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     gre -- encapsulating network device


     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
	     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 encapsula-
	     tion.  The original IP header is modified and the modifications
	     are inserted between the so modified header and the original pay-
	     load.  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 des-
		tination 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

     GREGPROTO	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 encapsu-
     lated), 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 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.*
			\			       /

	   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.  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 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 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 <hwr@pilhuhn.de>


     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 destina-
     tion 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 GRE RFCs are not yet fully implemented (no GRE options).


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