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GIF(4) Device Drivers Manual GIF(4)

gif — generic tunnel interface

pseudo-device gif

The gif interface is a generic tunnelling pseudo-device for IPv4 and IPv6. It can tunnel IPv[46] over IPv[46] 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[46] 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 netstart(8).

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[46] over IPv[46] 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 1.2.3.4 on fxp0, bridge2 on network2 has the external IP address 4.3.2.1 on fxp0, and both bridges have fxp1 on their internal network (network1 and network2, respectively), the following configuration can be used to bridge network1 and network2.

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 1.2.3.4 4.3.2.1
(on bridge 2) # ifconfig gif0 tunnel 4.3.2.1 1.2.3.4

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 1.2.3.4 to 4.3.2.1 spi 0x4242:0x4243 \
        authkey file "auth1:auth2" enckey file "enc1:enc2"
flow esp proto etherip from 1.2.3.4 to 4.3.2.1

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 4.3.2.1 to 1.2.3.4 spi 0x4243:0x4242 \
        authkey file "auth2:auth1" enckey file "enc2:enc1"
flow esp proto etherip from 4.3.2.1 to 1.2.3.4

And load them:

# ipsecctl -f ipsec.conf

To use dynamic (as opposed to static) keying, use this ipsec.conf(5) on bridge1:

ike esp proto etherip from 1.2.3.4 to 4.3.2.1

And on bridge2:

ike esp proto etherip from 4.3.2.1 to 1.2.3.4

Bring up the internal interface (if not already up) and encapsulation interface:

# ifconfig fxp1 up
# ifconfig gif0 up

Finally, bring the bridge interface up and allow it to start processing frames:

# 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), ifconfig(8), netstart(8)

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.

December 3, 2011 OpenBSD-5.1