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gif - generic tunnel interface


ECN friendly behavior
Route caching
See Also


.Cd "device gif"


The gif interface is a generic tunnelling device for IPv4 and IPv6. It can tunnel IPv[46] traffic over IPv[46]. Therefore, there can be four possible configurations. The behavior of gif is mainly based on RFC2893 IPv6-over-IPv4 configured tunnel. On
.Nx , gif can also tunnel ISO traffic over IPv[46] using EON encapsulation. Note that gif does not perform GRE encapsulation; use gre(4) for GRE encapsulation.

Each gif interface is created at runtime using interface cloning. This is most easily done with the "ifconfig create" command or using the ifconfig_<interface> variable in rc.conf(5).

To use gif, the administrator needs to configure the protocol and addresses used for the outer header. This can be done by using ifconfig(8) tunnel, or SIOCSIFPHYADDR ioctl. The administrator also needs to configure the protocol and addresses for the inner header, with ifconfig(8). Note that IPv6 link-local addresses (those that start with fe80::) will be automatically configured whenever possible. You may need to remove IPv6 link-local addresses manually using ifconfig(8), if you want to disable the use of IPv6 as the inner header (for example, if you need a pure IPv4-over-IPv6 tunnel). Finally, you must modify the routing table to route the packets through the gif interface.

The gif device can be configured to be ECN friendly. This can be configured by IFF_LINK1.

ECN friendly behavior

The gif device can be configured to be ECN friendly, as described in draft-ietf-ipsec-ecn-02.txt. This is turned off by default, and can be turned on by the IFF_LINK1 interface flag.

Without IFF_LINK1, gif will show normal behavior, as described in RFC2893. This can be summarized as follows:

Ingress Set outer TOS bit to 0.
Egress Drop outer TOS bit.

With IFF_LINK1, gif will copy ECN bits ( 0x02 and 0x01 on IPv4 TOS byte or IPv6 traffic class byte) on egress and ingress, as follows:

Ingress Copy TOS bits except for ECN CE (masked with 0xfe) from inner to outer. Set ECN CE bit to 0.
Egress Use inner TOS bits with some change. If outer ECN CE bit is 1, enable ECN CE bit on the inner.

Note that the ECN friendly behavior violates RFC2893. This should be used in mutual agreement with the peer.


A malicious party may try to circumvent security filters by using tunnelled packets. For better protection, gif performs both martian and ingress filtering against the outer source address on egress. Note that martian/ingress filters are in no way complete. You may want to secure your node by using packet filters. Ingress filtering can break tunnel operation in an asymmetrically routed network. It can be turned off by IFF_LINK2 bit.

Route caching

Processing each packet requires two route lookups: first on the packet itself, and second on the tunnel destination. This second route can be cached, increasing tunnel performance. However, in a dynamically routed network, the tunnel will stick to the cached route, ignoring routing table updates. Route caching can be enabled with the IFF_LINK0 flag.


By default, gif tunnels may not be nested. This behavior may be modified at runtime by setting the sysctl(8) variable net.link.gif.max_nesting to the desired level of nesting. Additionally, gif tunnels are restricted to one per pair of end points. Parallel tunnels may be enabled by setting the sysctl(8) variable net.link.gif.parallel_tunnels to 1.


gre(4), inet(4), inet6(4), ifconfig(8)
.Rs Transition Mechanisms for IPv6 Hosts and Routers
.Rs "IPsec Interactions with ECN"



Created by Blin Media, 2008-2013