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GRE(4)			 BSD Kernel Interfaces Manual			GRE(4)

     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


		to ifconfig(8).

		The link1 flag is not used to choose encapsulation, but	to
		modify the internal route search for the remote	tunnel end-
		point, see the BUGS section below.

     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.*
			\			       /
			 \			      /
			  +------ the Internet ------+

     Assuming router A has the (external) IP address A and the internal	ad-
     dress,	while router B has external address B and internal ad-
     dress,	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.  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 <>

     The compute_route() code in if_gre.c toggles the last bit of the IP-ad-
     dress to provoke the search for a less specific route than	the one	di-
     rectly 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).

BSD				 June 9, 2002				   BSD


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