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At home I have a laptop acting as a central mercurial server (from here on referenced as myserver). This is great, being able to synchronize my desktop and netbook through an intermediary gives me three hard drives on which my data is stored. The setup was, however, not especially useful since my netbook goes with me almost everywhere (read: outside the home network).

This presents a problem as when I clone a repository and my .hg/hgrc file now contain a link to the server using at best a name defined in my /etc/hosts file, and at worst a local IP address (192.168.0.xyz).

I had long since done away with the IP address links, replacing them with hostname links (i.e. ssh://hg@myserver/repo instead of ssh://hg@192.168.0.xyz/repo) which was the first step. Now all I needed to do was make myserver accessible through the firewall (forwarding the appropriate port (since we are talking ssh here that would be 22, unless you are paranoid and have switched the port which sshd listen on) in the router, make the server listen on that port, and finally come up with a way to dynamically switch what IP address myserver should point to.

There probably is a better solution for this, configuring the router to make all this would be one way, but I’m not sure that my router has that capability, and even if it did, I’m not sure I would have the skills to configure it. Software and shell scripting on the other hand, those are things I know how to work.

My solution, was simply to create two new hosts-files in /etc :

# touch /etc/hosts-{home,not-home}

and then insert appropriate information into them such as

# echo "myserver    192.168.0.xyz" > hosts-home
# echo "myserver    aaa.bbb.ccc.ddd" > hosts-not-home

(where aaa.bbb.ccc.ddd is my external IP as seen on http://whatismyip.org/).

I also made a backup of my current hosts file just to be on the safe side

# cp /etc/hosts /etc/hosts.bak

So, the overall plan is that once I’ve identified if I am at home, or somewhere else (i.e. not home) I will overwrite (copy) /etc/hosts with the “correct” hosts-file (-home or -not-home).

And this is where NetworkManager comes into play. Every time there is a change in the connection status of any identified network interface, NetworkManager reacts by executing every script found in /etc/NetworkManager/dispatcher.d/.

Every script is called with two parameters, the first being which interface changed (e.g. “eth0″, “wlan0″, etc.) and the second being how it changed (e.g. “up”, “down”). What I wanted to do was, every time a new interface went up, check if it had connected to a device in my home network (wlan or ethernet).

So, how do you distinguish different devices in a simple manner? For me, distinguishing different MAC addresses (two known addresses at home (wlan and ethernet), and every other address was essentially not home) would do nicely.

Enter the “arp” command. From it, using flag “-i” (for interface) I can discover the IP and MAC address  of the device I’m connected to. When I first discovered the example usage used flag “-a” as well which, according to the man-page, outputs results in an alternate BSD style format, using no fixed columns. This had some benefits, but as I read it again now, the “no fixed columns” remark begin to worry me. The fact that it has worked so far isn’t good enough proof that it will always work. Needless to say, ymmv.

Putting it all together:

#!/bin/sh
# filename: /etc/NetworkManager/dispatcher.d/99switchHosts.sh

# Unless the status is "up" I'm not interested
if [ "$2" != "up" ];
then
    exit 0;
fi

# I am interested in two interfaces, eth0 and wlan0
# each connect through different IP addresses and
# these devices have separate MAC addresses
if [ "$1" == "eth0" ];
then
    TARGETIP="192.168.0.1"
    TARGETMAC="aa:bb:cc:dd:ee:ff"
else
    TARGETIP="192.168.1.1"
    TARGETMAC="12:34:56:78:90:ab"
fi

# Sometimes arp hasn't gotten hold of any IP/MAC addresses
# pinging and waiting does wonders for this
ping -c 1 "$TARGETIP" 1>/dev/null 2>&1
sleep 5
CURMAC=$(arp -ai "$1" | grep "$TARGETIP" | awk '{ print $4 }')

while [ -z "$CURMAC" ];
do
    # Again, arp might not have gotten hold of IP/MAC
    sleep 3
    CURMAC=$(arp -ai "$1" | grep "$TARGETIP" | awk '{ print $4 }')
done

# If MACs match, it is a known address, i.e. home
if [ "$TARGETMAC" == "$CURMAC" ];
then
    cp /etc/hosts-home /etc/hosts
else
    cp /etc/hosts-not-home /etc/hosts
fi

exit 0

This should be put in /etc/NetworkManager/dispatcher.d/99switchHosts.sh (and the script should be made execute:able)

# chmod +x /etc/NetworkManager/dispatcher.d/99switchHosts.sh

All done, not too shabby eh?