IPTABLES and dial-ups

[Leigh]

Hi all,
Got an interesting problem here.
I'm setting up a firewall to share a dial-up connection onto a LAN. I think I can handle most of the IPTABLES stuff except for one or two issues:

I have two different connections - one when I dial the ISP, and one when they dial me. (long story). When the ISP dials me, I have a fixed IP address. I can quite happily NAT between inside and outside, but when I dial the ISP, using a different connection profile, I get a dynamic IP. How can I set up NAT rules in IPTABLES when I don't know the IP address? Alternatively, how can I find out the IP address on-the-fly? I'd like to use the ppp-up script or similar to set up IPTABLES rules, but i don't know how to set them up for a dynamic IP.
Other thing is: I have the box accepting a dial-in OK, but I've never set up dial-out before. Any pointers/gotchas? (This machine doesn't have a desktop or GUI or anything, so it's all gotta be done with scripts or command-line.)

[Jinx]

IPTABLES will accept adapters in it's rules... eg. IPTABLES -A INPUT -ppp0 -j ACCEPT

Where ppp0 can be a dialup adapter etc... the rule will only work when the adapter is up (obviously).

Have a read of the IPTABLES HOWTO at www.tldp.org or www.netfilter.org. The author (Rusty Russell) has written a pretty good HOWTO at www.netfilter.org, so try there first.

Cya round
Jinx

[Leigh]

It's not just the ACCEPT and DROP I'm after, its the SNAT for machines on the inside. What do I SNAT them to? I'll check out those docs. Thanks.

[Jinx]

It sound's like you want to MASQ (form of SNAT). Basically, you don't need to know the ppp adapter address (ie. dialup), as it works it out based on the adapter:

IPTABLES -A POSTROUTING -i ppp0 -j MASQUERADE

By now you have probably read the docs on www.netfilter.org. They would offer a better explaination than I could. :)

Cya round
Jinx