Identify GSM modem devices using udev

Again an interesting problem, I do have a Linux box and it has two GSM modems and a RS-232 FTDI USB device built in. Each GSM modem brings three USB serial devices. Now I do want to dial up using the first of these modems and therefore I do need the device name, e.g. /dev/ttyUSB2.

However, each time the box boots up, either the RS-232 device or the modems are first in the order or devices found by the kernel. This results in the modem to be either /dev/ttyUSB2 or /dev/ttyUSB3. Since this definitely is an issue when dialing up, I would like to keep these device names persistent.

udev can help here. It allows one to influence the way devices are created in userland. Depending on your distribution, the rules files are located at /etc/udev/rules.d/ (at least for Ubuntu).

Now my modems can be identified by vendor and product id (12d1, 1404) so a simply udev rule should be fine:

SUBSYSTEM=="tty", ATTRS{idVendor}=="12d1", ATTRS{idProduct}=="1404", SYMLINK+="gsm%s{bInterfaceNumber}"

In theory this should create additional entries under “/dev/” with map to the kernel assigned device names. For example /dev/gsm00 -> /dev/ttyUSBXX. So I could just access /dev/gsm01, whatever the boot order was.

The problem is that the device attribute bInterfaceNumber is not on the tty device, but on the usb device in the parent hierarchy.

Still it is possible to record the interface number of a first rule, and use it in a second one:

SUBSYSTEMS=="usb", ENV{.LOCAL_ifNum}="$attr{bInterfaceNumber}"
SUBSYSTEM=="tty", ATTRS{idVendor}=="12d1", ATTRS{idProduct}=="1404", SYMLINK+="gsm%E{.LOCAL_ifNum}"

This stores the attribute “bInterfaceNumber” into the environment variable .LOCAL_ifNum (the prefixed dot is a notation for temporary or hidden variables). In the second rule the same variable is pulled on using the %E syntax. Newer udev versions also support $env instead of %E.

Thanks to [1] for mentioning this trick!

[1] https://unix.stackexchange.com/questions/60154/udev-rule-file-for-modem-not-working

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