[Openswan Users] Problems with Large Packets? - ps ax hangs in ssh - tunnel over wireless network

Tomasz Grzelak tgrzelak at wktpolska.com.pl
Wed Apr 20 18:09:13 CEST 2005

Markus Meissner wrote:
> Hi,


> I have a very odd problem with a new tunnel. In quick words: I have a tunnel
> for two subnets over a wireless-link. Without the tunnel everything works
> like a charm, fast and reliable. After setting up the tunnel everything
> looks ok, I can ping a host from one subnet to another and I can log in via
> ssh. But if I try to execute commands on the remote host, the console
> "hangs". The problem is 100% reproduceable with the following commands:
> while true; do date; done -> creates much output, runs without problems
> "ps ax" or "find /" hangs after a few letters. Always. Sometimes after 10
> letters, sometimes after 100 letters, but never back again to my shell. 

it looks exactly like an mtu problem; I had the same

> Testing this on the console (with tunnel enabled) is ok, no problem. Testing
> it without tunnel per ssh is ok, no problem. Other services like http or smb
> behave the same: No problem without tunnel, hangs with tunnel.
> I found a hint in the faq that this might be an MTU problem. I tried to set
> overridemtu=1430 on both sides but is has no effects. 

I don't know why
I think we should ask Paul or Jacco if the option works in OpenSwan 2.x.y

> So, please help me: I don't know where to debug this problem. What can I do?

first read some articles, they may give you some answers and/or more 
light on the case:


check if you allow to pass icmp responsible for the MTU discovery; maybe 
you block them, and if you let them through it would solve the 
problem... maybe...
On the other hand you have another choice that worked for me - add the 
following rules to the iptables script:

$IPT -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1416

(apply it for all packets going through the tunnel)
I know it does not solve the problem for the upd "big" packets, but for 
tcp connections it really suites my needs.

Tomasz Grzelak

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