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):
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. Incoming
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.