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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: MTU problems with GRE tunnels (fwd)
> 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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