Two PPP interfaces

[Graham Murray]

I currently administer a LAN gateway system running netfilter which
performs NAT and firewalling for an outgoing (dialup) PPP
connection. I have now been to add the capability of having a dial-in
PPP connection, which will need completely different netfilter rules.

As I understand it the PPP interface number is allocated dynamically,
so it will not possible to always have ppp0 as the dial-out interface
and ppp1 as the dial-in. Currently all my rules for the PPP interface
use "-i ppp+" to identify them. 

Can anyone offer advice as to how I can associate the correct
netfilter rules with the appropriate interface?