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gre - encapsulating network device
To compile the gre device into the kernel, place the following line in
the kernel configuration file:
Alternatively, to load the gre device 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 currently supports the following modes of operation:
GRE encapsulation (IP protocol number 47)
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 tunnel mode on
Cisco routers. This is also the default mode of operation of the
gre interfaces. As part of the GRE mode, gre also supports Cisco
WCCP protocol, both version 1 and version 2. Since there is no
reliable way to distinguish between WCCP versions, it should be
configured manually using the link2 flag. If the link2 flag is
not set (default), then WCCP version 1 is selected.
MOBILE encapsulation (IP protocol number 55)
Datagrams are encapsulated into IP, but with a shorter
encapsulation. The original IP header is modified and the
modifications are inserted between the so modified header and the
original payload. Like gif(4), only for IP-in-IP encapsulation.
The gre interfaces support a number of ioctl(2)s, such as:
GRESADDRS Set the IP address of the local tunnel end. This is the
source address set by or displayed by ifconfig(8) for the gre
GRESADDRD Set the IP address of the remote tunnel end. This is the
destination address set by or displayed by ifconfig(8) for the
GREGADDRS Query the IP address that is set for the local tunnel end.
This is the address the encapsulation header carries as local
address (i.e., the real address of the tunnel start point).
GREGADDRD Query the IP address that is set for the remote tunnel end.
This is the address the encapsulated packets are sent to
(i.e., the real address of the remote tunnel endpoint).
GRESPROTO Set the operation mode to the specified IP protocol value.
The protocol is passed to the interface in (struct
ifreq)->ifr_flags. The operation mode can also be given as
The link1 flag is not used to choose encapsulation, but to
modify the internal route search for the remote tunnel
endpoint, see the BUGS section below.
GREGPROTO Query operation mode.
Note that the IP addresses of the tunnel endpoints may be the same as the
ones defined with ifconfig(8) for the interface (as if IP is
encapsulated), but need not be, as e.g. when encapsulating AppleTalk.
Host X-- Host A ----------------tunnel---------- Cisco D------Host E
+------Host B----------Host C----------+
On host A (FreeBSD):
route add default B
ifconfig greN create
ifconfig greN A D netmask 0xffffffff linkX up
ifconfig greN tunnel A D
route add E D
On Host D (Cisco):
ip unnumbered D ! e.g. address from Ethernet interface
tunnel source D ! e.g. address from Ethernet interface
tunnel destination A
ip route C <some interface and mask>
ip route A mask C
ip route X mask tunnelX
On Host D (FreeBSD):
route add default C
ifconfig greN create
ifconfig greN D A
ifconfig greN tunnel D A
If all goes well, you should see packets flowing ;-)
If you want to reach Host A over the tunnel (from Host D (Cisco)), then
you have to have an alias on Host A for e.g. the Ethernet interface like:
ifconfig <etherif> alias Y
and on the Cisco:
ip route Y mask tunnelX
A similar setup can be used to create a link between two private networks
(for example in the 192.168 subnet) over the Internet:
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 192.168.1.1 192.168.2.1 link1
ifconfig greN 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 192.168.2.1 192.168.1.1 link1
ifconfig greN tunnel B A
route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1
Note that this is a safe situation where the link1 flag (as discussed in
the BUGS section below) may (and probably should) be set.
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 destination
that is less specific than the one over the tunnel. (Basically, there
needs to be a route to the decapsulating host that does not run over the
tunnel, as this would be a loop.) If the addresses are ambiguous, doing
the ifconfig tunnel step before the ifconfig(8) call to set the gre IP
addresses will help to find a route outside the tunnel.
In order to tell ifconfig(8) to actually mark the interface as “up”, the
keyword up must be given last on its command line.
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), netintro(4), protocols(5), ifconfig(8), sysctl(8)
A description of GRE encapsulation can be found in RFC 1701 and RFC 1702.
A description of MOBILE encapsulation can be found in RFC 2004.
Heiko W.Rupp 〈email@example.com〉
The compute_route() code in if_gre.c toggles the last bit of the IP-
address to provoke the search for a less specific route than the one
directly over the tunnel to prevent loops. This is possibly not the best
To avoid the address munging described above, turn on the link1 flag on
the ifconfig(8) command line. This implies that the GRE packet
destination and the ifconfig remote host are not the same IP addresses,
and that the GRE destination does not route over the gre interface
The GRE RFCs are not yet fully implemented (no GRE options).