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GRE(4)                 FreeBSD Kernel Interfaces Manual                 GRE(4)

     gre - encapsulating network device

     pseudo-device gre

     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 greX

     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 greX 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 for the greX interface.

          Set the IP address of the remote tunnel end.  This is the
          destination address set by or displayed by ifconfig for the greX

          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 (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.

          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.

     Configuration example:

     Host X-- Host A  ----------------tunnel---------- cisco D------Host E
               \                                          |
                \                                        /
                  +------Host B----------Host C----------+
     On host A (NetBSD):

        # 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
     OR On Host D (NetBSD):

        # route add default C
        # ifconfig greN create
        # ifconfig greN D A
        # ifconfig tunnel greN 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 greX interfaces is set to 1476 by default to match the value
     used by Cisco routers.  This may not be an optimal value, depending on
     the link between the two tunnel endpoints.  It can be adjusted via

     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(8) tunnel step before the ifconfig(8) call to set the greX
     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 either option GATEWAY in
     the kernel config file or by issuing the appropriate option to sysctl(8).

     atalk(4), gif(4), inet(4), ip(4), netintro(4), options(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 greX interface

     The GRE RFCs are not yet fully implemented (no GRE options).

FreeBSD 11.0-PRERELEASE          June 9, 2002          FreeBSD 11.0-PRERELEASE


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