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

     gre -- encapsulating network device

     To	compile	the driver into	the kernel, place the following	line in	the
     kernel configuration file:

	   device gre

     Alternatively, to load the	driver as a module at boot time, place the
     following line in loader.conf(5):


     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 corresponds to	RFC 2784.  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 tun-
     nel mode on Cisco routers.	 gre also supports Cisco WCCP protocol,	both
     version 1 and version 2.

     The gre interfaces	support	a number of additional parameters to the

     grekey	  Set the GRE key used for outgoing packets.  A	value of 0
		  disables the key option.

     enable_csum  Enables checksum calculation for outgoing packets.

     enable_seq	  Enables use of sequence number field in the GRE header for
		  outgoing packets.

     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 inet
	   ifconfig greN inet tunnel A B
	   route add -net 192.168.2 -netmask

     On	router B:

	   ifconfig greN create
	   ifconfig greN inet
	   ifconfig greN inet tunnel B A
	   route add -net 192.168.1 -netmask

     In	case when internal and external	IP addresses are the same, different
     routing tables (FIB) should be used.  The default FIB will	be applied to
     IP	packets	before GRE encapsulation.  After encapsulation GRE interface
     should set	different FIB number to	outgoing packet.  Then different FIB
     will be applied to	such encapsulated packets.  According to this FIB
     packet should be routed to	tunnel endpoint.

     Host X -- Host A ( ---tunnel--- Cisco	D (	-- Host	E
			\				    /
			 \				   /
			  +----- Host B	----- Host C -----+

     On	Host A (FreeBSD):

     First of multiple FIBs should be configured via loader.conf:


     Then routes to the	gateway	and remote tunnel endpoint via this gateway
     should be added to	the second FIB:

	   route add -net -netmask -fib 1 -iface em0
	   route add -host -fib 1

     And GRE tunnel should be configured to change FIB for encapsulated	pack-

	   ifconfig greN create
	   ifconfig greN inet
	   ifconfig greN inet tunnel tunnelfib	1

     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	decapsulating
     host that does not	run over the tunnel, as	this would be a	loop.

     The kernel	must be	set to forward datagrams by setting the
     net.inet.ip.forwarding sysctl(8) variable to non-zero.

     gif(4), inet(4), ip(4), me(4), netintro(4), protocols(5), ifconfig(8),

     A description of GRE encapsulation	can be found in	RFC 2784 and RFC 2890.

     Andrey V. Elsukov <>
     Heiko W.Rupp <>

     The current implementation	uses the key only for outgoing packets.	 In-
     coming packets with a different key or without a key will be treated as
     if	they would belong to this interface.

     The sequence number field also used only for outgoing packets.

BSD				 June 2, 2015				   BSD


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