MTU problems with GRE tunnels (fwd)

Dennis Ferguson dennis at
Tue Jul 7 19:25:09 UTC 1998

> Here's something very strange I observe with GRE tunnels (the default
> tunnel mode). It looks like cisco routers send IP datagrams violating RFC 791
> [Internet Protocol] over GRE tunnels. In particular, the length field of
> the IP header is computed incorrectly to *not* include the size of the
> IP header. RFC 791 says about the length field:
> I have an application on my workstation that serves as one endpoint
> of a GRE tunnel. In fact, it's such a tiny perl program that I have
> appended it at the end of this mail.
> We note that the length as reported in the IP header is
> always 20 octets less than what we receive on the socket.
> This leads me to the question
>   Do you cisco guys read RFCs? :-)

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.

Dennis Ferguson

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