I have the weirest problem with kernel-mode PPPoE.
I have tried 2 configurations on 2 entierly different boxes with different internet connections:
FreeBSD with ipfw for firewalling and mpd for kernel-mode pppoe
Gentoo with iptables for firewalling and rp-pppoe for kernel-mode pppoe

Now when running these systems in userland pppoe they both work fine, but as soon as i change to kernel-mode (in fbsd from ppp to mpd and in gentoo from rp-pppoe userland to rp-pppoe kernel-mode) and use the exact same rules for iptables\ipfw (and natd settings), then things get weird. I can use lynx from the box itself to open any of these pages: www \ passport \ com www \ microsoft \ com www \ steampowered \ com , while any machine that is using this box as a gateway cannot open any of these sites, and cannot connect to MSN chat thing aswell. It is possible that there are more pages that don't work on NATed machines but work on the box itself, but i only know of these three. Now when i switch back to userland pppoe those pages work on NATed machines too. No settings changed, just switched to userland.

(i had to reorder the urls a bit because i don't have enough posts to post an url)

Any ideas?