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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
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 (198.51.100.1) ---tunnel--- Cisco D (203.0.113.1) -- Host E
\ /
\ /
+----- Host B ----- Host C -----+
(198.51.100.254)
On Host A (FreeBSD):
First of multiple FIBs should be configured via loader.conf:
net.fibs=2
net.add_addr_allfibs=0
Then routes to the gateway and remote tunnel endpoint via this gateway should be added to the second FIB:
route add -net 198.51.100.0 -netmask 255.255.255.0 -fib 1 -iface em0
route add -host 203.0.113.1 -fib 1 198.51.100.254
And GRE tunnel should be configured to change FIB for encapsulated packets:
ifconfig greN create
ifconfig greN inet 198.51.100.1 203.0.113.1
ifconfig greN inet tunnel 198.51.100.1 203.0.113.1 tunnelfib 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.
Debian June 2, 2015 GRE(4)