Home page logo
/

nanog logo nanog mailing list archives

Re: I'm missing 2 bytes (GRE implementation)
From: William Herrin <bill () herrin us>
Date: Wed, 10 Aug 2011 13:26:02 -0400

On Wed, Aug 10, 2011 at 12:36 PM, Daniel Roesen <dr () cluenet de> wrote:
On Wed, Aug 10, 2011 at 12:57:44AM +0000, Franck Martin wrote:
I'm using a GRE IPv4 tunnel between a cisco and linux machines
So why Cisco is off by 2 Bytes?

The only GRE options using 2 bytes are GRE checksum and offset. Haven't
seen any of them being used by default by IOS. IOS default GRE payload
MTU traversing an IPv4 MTU 1500 egress interface is 1476 (1500 minus 20
octets IPv4 header, 4 octets GRE header).

Handy reference:
http://en.wikipedia.org/wiki/Generic_Routing_Encapsulation#Packet_header

The GRE header length will be evenly divisible by 4. If the checksum
is present then so is the offset, and vice versa. So if you're seeing
a 2 byte (not 4 byte) difference that's coming from somewhere else.

Regards,
Bill Herrin




-- 
William D. Herrin ................ herrin () dirtside comĀ  bill () herrin us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004


  By Date           By Thread  

Current thread:
[ Nmap | Sec Tools | Mailing Lists | Site News | About/Contact | Advertising | Privacy ]
AlienVault