How to forward all GRE traffic to a particular network interface?

Question

I want to forward all GRE traffic (IP protocol 47) coming on eth2 to br0 on my machine. How can I do that?

Answer 1

I've never tried it, but first you need to match the traffic you want. You should be able to do so using iptables and the -p option with the protocol number:

PARAMETERS The following parameters make up a rule specification (as used in the add, delete, insert, replace and append commands).

   [!] -p, --protocol protocol
          The  protocol of the rule or of the packet to check.  The speci‐
          fied protocol can be one of tcp, udp, udplite,  icmp,  esp,  ah,
          sctp  or  all, or it can be a numeric value, representing one of
          these protocols or  a  different  one.   A  protocol  name  from
          /etc/protocols  is also allowed.  A "!" argument before the pro‐
          tocol inverts the test.  The number zero is equivalent  to  all.
          Protocol  all  will  match  with  all  protocols and is taken as
          default when this option is omitted.