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iptables, MTU problem, NETDEV WATCHDOG errors



Hi,
 
My Debian box has 2 network cards, eth0 connected to my ADSL modem and eth1 to my local LAN via a hub.  I have been using iptables / NAT to get the local LAN on the 'net for a couple of years now with great success.  Unfortunately I fried a hard drive and had to start from scratch so thought I would use the newest Debian release.  That's when my problems began.
 
The box is reliable and NIC's are stable before any iptables rules are applied.  I know this because I am driving it through an SSH session on eth1 and using aptitude to get some quite large files of the 'net (kernel-source etc.).
 
When I apply iptables rules for NAT etc I get the classic MTU problem where I can't load some webpages etc.  Part of the problem is I don't know how to set MTU any more!  There is no more pppoe.conf file and it seems the the kernel based pppd / pppoe plugin and the roaring penguin one are configured using different methods.  How do I know which one I have?  How do I find out how to configure MTU etc?  I tried setting it in dsl-provider but it has no impact on the MTU shown for ppp0.  I have also used the iptables TCPMSS clamp but the result remains the same.
 
More troubling again is that after a while I get NETDEV WATCHDOG transmit timeout errors that eventuate in the box hanging totally.  This only happens after I have run my iptables script though!
 
I have tried installing a 2.4.27 kernel and a 2.4.25 kernel I compiled myself (to get rid of APIC) and get the same symptoms.  I am obviously missing something very basic!  Can anybody shed any light on this?
 
Thanks in advance 
 
Chris
 
 



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