Manual Pages  — GRE

gre – encapsulating network device

SYNOPSIS DESCRIPTION GRE-IN-UDP ENCAPSULATION EXAMPLES NOTES SEE ALSO STANDARDS AUTHORS BUGS

The gre supports GRE in UDP encapsulation as defined in RFC 8086. A GRE in UDP tunnel offers the possibility of better performance for load-balancing GRE traffic in transit networks. Encapsulating GRE in UDP enables use of the UDP source port to provide entropy to ECMP hashing.

The GRE in UDP tunnel uses single value 4754 as UDP destination port. The UDP source port contains a 14-bit entropy value that is generated by the encapsulator to identify a flow for the encapsulated packet. The udpport option can be used to disable this behaviour and use single source UDP port value. The value of udpport should be within the ephemeral port range, i.e., 49152 to 65535 by default.

Note that a GRE in UDP tunnel is unidirectional; the tunnel traffic is not expected to be returned back to the UDP source port values used to generate entropy. This may impact NAPT (Network Address Port Translator) middleboxes. If such tunnels are expected to be used on a path with a middlebox, the tunnel can be configured either to disable use of the UDP source port for entropy or to enable middleboxes to pass packets with UDP source port entropy.

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

S. Hanks, T. Li, D. Farinacci, P. Traina, RFC 1701, Generic Routing Encapsulation (GRE), October 1994.

S. Hanks, T. Li, D. Farinacci, P. Traina, RFC 1702, Generic Routing Encapsulation over IPv4 networks, October 1994.

D. Farinacci, T. Li, S. Hanks, D. Meyer, P. Traina, RFC 2784, Generic Routing Encapsulation (GRE), March 2000.

G. Dommety, RFC 2890, Key and Sequence Number Extensions to GRE, September 2000.

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

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.