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

NAME

     gre -- encapsulating network device

SYNOPSIS

     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):

	   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 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.  GRE mode is also the default tunnel 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 ifconfig(8):

     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.

EXAMPLES

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

NOTES

     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 ifconfig(8).

     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.

SEE ALSO

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

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

AUTHORS

     Andrey V. Elsukov <ae@FreeBSD.org>
     Heiko W.Rupp <hwr@pilhuhn.de>

BUGS

     The current implementation uses the key only for outgoing packets.  Incoming 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
								 November 7, 2014							       BSD