[Openswan Users] MTU Problem on LAN <-> LAN Tunnel

dt at LF.net dt at LF.net
Mon Feb 14 15:50:24 CET 2005


I have a strange problem I don't really understand and I need some
help to fix it. I'm using a LAN <-> LAN Tunnel makeing a VPN
connection between the network and 
The site uses a 2.6.8 Kernel with U2.2.0/K2.6.8masq-patch 
from debian sarge. The other site is a Redhat 9 using Superfreeswan 1.99.8. 
Everything works fine until I try to transfer big files using samba from to The transfer freezes and I see the following 
fragment requests on the machine:

15:31:01.418942 IP <ext-gw-ip> > icmp 556: unreachable - need to frag (mtu 1500)
15:31:09.419100 IP <ext-gw-ip> > icmp 556: unreachable - need to frag (mtu 1500)
15:31:25.420608 IP <ext-gw-ip> > icmp 556: unreachable - need to frag (mtu 1500)

Of course it never reaches the remote site, because the target IP is not reachable via the Internet. If I lower the MTU
Size @ erverything works fine. But I'd like to avoid
changing the MTU by hand. Is there any chance to fix this? 

Please CC: to my address because I'm not on the lists, yet.



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dt at LF.net             |  D-70565 Stuttgart  |  http://www.lf.net 

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