MTU problems with GRE tunnels (fwd)

Dave Thaler thalerd at
Tue Jul 7 21:08:36 UTC 1998

Dennis Ferguson
> I can tell you for sure that the Cisco routers do send the packets (GRE
> or IP protocol 4) with a length which includes the IP header, just like
> the RFC.  If you look I think you'll find that it is your kernel which is
> subtracting out the IP header length before it hands the packet to you on
> the raw socket.

BSD Unix converts the length to host order and subtracts the IP header 
Linux leaves the length in net order (don't know about subtracting).
Windows leaves the length in net order and does not subtract the IP 
   header length.


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