[Openswan Users] (p)mtu problem between winxp and openswan2.3.0

Piero Filippin filippinp at yahoo.co.uk
Thu Mar 31 11:38:34 CEST 2005

I think the correct way to set the MTU foran interface is using ifconfig.


>From the ifconfig manpage:

mtu n
Set the maximum transmission unit of the  interface to n, default is interface  specific. The MTU is used to limit the size of packets that are transmitted  on an interface. Not all interfaces support setting the MTU, and some  interfaces have range restrictions.

Changing the MTU in Windows is a littlebit trickier, as it requires some registry editing (but most network carddrivers let you modify the MTU from their properties page).



From: users-bounces at openswan.org[mailto:users-bounces at openswan.org] On BehalfOf VRZ (VRZ Koblenz)
Sent: 30 March 2005 13:29
To: users at openswan.org
Subject: [Openswan Users] (p)mtuproblem between winxp and openswan2.3.0



i am using openswan 2.3.0 under suse9.3 (kernel 2.6.10).  If i start aconnection from my winxp pro workstation (sp2) via adsl a vpn-connection everything works fine until i start a vnc-session. Then the screen is frozen andnothing works. This is a classical (p)mtu-prohlem as i read but until now ihaven'f found any solution for this problem.

Here is  what i did:

Under windows xp the largest packet i can ping without defragmentation is 1395bytes (ping -f -l 1395 xx.xx.xx.xx). The mtu of my linuxbox is 1492 bytes. I amtuning via iptables the mtu:

#iptables -A FORWARD -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss1452
iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS--clamp-mss-to-pmtu

none of them fixed the problem.

During the session setup my linux box writes the following message to thesystemlog:
kernel: pmtu discovery on SA ESP/54c693d0/54a99ca9

Do you have any suggestions?

Regards from germany

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