple virtual interfaces. The secondary interfaces are skipped for the same reason that

VLAN and bridge sub-interfaces are skipped: there would be a name collision otherwise.

• KERNEL=="eth*" - This key was added to the Udev rule generator to handle machines that have multiple

network interfaces, all with the same MAC address (the PS3 is one such machine). If the independent interfaces

have different basenames, this key will allow Udev to tell them apart. This is generally not necessary for most

Linux From Scratch users, but does not hurt.

• NAME - The value of this key is the name that Udev will assign to this interface.

The value of NAME is the important part. Make sure you know which name has been assigned to each of your network

cards before proceeding, and be sure to use that NAME value when creating your configuration files below.

7.2.2. Creating Network Interface Configuration Files

Which interfaces are brought up and down by the network script depends on the files in /etc/sysconfig/.

This directory should contain a file for each interface to be configured, such as ifconfig.xyz, where “xyz” is

meaningful to the administrator such as the device name (e.g. eth0). Inside this file are attributes to this interface,

such as its IP address(es), subnet masks, and so forth. It is necessary that the stem of the filename be ifconfig.

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