Encapsulated datagrams are prepended an outer datagram and a GRE header. The GRE header specifies the type of the encapsulated datagram and thus allows for tunneling other protocols than IP like e.g. AppleTalk. GRE mode is also the default tunnel mode on Cisco routers. This is also the default mode of operation of the gre interfaces. As part of the GRE mode, gre also supports Cisco WCCP protocol, both version 1 and version 2. Since there is no reliable way to distinguish between WCCP versions, it should be configured manually using the link2 flag. If the link2 flag is not set (default), then WCCP version 1 is selected.
"MOBILE encapsulation (IP protocol number 55)"
Datagrams are encapsulated into IP, but with a shorter encapsulation. The original IP header is modified and the modifications are inserted between the so modified header and the original payload. Like gif(4), only for IP-in-IP encapsulation.
The gre interfaces support a number of ioctl 2 s, such as:
Set the IP address of the local tunnel end. This is the source address set by or displayed by ifconfig(8) for the gre interface.
Set the IP address of the remote tunnel end. This is the destination address set by or displayed by ifconfig(8) for the gre interface.
Query the IP address that is set for the local tunnel end. This is the address the encapsulation header carries as local address (i.e., the real address of the tunnel start point).
Query the IP address that is set for the remote tunnel end. This is the address the encapsulated packets are sent to (i.e., the real address of the remote tunnel endpoint).
Set the operation mode to the specified IP protocol value. The protocol is passed to the interface in (Vt "struct ifreq"-> ifr_flags). The operation mode can also be given as
The link1 flag is not used to choose encapsulation, but to modify the internal route search for the remote tunnel endpoint, see the BUGS section below.
Query operation mode.
Note that the IP addresses of the tunnel endpoints may be the same as the ones defined with ifconfig(8) for the interface (as if IP is encapsulated), but need not be, as e.g. when encapsulating AppleTalk.