Made of Bugs

It's software. It's made of bugs.

wpa_supplicant: GUI and wpa_action

I’ve made two new interesting discoveries about wpa_supplicant since writing my last blog post on the subject. (Actually, I pretty much made both of them while reading documentation in order to write it, and have been lame about writing them up).

Using wpa_gui  🔗︎

It turns out that wpa_gui not only allows you to select existing networks, but also to scan for and add new networks to your configuration file. In addition, you can run it as yourself, without needing to sudo it. In order to do so, you need to add two lines to /etc/wpa_supplicant/wpa_supplicant.conf:


ctrl_interface_group selects a UNIX group that will be given permission to read/write the control socket. I chose netdev because it seems like it’s supposed to be networking-related, and my login user was already in it on my Ubuntu machine.

update_config allows wpa_supplicant to write back to its conf file if instructed to configure new networks by a UI (wpa_cli or wpa_gui). Note that this will squash any comments you have in the file.

wpa_action — a mostly-baked roaming solution  🔗︎

The setup I described in the previous post causes wpa_supplicant to manage associating with access points, while Debian’s ifupdown request DHCP independently. There’s no communication between the layer, so if you switch networks, or associate sometime after we bring up the interface, nothing tells dhclient to request a new lease. It turns out we can turn this picture inside-out, and make wpa_supplicant responsible for bringing up and down a virtual interface, whenever it associates or loses association.

To make this work, we’re going to need to edit /etc/network/interface again. Our wpa_supplicant.conf can stay unchanged; Debian’s wrapper scripts do all the magic. Replace your ath0 block and add a virtual default interface as follows:

iface ath0 inet manual
  wpa-driver wext
  wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf

iface default inet dhcp

The way this is going to work is that, whenever wpa_supplicant associates to a network, it will bring up the virtual default interface, causing ifupdown to spawn dhclient and request DHCP. When it loses association, it brings it down, killing the DHCP daemon.

Furthermore, we can associate different virtual interfaces with different networks. Suppose that I usually want DHCP, but at home (essid nelhage) I don’t run a DHCP server, and just want my laptop to always grab I can add an interface to wpa_supplicant.conf:


And then I add a new virtual interface to interfaces, corresponding to the id_str:

iface nelhage inet static

Now, if wpa_supplicant associates to the nelhage network, it will bring up the nelhage interface, binding ath0 to the static configuration there listed.

For documentation, check out the third section of /usr/share/doc/wpasupplicant/README.modes.gz on your Debian or Ubuntu machine.

In conclusion…  🔗︎

This setup actually seems pretty close to the correct design for a roaming wifi architecture, to me. Unfortunately, my experience is that it hasn’t worked well for me; For some reason, when I put it in roaming mode, it fails to associate with networks that it otherwise works fine with. I suspect that this is related to madwifi suckage as much as wpa_supplicant suck, though, so I’d encourage everyone else who’s been fighting with wifi to try it out and report back if it works for them.