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NAME

     gre — encapsulating network device

SYNOPSIS

     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):

           if_gre_load="YES"

DESCRIPTION

     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 interface.

     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.

EXAMPLES

     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

     OR

     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 192.168.1.1,
     while router B has external address B and internal address 192.168.2.1, the following
     commands will configure the tunnel:

     On router A:

           ifconfig greN create
           ifconfig greN 192.168.1.1 192.168.2.1 link1
           ifconfig greN tunnel A B
           route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1

     On router B:

           ifconfig greN create
           ifconfig greN 192.168.2.1 192.168.1.1 link1
           ifconfig greN tunnel B A
           route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1

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

NOTES

     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.

SEE ALSO

     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.

AUTHORS

     Heiko W.Rupp <hwr@pilhuhn.de>

BUGS

     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(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 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.