Odd pmtu network problem with 2.6.8? no problem with 2.4.25

[SockToy]

Hi,

First the system details: VIA EPIA MII with the via rhine network and 3com 3c574 cardbus adapter, 100mb twisted pair to a hub, leading to an openbsd router with an adsl modem, and the internet. 2 local lans firewalled by the router.

With 2.4.25 networking works fine - no problems. With 2.6.8, however, networking works 'odd'. TCP sessions get established and get well past the handshake, but then transfer about 5.4KiB before stalling, completely. The amount transferred is exactly the same every time; if I lower the MTU on the interface, the amount is consequently lowered as well.

This happens with and without iptables compiled in and still happens if I use the IPtables command to tie the mss to the interfaces mtu. There is no filtering on any ICMP router messages that might come back to the system, but I can post traces if required. All other systems in the flat work fine, and this sytem works fine with 2.4.25 kernel.

Now the really odd bit.

This system works fine to other systems on the local LAN and the lan routed 1 hop away locally via the router... but has this problem to any systems out on the internet. I'm damned if I understand why.

I have most stuff compiled directly into the kernel rather than as modules; dmesg has no obviously network related errors and the interface itself is using the right duplex and has no xmit/receive errors listed.

It looks for all the world like a path MTU discovery issue, and happens with both network cards using (obviously completely seperate kernel drivers but... I aint filtering ICMP and it works on 2.4.25 so...

Hlep me! *tears hair out* And thanks

Matt
_________________
--
EPIA MII, Gentoo, No brain

[SockToy]

Oh, the router is an old openBSD box doing filtering/natting - but its using state and passes ICMP back ok.
_________________
--
EPIA MII, Gentoo, No brain

[SockToy]

Problem solved:

net.ipv4.tcp_default_win_scale = 0
_________________
--
EPIA MII, Gentoo, No brain