What is udev and how do you write custom udev rules in systemv environments (RHEL 6 and earlier)?

Solution Verified - Updated 7 Aug 2024

Environment

Red Hat Enterprise Linux 6
Red Hat Enterprise Linux 5
Red Hat Enterprise Linux 4
udev

For systemd (RHEL 7 and later):
- "What is udev and how do you write custom udev rules in systemd environment (RHEL 7 and later)?"

Issue

What is udev used for?
How do you write custom udev rules?

Resolution

Udev is the mechanism used to create and name /dev device nodes corresponding to the devices that are present in the system. Udev uses matching information provided by sysfs with rules provided by the user to dynamically add the required device nodes.

Writing `udev` rules

Udev rule files are kept in the /etc/udev/rules.d/ directory.
Files in /etc/udev/rules.d/ are parsed in lexical order, and in some circumstances, the order in which rules are parsed is important.
In general, custom rules should be parsed before the defaults, so it's common to create a file at /etc/udev/rules.d/10-local.rules and write all custom rules into that file.
The basic format of a rule is:
- key1="value", key2="value", ... keyN="value", name="value", symlink="value"
- At least one key and a name must be provided. Extra keys are optional, but all must match for the rule to be applied.

Examples

Notification on device add/remove.

$ cat /etc/udev/rules.d/10-custom.rules

ACTION=="add", KERNEL=="sdb[1-9]", RUN="/usr/bin/wall  SCSI DEVICE ADDED"
ACTION=="remove", KERNEL=="sdb[1-9]", RUN="/usr/bin/wall SCSI DEVICE REMOVED"

To setup symlinks to devices. The %n is the partition number.

$ cat /etc/udev/rules.d/75-custom.rules

ACTION=="add", KERNEL=="sdb[1-9]", SYMLINK="scsi%n"
ACTION=="remove", KERNEL=="sdb[1-9]", SYMLINK="scsi%n"

Apply a default setting for max_sectors_kb to newly discovered devices.

$ cat /etc/udev/rules.d/54-custom.rules

ACTION=="add", KERNEL=="sd*[!0-9]", SYSFS{vendor}=="WDC WD32", RUN+="/bin/sh -c 'echo 128 > /sys/block/%k/queue/max_sectors_kb'"
ACTION=="add", KERNEL=="sd*[!0-9]", SYSFS{vendor}=="WDC WD32", RUN+="/usr/bin/wall /sys/block/%k/queue/max_sectors_kb set to 128"

The 2nd action line is for debug so that a broadcast message that this rule was being applied is sent to the system. Only the first line is needed to apply the desired setting to max_sectors_kb upon device discovery.

The rules can have owner/group/modes set as well.

KERNEL=="sdc5", OWNER="student", GROUP="student", MODE="0600"

Here, a specific USB will get /dev/fedora%n symlinks for each partition.
```
ACTION=="add", KERNEL=="sd*", SYSFS=="4317210A2880EF89", SYMLINK+="fedora%n"
```
- The serial number above was grabbed from the output of udevinfo -a -p $(udevinfo -q path -n /dev/sdc).
There are several keys available but the three most useful ones are BUS, KERNEL, and SYSFS:
- BUS: Covers how the device is connected (usb, scsi, etc..)
- KERNEL: Refers to the standard kernel identification of the device (as used by devfs or a static /dev)
- SYSFS: Keys use the information on each device that appears in the /sys directory

Useful Commands To Use With Udev:

RHEL6

# udevadm info -a -p $(udevadm info -q path -n /dev/sda)
# udevadm info -a -p /sys/class/net/eth0
# udevadm info -q path -n /dev/sda1  
# udevadm control --reload-rules

RHEL5

# udevinfo -a -p $(udevinfo -q path -n /dev/sda)
# udevinfo -a -p /sys/class/net/eth0
# udevinfo -q path -n /dev/sda1  
# udevtrigger

Note: udev accesses device info from sysfs using libsysfs library calls.