[Openswan dev] [Openswan Users] Fragmentation/reassembly bad behaviour (fwd)

Henrik Nordstrom hno at marasystems.com
Tue Jan 11 18:05:51 CET 2005

On Tue, 11 Jan 2005, Marcus Better wrote:

> I and others have had a similar problem with fragmentation. It might be the
> same bug. It has also been reported here:
> http://www.uwsg.iu.edu/hypermail/linux/net/0401.3/0057.html
> http://www.uwsg.iu.edu/hypermail/linux/net/0402.2/0000.html
> The temporary fix is to use Netfilter to force the MSS to something smaller:
> $IPTABLES -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1404

This approach works around PMTU Discovery malfunctions in the network by 
forcing TCP to only send small segments.

It is a somewhat ugly thing to do, and only works for TCP traffic (not 
UDP), but sometimes workarounds is the best one can do..


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