NAME¶
gre —
encapsulating network
device
SYNOPSIS¶
To compile the
gre device into the kernel, place the following
line in the kernel configuration file:
device gre
Alternatively, to load the
gre device as a module at boot
time, place the following line in
loader.conf(5):
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 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 interface.
GRESADDRD
- Set the IP address of the remote tunnel end. This is the
destination address set by or displayed by ifconfig(8)
for the gre interface.
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
- link0
IPPROTO_GRE
- -link0
IPPROTO_MOBILE
to ifconfig(8).
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.
GRESKEY
- Set the GRE key used for outgoing packets. A value of 0
disables the key option.
GREGKEY
- Get the GRE key currently used for outgoing packets. 0
means no outgoing key.
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.
EXAMPLES¶
Configuration example:
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):
Interface TunnelX
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
OR
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.
NOTES¶
The MTU of
gre interfaces is set to 1476 by default, to match
the value used by Cisco routers. If grekey is set this is lowered to 1472.
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
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.
SEE ALSO¶
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.
AUTHORS¶
Heiko W.Rupp ⟨hwr@pilhuhn.de⟩
BUGS¶
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 solution.
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 itself.
The current implementation uses the key only for outgoing packets. Incomming
packets with a different key or without a key will be treated as if they would
belong to this interface.
RFC1701 is not fully supported, however all unsupported features have been
deprecated in RFC2784.