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That's the best way I found to see if GRE is up, just watching for two way
"proto gre" traffic.

Here's how you can match the IP addresses *inside* the GRE packet, which
you probably will want.  Note that 0x0a050505 is hexadecimal for my desired
IP address of 10.5.5.5:

root at os-network:~# tcpdump -i eth1 'proto gre and ( ip[58:4] = 0x0a050505
or ip[62:4] = 0x0a050505 )'
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
16:18:17.434378 IP os-network-d > os-compute-01-d: GREv0, key=0x0, length
110: IP opskzlp119.snops.net > 10.5.5.5: ICMP echo request, id 21321, seq
488, length 64
16:18:17.436190 IP os-compute-01-d > os-network-d: GREv0, key=0x0, length
110: IP 10.5.5.5 > opskzlp119.snops.net: ICMP echo reply, id 21321, seq
488, length 64
16:18:18.435750 IP os-network-d > os-compute-01-d: GREv0, key=0x0, length
110: IP opskzlp119.snops.net > 10.5.5.5: ICMP echo request, id 21321, seq
489, length 64
16:18:18.437798 IP os-compute-01-d > os-network-d: GREv0, key=0x0, length
110: IP 10.5.5.5 > opskzlp119.snops.net: ICMP echo reply, id 21321, seq
489, length 64
:


On Thu, Mar 14, 2013 at 9:55 AM, Robert van Leeuwen <
Robert.vanLeeuwen at spilgames.com> wrote:

> > Thanks for the reply. I have one more question.
> > How we will check whether the tunneling is established or not?
>
> tcpdump can show you the GRE traffic:
> tcpdump -i ethX proto gre
>
> Cheers,
> Robert
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