9.4. Managing Devices

9.4.1. Network Devices

Udev, by default, names network devices according to Firmware/BIOS data or physical characteristics like the bus, slot, or MAC address. The purpose of this naming convention is to ensure that network devices are named consistently, not based on when the network card was discovered. In older versions of Linux—on a computer with two network cards made by Intel and Realtek, for instance—the network card manufactured by Intel might have become eth0 while the Realtek card became eth1. After a reboot, the cards would sometimes get renumbered the other way around.

In the new naming scheme, typical network device names are something like enp5s0 or wlp3s0. If this naming convention is not desired, the traditional naming scheme, or a custom scheme, can be implemented.

9.4.1.2. Creating Custom Udev Rules

The naming scheme can be customized by creating custom udev rules. A script has been included that generates the initial rules. Generate these rules by running:

bash /usr/lib/udev/init-net-rules.sh

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

Note

In some cases, such as when MAC addresses have been assigned to a network card manually, or in a virtual environment such as Qemu or Xen, the network rules file may not be generated because addresses are not consistently assigned. In these cases, this method cannot be used.

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 keywords, separated by commas and optional whitespace. Here are the keywords, and an explanation of each one:

• 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 the parent devices.

• ATTR{address} - The value of this keyword 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.

• NAME - The value of this keyword 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 network configuration files.

Even if the custom udev rule file is created, udev may still assign one or more alternative names for a NIC based on physical characteristics. If a custom udev rule would rename some NIC using a name already assigned as an alternative name of another NIC, this udev rule will fail. If this issue happens, you may create the /etc/udev/network/99-default.link configuration file with an empty alternative assignment policy, overriding the default configuration file /usr/lib/udev/network/99-default.link:

sed -e '/^AlternativeNamesPolicy/s/=.*$/=/'  \
       /usr/lib/udev/network/99-default.link \
     > /etc/udev/network/99-default.link

9.4.2. CD-ROM Symlinks

Some software that you may want to install later (e.g., various media players) expects the /dev/cdrom and /dev/dvd symlinks to exist, and to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put references to those symlinks into /etc/fstab. Udev comes with a script that will generate rules files to create these symlinks for you, depending on the capabilities of each device, but you need to decide which of two modes of operation you wish to have the script use.

First, the script can operate in “by-path” mode (used by default for USB and FireWire devices), where the rules it creates depend on the physical path to the CD or DVD device. Second, it can operate in “by-id” mode (default for IDE and SCSI devices), where the rules it creates depend on identification strings stored on the CD or DVD device itself. The path is determined by udev's path_id script, and the identification strings are read from the hardware by its ata_id or scsi_id programs, depending on which type of device you have.

There are advantages to each approach; the correct approach depends on what kinds of device changes may happen. If you expect the physical path to the device (that is, the ports and/or slots that it plugs into) to change, for example because you plan on moving the drive to a different IDE port or a different USB connector, then you should use the “by-id” mode. On the other hand, if you expect the device's identification to change, for example because it may die, and you intend to replace it with a different device that plugs into the same connectors, then you should use the “by-path” mode.

If either type of change is possible with your drive, then choose a mode based on the type of change you expect to happen more often.

Important

External devices (for example, a USB-connected CD drive) should not use by-path persistence, because each time the device is plugged into a new external port, its physical path will change. All externally-connected devices will have this problem if you write udev rules to recognize them by their physical path; the problem is not limited to CD and DVD drives.

If you wish to see the values that the udev scripts will use, then for the appropriate CD-ROM device, find the corresponding directory under /sys (e.g., this can be /sys/block/hdd) and run a command similar to the following:

udevadm test /sys/block/hdd

Look at the lines containing the output of various *_id programs. The “by-id” mode will use the ID_SERIAL value if it exists and is not empty, otherwise it will use a combination of ID_MODEL and ID_REVISION. The “by-path” mode will use the ID_PATH value.

If the default mode is not suitable for your situation, then the following modification can be made to the /etc/udev/rules.d/83-cdrom-symlinks.rules file, as follows (where mode is one of “by-id” or “by-path”):

sed -e 's/"write_cd_rules"/"write_cd_rules mode"/' \
    -i /etc/udev/rules.d/83-cdrom-symlinks.rules

Note that it is not necessary to create the rules files or symlinks at this time because you have bind-mounted the host's /dev directory into the LFS system and we assume the symlinks exist on the host. The rules and symlinks will be created the first time you boot your LFS system.

However, if you have multiple CD-ROM devices, then the symlinks generated at that time may point to different devices than they point to on your host because devices are not discovered in a predictable order. The assignments created when you first boot the LFS system will be stable, so this is only an issue if you need the symlinks on both systems to point to the same device. If you need that, then inspect (and possibly edit) the generated /etc/udev/rules.d/70-persistent-cd.rules file after booting, to make sure the assigned symlinks match your needs.

9.4.3. Dealing with Duplicate Devices

As explained in Section 9.3, “Overview of Device and Module Handling,” the order in which devices with the same function appear in /dev is essentially random. E.g., if you have a USB web camera and a TV tuner, sometimes /dev/video0 refers to the camera and /dev/video1 refers to the tuner, and sometimes after a reboot the order changes. For all classes of hardware except sound cards and network cards, this is fixable by creating udev rules to create persistent symlinks. The case of network cards is covered separately in Section 9.5, “General Network Configuration,” and sound card configuration can be found in BLFS.

For each of your devices that is likely to have this problem (even if the problem doesn't exist in your current Linux distribution), find the corresponding directory under /sys/class or /sys/block. For video devices, this may be /sys/class/video4linux/videoX. Figure out the attributes that identify the device uniquely (usually, vendor and product IDs and/or serial numbers work):

udevadm info -a -p /sys/class/video4linux/video0

Then write rules that create the symlinks, e.g.:

cat > /etc/udev/rules.d/83-duplicate_devs.rules << "EOF"

# Persistent symlinks for webcam and tuner
KERNEL=="video*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", SYMLINK+="webcam"
KERNEL=="video*", ATTRS{device}=="0x036f",  ATTRS{vendor}=="0x109e", SYMLINK+="tvtuner"

EOF

The result is that /dev/video0 and /dev/video1 devices still refer randomly to the tuner and the web camera (and thus should never be used directly), but there are symlinks /dev/tvtuner and /dev/webcam that always point to the correct device.