At the moment I just have a masquerading firewall but I'm hoping to add a shell script containing ipchains rules onto it soon. At the moment the script defines variables at the beginning and uses them later. ie.

#!/bin/sh

EXTERNAL_INTERFACE="eth0"

IP_ADDR="192.168.1.20"

ipchains -A input $EXTERNAL_INTERFACE -s $IP_ADDR etc.etc.

Although there is a problem.

I've got a dynamically assigned IP address, so in theory I'd have to re-configure the script every time PPP was started. Is there and environment variable in BASH to do the job $IP_ADDR or something like that, but then how does it know which? eth0 or ppp0? I feel I'm going to have to write a more cunning piece of script for this one. Any ideas?