NAME
gre —
encapsulating network device
SYNOPSIS
To compile the driver into the kernel, place the following line in the kernel configuration file:
device greAlternatively, to load the driver as a module at boot time, place the following line in loader.conf(5):
if_gre_load="YES"
DESCRIPTION
Thegre 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.