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     gre — encapsulating network device


     To compile the gre device into the kernel, place the following line in
     the kernel configuration file:

           device gre

     Alternatively, to load the gre device as a module at boot time, place the
     following line in loader.conf(5):



     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(8)
     create and destroy subcommands.

     This driver currently supports the following modes of operation:

     GRE encapsulation (IP protocol number 47)
             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:

     GRESADDRS  Set the IP address of the local tunnel end.  This is the
                source address set by or displayed by ifconfig(8) for the gre

     GRESADDRD  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.

     GREGADDRS  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).

     GREGADDRD  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).

     GRESPROTO  Set the operation mode to the specified IP protocol value.
                The protocol is passed to the interface in (struct
                ifreq)->ifr_flags.  The operation mode can also be given as

                link0   IPPROTO_GRE
                -link0  IPPROTO_MOBILE

                to ifconfig(8).

                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.

     GREGPROTO  Query operation mode.

     GRESKEY    Set the GRE key used for outgoing packets.  A value of 0
                disables the key option.

     GREGKEY    Get the GRE key currently used for outgoing packets.  0 means
                no outgoing key.

     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.


     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(8).

     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(8) 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(8) variable to non-zero.


     gif(4), inet(4), ip(4), netintro(4), protocols(5), ifconfig(8), sysctl(8)

     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.


     Heiko W.Rupp ⟨


     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

     To avoid the address munging described above, turn on the link1 flag on
     the ifconfig(8) 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

     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.