Manually connecting to each got old very quickly. So, to automate those connections I settled on wpa_supplicant, (wpasupplicant is the Debian package name,) which is quite easy to set up in roaming mode, as outlined in /usr/share/doc/wpasupplicant/README.modes.gz. All you need is the following in /etc/network/interfaces:

allow-hotplug ath0
iface ath0 inet manual
        wpa-driver wext
        wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp

Then create your /etc/wpa_supplicant/wpa_supplicant.conf as follows:

ctrl_interface=/var/run/wpa_supplicant
network={
     key_mgmt=NONE
}

That’s it. You can now roam from one open network to the next and the supplicant will connect you to each one in turn. I have used this to reconnect to my screen session each time the bus slows down for a stop or gets stuck in traffic to carry on conversations on irc for the whole length of the commute.

If you also connect to networks requiring authentication, you’ll need to add a “network” clause for each network. See /usr/share/doc/wpasupplicant/examples/ for help with that, particularly the annotated conf file, README.wpa_supplicant.conf.gz.

For finer control over the process, I use sudo wpa_cli so i can:

• keep an eye on what networks are around: scan, then scan_results,
• disconnect from a network once I know it’s out of range
• and reassociate as needed.

The only things that I haven’t figured out how to do yet are to make the process of switching from one network to the next a bit faster (it normally takes about a minute; precious seconds of online time are lost when the bus is in motion,) and to blacklist certain essids, e.g. commercial hotspots that make you pay before they’ll route your packets to the Internet. For the first problem, I have already tweaked my dhcp settings, which helps a bit. For the second, I tried adding network blocks for specific networks and setting them to a lower priority, but that doesn’t work because then the ‘catch all’ open network block kicks in and picks them up anyway.