nanog mailing list archives
Re: MTU problems with GRE tunnels (fwd)
From: Dennis Ferguson <dennis () juniper net>
Date: Tue, 07 Jul 1998 12:25:09 -0700
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
Current thread:
- MTU problems with GRE tunnels (fwd) Jens Schweikhardt (Jul 06)
- IpinIP tunnels - was Re: MTU problems with GRE tunnels (fwd) Paul Mansfield (Jul 06)
- Re: MTU problems with GRE tunnels (fwd) Alex P. Rudnev (Jul 06)
- Re: MTU problems with GRE tunnels (fwd) Dennis Ferguson (Jul 07)
- Re: MTU problems with GRE tunnels (fwd) Dave Thaler (Jul 09)
- Re: MTU problems with GRE tunnels (fwd) Jens Schweikhardt (Jul 09)
- Re: MTU problems with GRE tunnels (fwd) Dave Thaler (Jul 09)
- <Possible follow-ups>
- Re: MTU problems with GRE tunnels (fwd) BrandonButterworth (Jul 06)