IP Tunneling (Generic Routing Encapsulation)

GRE (Generic Routing Encapsulation) or IP tunneling (IP encapsulation) is a technique that encapsulates IP datagrams within IP datagrams. GRE is a technique that allows datagrams to be encapsulated into IP packets and then redirected to an intermediate host. At this intermediate destination, the datagrams are decapsulated and then routed to the next leg. In doing so, the trip to the intermediate host appears to the inner datagrams as one hop. The general outline of GRE can be found in RFC 1701.

Note: In order to use IP tunneling, you must be running io-pkt-v4-hc or io-pkt-v6-hc; io-pkt-v4 doesn't support GRE.

In the current stack, GRE is performed via GRE pseudo interfaces which simulate point-to-point connections. You can use the ifconfig utility to create GRE interfaces:

# ifconfig gre0 create

Each GRE interface supports the following modes of operation:

GRE encapsulation (default): Outgoing datagrams are encapsulated by an IP header of protocol type 47, and a GRE header specifying the type of the encapsulated datagram (currently only IP). This mode is described in RFC 1702. It's also the default mode on Cisco routers.
MOBILE encapsulation (IP protocol number 55): Applicable for IP encapsulation only. Outgoing IP datagrams are encapsulated by a smaller header, and the original IP header is modified. This mode is described in RFC 2004.