Internet Sharing: OS X with Linux clients

Today I hauled my old machines out of the cupboard and had the following set-up working:

Things work the way they still used to, e.g. I can mount across the network soupdragon's hard-drive from mogwai, and soupdragon is still cranky refusing all connect requests except from mogwai. Because it's been about 4 years since I last used these machines properly I can't quite remember how I set it all together.

Anyway, when I was at University I used to go into the Library and connect the laptop, mogwai, to the University LAN using DHCP. So, I turned on Internet Sharing on the iBook, laddhu. After a bit of faffing I was able to get laddhu to grant mogwai a DHCP lease, and it seemed to update everything. The only problem was that I could, e.g. type ssh telnet3.ukshells.co.uk, or ping www.redhat.com on mogwai, and it would successfully use laddhu to get the IP address, etc., but whatever protocol I tried anything on, it all got swallowed into a blackhole. I know it was doing something because it would say 'trying' and then the non-local IP address...

So here's the deal: has anyone got an Apple iBook (10.3) to share its Internet connection with Linux boxen successfully? (I'm looking for a solution that works across Linux flavours.) And if so, does anyone know of any easy tutorial for it? I know it's not very useful right now (because I'm still on dialup, though I could use it to get apt-get working to update soupdragon), but I will have to do this kind of thing when (eventually) we get broadband.

(x-posted to computergeeks, linux, and macosx)

Update:

Installed DHCP on soupdragon - same thing happening. Turn off the firewall on the iBook and it lets the traffic through! So, the real question is which port do I allow traffic on so I don't have to keep the whole firewall down?