Automatic USB Tethering on OpenBSD

> hotplug my lazyness

Published: 2020-09-21

Tagged with:

#OpenBSD

Relevant XKCD – #1319

2024/02/29 edit: update the samples to use dhcpleasectl

These days, for various reason, I'm using USB tethering very often. Enabling it it's not difficult at all, it as simple as

doas dhcpleasectl urndis0

But it's tedious. Especially if you need to do it multiple times per day. I needed something to save me from filling my shell history of dhclient urndis0.

Enter hotplugd(8)!

hotplugd is a daemon that will listen for the attach/detach of various devices and execute a script. It's that simple, and at the same time really useful.

Disclaimer: I don't like to write howtos because the man pages are generally better, and the information in blog like this tends to rot sooner or later. I encourage you to go read the hotplugd(8) man page (it's really short, simple and understandable -- it even has some examples!) and consider this post as a "did you know?"-sort of thing.

hotplugd(8) manpage

With the disclaimer in place, let's continue. The idea is that hotplugd will execute /etc/hotplug/attach and detach script when a device is attached or detached. It doesn't need to be a shell script, any executable file will do, but will stick with a plain old sh script.

When you enable your phone USB tethering, a new device called urndisN is created. So, knowing this, we just need to execute dhclient(8) inside the attach script on the correct urndis(4) devices. Easy peasy:

#!/bin/sh
# /etc/hotplug/attach

DEVCLASS=$1
DEVNAME=$2

case $DEVCLASS in
# network devices
3)
    case $DEVNAME in
        # USB tethering
        urndis*) dhcpleasectl $DEVNAME ;;
    esac
esac

Remember to make the script executable and to enable hotplugd(8):

# chmod +x /etc/hotplug/attach
# rcctl enable hotplugd
# rcctl start hotplugd

Every time you enable the tethering on your phone your computer will automatically connect to it. In theory the same principle can also be used to automatically mount discs when plugged, but I haven't tried yet.

NB: I'm not paranoid enough to worry about accidentally connect to a stranger's phone. You have been warned.

-- text: CC0 1.0; code: public domain (unless specified otherwise). No copyright here.

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