Routing question (GRE packet vs normal traceroute)?

[Julian Elischer]

Xin LI wrote:
> Hi,
> 
> A friend of mine has encountered some problem in his setup which
> consists a pair of GRE peer, one running on OpenBSD and another
> running FreeBSD 7.2-RELEASE; with 7.2-STABLE, there is no improvement
> over the situation.  The problem we have observed seems to be related
> to GRE packet not being routed as observed, here is some details:
> 
>  - The FreeBSD box has one network interface connected to two (2)
> upstream network, with different IP and does not belong to the same
> subnet, say, one is 1.2.3.4/24 and another is 5.6.7.8/24
>  - The default gateway can be reached through the first IP address
> bound to the network interface;
>  - An explicit route has been configured to the OpenBSD host, the
> gateway being used can be reached directly via the secondary (aliased
> 5.6.7.8/24) IP.
>  - Both the default gateway and the explicit host route can reach the
> OpenBSD route.
> 
> The problem they had is, while traceroute to the OpenBSD host can give
> the desired result, however, packets that is supposed to be
> transferred through the GRE tunnel, while they will be encapsulated
> into a GRE packet, the GRE packet itself won't go to the explicit host
> route, but end up going to the default gateway.
> 
> The friend has configured his switch to "bounce" the packet back to
> the server by configuring a host route on L3 switch, and it seems that
> the FreeBSD box is able to route the GRE packet to its desired gateway
> this time.
> 
> Any suggestions?

there is a hack in the GRE code "that you can turn off" where the GRE
envelope is looking up the address of the peer *WITH THE LAST BIT 
SWITCHED*

try adding a route to the address of the openBSD host with /31 (not 32)

I forget how to turn it off but th man page says.

there IS a good reason for it if you want packets for the OpenBSD host 
itself to go through the tunnel.. Then you need to not use that 
address itself or you get a routing loop.




> 
> Cheers,