GRE and BPF

[Brian Hechinger]

On Wed, May 22, 2013 at 08:48:36AM -0700, Michael Sierchio wrote:
> On Wed, May 22, 2013 at 8:39 AM, Brian Hechinger <wonko at 4amlunch.net> wrote:
> 
> > Hello all,
> >
> > I've been having some trouble with a GRE tunnel. Specifically with
> > non-IP traffic (DECnet, in this case) and BPF.
> >
> > I can see the GRE packets containing the DECnet packets coming over the
> > physical interface but when I do a tcpdump of bge0 I never see them.
> >
> > This is an issue because the program I want to use will be using pcap to
> > pull packets off of the bge0 interface.
> >
> > I can get IP packets off of the bge0 interface with tcpdump, however.
> >
> > Is this just never going to work or am I missing something here?
> >
> >
> What does ifconfig report for the MTU of the gre interface?  Does it match
> the Cisco?  Is grekey set?

On the Cisco:

  Tunnel transport MTU 1476 bytes

On the FreeBSD box:

gre0: flags=9051<UP,POINTOPOINT,RUNNING,LINK0,MULTICAST> metric 0 mtu 1476

No grekey set.

> Do you have a route to the network that is encapsulated on the other side?

It will once communication is established. DECnet does dynamic routing.

> is net.inet.ip.forwarding set to 1?

I don't see how that's relevant unless that turns on some sort of magic?
I'm not trying to do IP forwarding at all. In fact, I'm not trying to do
any forwarding. I have a user mode application that just wants to read
and write packets off the the GRE tunnel.

-brian