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gre (Encapsulating network device)
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The gre network interface pseudo device encapsulates datagrams into IP. These encapsulated datagrams are routed to a destination host, where they are decapsulated and further routed to their final destination. The 'tunnel' appears to the inner datagrams as one hop.

gre interfaces are dynamically created and destroyed with the ifconfig create and destroy subcommands.

This driver currently supports the following modes of operation:

The gre interfaces support a number of ioctl's, such as:

Note that the IP addresses of the tunnel endpoints may be the same as the ones defined with ifconfig for the interface (as if IP is encapsulated), but need not be, as e.g. when encapsulating AppleTalk.


Configuration example:

     Host X-- Host A  ----------------tunnel---------- Cisco D------Host E
               \                                          |
                \                                        /
                 +------Host B----------Host C----------+

     On host A (FreeBSD):

           route add default B
           ifconfig greN create
           ifconfig greN A D netmask 0xffffffff linkX up
           ifconfig greN tunnel A D
           route add E D

     On Host D (Cisco):

           Interface TunnelX
            ip unnumbered D   ! e.g. address from Ethernet interface
            tunnel source D   ! e.g. address from Ethernet interface
            tunnel destination A
           ip route C <some interface and mask>
           ip route A mask C
           ip route X mask tunnelX


     On Host D (FreeBSD):

           route add default C
           ifconfig greN create
           ifconfig greN D A
           ifconfig greN tunnel D A

     If all goes well, you should see packets flowing ;-)

     If you want to reach Host A over the tunnel (from Host D (Cisco)), then
     you have to have an alias on Host A for e.g. the Ethernet interface like:

           ifconfig <etherif> alias Y

     and on the Cisco:

           ip route Y mask tunnelX

     A similar setup can be used to create a link between two private networks
     (for example in the 192.168 subnet) over the Internet:

     192.168.1.* --- Router A  -------tunnel-------- Router B --- 192.168.2.*
                        \                              /
                         \                            /
                          +------ the Internet ------+

     Assuming router A has the (external) IP address A and the internal
     address, while router B has external address B and internal
     address, the following commands will configure the tunnel:

     On router A:

           ifconfig greN create
           ifconfig greN link1
           ifconfig greN tunnel A B
           route add -net 192.168.2 -netmask

     On router B:

           ifconfig greN create
           ifconfig greN link1
           ifconfig greN tunnel B A
           route add -net 192.168.1 -netmask

     Note that this is a safe situation where the link1 flag (as discussed in
     the BUGS section below) may (and probably should) be set.


The MTU of gre interfaces is set to 1476 by default, to match the value used by Cisco routers. If grekey is set this is lowered to 1472. This may not be an optimal value, depending on the link between the two tunnel endpoints. It can be adjusted via ifconfig.

For correct operation, the gre device needs a route to the destination that is less specific than the one over the tunnel. (Basically, there needs to be a route to the decapsulating host that does not run over the tunnel, as this would be a loop.) If the addresses are ambiguous, doing the ifconfig tunnel step before the ifconfig call to set the gre IP addresses will help to find a route outside the tunnel.

In order to tell ifconfig(8) to actually mark the interface as 'up', the keyword up must be given last on its command line.

The kernel must be set to forward datagrams by setting the net.inet.ip.forwarding sysctl variable to non-zero.


The compute_route() code in if_gre.c toggles the last bit of the IP-address to provoke the search for a less specific route than the one directly over the tunnel to prevent loops. This is possibly not the best solution.

To avoid the address munging described above, turn on the link1 flag on the ifconfig command line. This implies that the GRE packet destination and the ifconfig remote host are not the same IP addresses, and that the GRE destination does not route over the gre interface itself.

The current implementation uses the key only for outgoing packets. Incomming packets with a different key or without a key will be treated as if they would belong to this interface.

RFC1701 is not fully supported, however all unsupported features have been deprecated in RFC2784.


Heiko W.Rupp

See Also

A description of GRE encapsulation can be found in RFC 1701 and RFC 1702.

A description of MOBILE encapsulation can be found in RFC 2004.

See Also