User Stories: ifstated & !carp

A long while ago I found myself wondering if I could make my OpenBSD laptop just up and grab an IP whenever I plugged the damned ethernet in.

Most of the time the machine just sits in the basement plugged in and either is building a kernel from the -current tree or the corresponding user land. Occasionally I unplug it to test out something, play with some Soekris boxes or even do actual work. In the event that I unplug the ethernet, I didn't want to have to deal running dhclient manually or at all.

The following is my solution (using ifstated) and how I came by it.

It turns out the solution to my problem is a utility called ifstated which is typically used in CARP situations where you watch when one interface fails, and swap over to another. A bit of example reading (for which there is a dearth on Google), some man page reading and a bit of thinking, I scraped up the following configuration (and put it in /etc/ifstated.conf):

ethernet_up = "xl0.link.up"
ethernet_down = "!xl0.link.up"

state auto {
        if $ethernet_up
                set-state main
}

state main {
        init { # do any network detection you care for here.
                run "dhclient xl0"
        }
        if $ethernet_down {
                run "ifconfig xl0 delete ; route flush >/dev/null"
                set-state auto
        }
}

init-state auto

This is about as simple as I can make it. The first two lines are macros/variables which are not all that necessary in this case but make things a tad more readable. I'm not sure if they can be nested (if so then I could make things easier and note the interface only once).

Then I define the two possible states I wish to handle. The first (auto) is a priming state where I either start out or return to on link failure. All this state does is wait for the interface to come back up so it can switch over to the second state (namely main). This second state can only be reached on a link coming up so we start out by grabbing a dhcp lease. At this state you can do some funky magic (arping et al.) and figure out 'where you are' and setup accordingly, but for my purposes simply running dhclient is sufficient. From this point we stick around in this state until the link fails. Then we reset the interface and go back into the auto state for waitin on another link. This completes the loop and now I don't have to do anything but plug and unplug the cable.

Now that we have a configuration we should probably just use it. This is really easy to as we just invoke ifstated -f /etc/ifstated.conf and we're good to go. A nice way to enable ifstated on boot up is to just add the following to rc.local:

if [ X"${ifstated_flags}" != X"NO" ]; then 
        /usr/sbin/ifstated $ifstated_flags
fi

Then pop the following line in /etc/rc.conf.local

ifstated_flags="-f /etc/ifstated.conf"

At this point you can either reboot or just run ifstated and be up and running.

1. Use the -D flag to set a macro for the relevant interface. Makes things a lot more generic and I would even like to see a similar setup when you install the OS from CD (grab yours in N.A. or world-wide) but that is neither here or there.
2. Since I also have a wireless card, (whose MAC is used to give it the same IP as the wired NIC) I could setup a more complicated state such that I would flip flop between either NIC based on the link state of the wired ethernet adapter.
3. Find a way to try using my home wireless AP by default but fall back on a 'scan for open APs' and then connect to the first open AP with the best signal (if there are multiples).

ifstated is a very useful tool, even outside of the typical CARP scenario.

What have use used ifstated for?
How would you improve on my solution?