pppoe and mss clamping via iptables

[Declan Mullen <declan@mullen.net.au>]

Hi,

pppoeconf has arranged for the following iptables rule to be 
added when my Debian (sarge) firewall connects to the Internet 
via my ADSL modem:

  iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -m tcpmss
           --mss 1400:1536 -j TCPMSS --clamp-mss-to-pmtu

This rule clamps the mss regardless of which interface the packet 
will be transmitted out through (ie not just the Internet ppp0 
interface). Seeing as the firewall needs to "port forward" some 
TCP services from the Internet to servers on my internal LAN, is 
this appropriate ? Or should the clamping be applied ONLY to 
packets going out to the Internet through ppp0 ? Ie would the 
following rule be more appropriate ?

  iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -m tcpmss
           --mss 1400:1536 -j TCPMSS --clamp-mss-to-pmtu
           -o ppp0

Guess if I new more about TCP/IP I'ld know the answer, but I don't :(

Regards,
Declan