GIF(4) OpenBSD Programmer's Manual GIF(4)
gif - generic tunnel interface
The gif interface is a generic tunnelling pseudo-device for IPv4 and
IPv6. It can tunnel IPv over IPv with behavior mainly based on
RFC 1933 IPv6-over-IPv4, for a total of four possible combinations. When
instead used as a member in a bridge(4), it will tunnel Ethernet packets
over IPv using RFC 3378 EtherIP encapsulation (version 3), providing
two more combinations.
A gif interface can be created at runtime using the ifconfig gifN create
command or by setting up a hostname.if(5) configuration file for
For all six modes the gif interface must be configured with the addresses
used for the outer header. This can be done by using ifconfig(8)'s
tunnel command (which uses the SIOCSIFPHYADDR ioctl).
For the IPv over IPv modes the addresses of the inner header must
be configured by using ifconfig(8) in the normal way. Note that IPv6
link-local address (those start with fe80::) will be automatically
configured whenever possible. One may need to remove any IPv6 link-local
address manually using ifconfig(8), to disable the use of IPv6 as inner
header, for example when a pure IPv4-over-IPv6 tunnel is required. The
routing table can be used to direct packets toward the gif interface.
For the Ethernet-over-IP modes the gif interface must be made a member of
a bridge(4). The sysctl(3) variable net.inet.etherip.allow must be set
to 1, unless ipsec(4) is being used to protect the traffic. Ethernet
frames are then encapsulated and sent across the network to another
bridge(4), which decapsulates the datagram and processes the resulting
Ethernet frame as if it had originated on a normal Ethernet interface.
This effectively allows a layer 2 network to be extended from one point
to another, possibly through the Internet. This mechanism may be used in
conjunction with IPsec by specifying the appropriate IPsec flows between
the two bridges. To only protect the bridge traffic between the two
bridges, the transport protocol 97 (etherip) selector may be used in
ipsec.conf(5). Otherwise, the Ethernet frames will be sent in the clear
between the two bridges.
Given two physically separate Ethernet networks, a bridge can be used as
follows to make them appear as the same local area network. If bridge1
on network1 has the external IP address 18.104.22.168 on fxp0, bridge2 on
network2 has the external IP address 22.214.171.124 on fxp0, and both bridges
have fxp1 on their internal network (network1 and network2,
respectively), the following configuration can be used to bridge network1
First create the bridge interface, adding the encapsulation interface and
internal Ethernet interface to the bridge interface:
# ifconfig bridge0 add gif0 add fxp1
Create and configure the gif0 interface:
(on bridge 1) # ifconfig gif0 tunnel 126.96.36.199 188.8.131.52
(on bridge 2) # ifconfig gif0 tunnel 184.108.40.206 220.127.116.11
Create Security Associations (SAs) between the external IP address of
each bridge and matching ingress flows by using the following
ipsec.conf(5) file on bridge1:
esp from 18.104.22.168 to 22.214.171.124 spi 0x4242:0x4243 \
authkey file "auth1:auth2" enckey file "enc1:enc2"
flow esp proto etherip from 126.96.36.199 to 188.8.131.52
Now load these rules into the kernel by issuing the ipsecctl(8) command:
# ipsecctl -f ipsec.conf
Appropriate ipsec.conf(5) for bridge2:
esp from 184.108.40.206 to 220.127.116.11 spi 0x4243:0x4242 \
authkey file "auth2:auth1" enckey file "enc2:enc1"
flow esp proto etherip from 18.104.22.168 to 22.214.171.124
And load them:
# ipsecctl -f ipsec.conf
To use dynamic (as opposed to static) keying, use this ipsec.conf(5) on
ike esp proto etherip from 126.96.36.199 to 188.8.131.52
And on bridge2:
ike esp proto etherip from 184.108.40.206 to 220.127.116.11
Bring up the internal interface (if not already up) and encapsulation
# ifconfig fxp1 up
# ifconfig gif0 up
Finally, bring the bridge interface up and allow it to start processing
# ifconfig bridge0 up link2
The internal interface on each bridge need not have an IP address: the
bridge can function without it.
Note: It is possible to put the above commands in the hostname.if(5)
files, using the `!' operator.
sysctl(3), bridge(4), inet(4), inet6(4), ipsec(4), hostname.if(5),
R. Gilligan and E. Nordmark, "Transition Mechanisms for IPv6 Hosts and
Routers", RFC 1933, April 1996.
R. Housley and S. Hollenbeck, "EtherIP: Tunneling Ethernet Frames in IP
Datagrams", RFC 3378, September 2002.
The gif device first appeared in WIDE hydrangea IPv6 kit.
There are many tunnelling protocol specifications, defined differently
from each other. gif may not interoperate with peers which are based on
different specifications, and are picky about outer header fields. For
example, you cannot usually use gif to talk with IPsec devices that use
IPsec tunnel mode.
The current code does not check if the ingress address (outer source
address) configured to gif makes sense. Make sure to configure an
address which belongs to your node. Otherwise, your node will not be
able to receive packets from the peer, and your node will generate
packets with a spoofed source address.
If the outer protocol is IPv6, path MTU discovery for encapsulated packet
may affect communication over the interface.
When used in conjunction with a bridge(4) interface, only one bridge
tunnel may be operational for every pair of source/destination addresses.
If more than one gif interface is configured with the same pair of outer
addresses, the one with the lowest index number will receive all traffic.
OpenBSD 5.1 December 3, 2011 OpenBSD 5.1
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