WEBlog -- Wouter's Eclectic Blog

Mon, 14 Nov 2005

Automatic networking

A long time ago, I wrote:

In these days of VPN, IPsec, dynamic routing, tunnels, IPv6, and autoconfiguration protocols, one would expect that it would be possible to configure a laptop to find all kinds of hosts automatically, both for those that are network-specific (proxy servers, NNTP-servers), and for those that are not (mail servers, version control systems, ...).

I then went on to say that I could not, at the time, expect my laptop to find those networks by itself; every time I logged on to some network, I had to set up at least two SSH tunnels.

I received a lot of feedback to that post, and I now do have my laptop set up so that it configures itself automatically if necessary. Well, almost; see below. This is what I've done:

I don't use ifupdown on my laptop anymore. The package is installed and configures my lo interface, but that's it—nothing else. Instead, I'm using whereami to configure my network cards. Whereami is an excellent piece of software that is configured through two configuration files: detect.conf and whereami.conf. The former has a pretty peculiar syntaxis, which you'll need to get used to at first; but it's very powerful. There are a few things you can do in detect.conf:

1. Run tests. A test will check whether a condition is true, and set a location based on the result. An example of a test could be

   testmii eth0 lan
   

   which checks whether the MII detects a cable (testmii) on eth0, and will set the lan location if that is true. Tests are the raison d'être of detect.conf; the point is to run as many relevant tests as possible, to get as much information as possible about the location where you are; that way, you can configure the system later on from whereami.conf (see below). There's other things like testdhcp (check for the presence and/or the returned IP address of a DHCP server), testap (test for the presence of an Access Point), testmodule (check whether something loaded a given kernel module), or even testarp if you're desperate (to test whether a node with a given MAC address lives on the network).
2. Checking locations. For instance, you might want to run testdhcp on eth0 only if the location lan is set already. This way, you don't get timeouts if you're on the train or anything. Checking locations is done with if <location> ... elif <location> ... else ... fi. Which is to say, you get a multi-line block of if structures, in which you can then run other tests conditionally. I'm just too lazy to write it out here :-)
   Unfortunately, it is not possible to nest if structures in detect.conf; but I don't think it's actually necessary for the detection (it might be nice for whereami.conf later on, but there the issues can be worked around).
3. There's also the possibility to do things like setting environment variables, default locations, and some similar housekeeping things.

It is possible to set more than one location from detect.conf; for example, right now my laptop claims it's at werk,dhcp,lan. At home, it detects it's at thuis,dhcp,lan—unless I use my wireless interface, in which case it detects it's at zd1211,thuis,wlan,dhcp,wdhcp

After whereami finished the tests in detect.conf, your network should be working (provided you have a network cable plugged in, or are in the vicinity of an AP that you can connect to). Next, it will process whereami.conf. The genius of whereami probably lies in whereami.conf: this is a simple shellscript, but every line can be preceded by one of +, -, or =, followed by a location. For example, I have (amongst other things) this set up in my whereami.conf:

+lan ln -sf /etc/news/leafnode/config.generic /etc/news/leafnode/config
+werk ln -sf /etc/news/leafnode/config.werk /etc/news/leafnode/config
+thuis ln -sf /etc/news/leafnode/config.thuis /etc/news/leafnode/config

Whereami will check this file for any lines that start with '+' followed by a location it's going to, or '-' followed by a location it's leaving from, or '=' followed by a location it's either leaving from, going to, or staying at. It then removes the +/-/= plus the location, and dumps the rest of the line to a file; that file is then ran as a shell script. In other words, the above code will overwrite my /etc/news/leafnode/config with an appropriate configuration, depending on what network I'm entering.

I don't have to reconfigure NNTP servers anymore. Nor do I need to set up any tunnels—whereami does all of this for me, and more.

Next, since a few weeks, I've also added ifplugd. Previously, when I would get my laptop to resume from swsusp, I would need to manually run whereami afterwards; with ifplugd, this is no longer necessary—I just removed the ifupdown script in /etc/ifplugd/action.d, and replaced it with a whereami script that I quickly wrote myself (not as if it's hard, obviously).

There's just one thing left: the proxy server. There are just way too many programs that one needs to configure to use a proxy server (or not) in Linux; and though it's possible to configure a proxy server by letting firefox "detect" it, I have no clue how that's done. Additionally, programs that use proxy servers through environment variables can not be reconfigured, unless one logs off and on again.

I guess I'm looking for something that will do transparent proxying if there's a proxy server on the network, or just forward the requests to the remote server if not. But that's for another time.

/en/computer

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