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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 data-
	  gram 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 encapsula-
	  tion.	 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 destina-
	  tion address set by or displayed by ifconfig for the greX interface.

	  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 pro-
	  tocol	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 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 (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 destina-
     tion and the ifconfig remote host are not the same	IP addresses, and that
     the GRE destination does not route	over the greX interface	itself.

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

FreeBSD	11.1			 June 9, 2002			  FreeBSD 11.1


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