GIF(4) BSD Kernel Interfaces Manual GIF(4)
gif -- generic tunnel interface
The gif interface is a generic tunnelling pseudo device for IPv4 and IPv6. It can tunnel IPv traffic over IPv. Therefore, there can
be four possible configurations. The behavior of gif is mainly based on RFC2893 IPv6-over-IPv4 configured tunnel. On NetBSD, gif can also
tunnel ISO traffic over IPv using EON encapsulation.
Each gif interface is created at runtime using interface cloning. This is most easily done with the ifconfig(8) create command or using the
gifconfig_<interface> variable in rc.conf(5).
To use gif, administrator needs to configure protocol and addresses used for the outer header. This can be done by using gifconfig(8), or
SIOCSIFPHYADDR ioctl. Also, administrator needs to configure protocol and addresses used for the inner header, by using ifconfig(8). Note
that IPv6 link-local address (those start with fe80::) will be automatically configured whenever possible. You may need to remove IPv6 link-
local address manually using ifconfig(8), when you would like to disable the use of IPv6 as inner header (like when you need pure IPv4-over-
IPv6 tunnel). Finally, use routing table to route the packets toward gif interface.
gif can be configured to be ECN friendly. This can be configured by IFF_LINK1.
ECN friendly behavior
gif 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 IFF_LINK1 interface flag.
Without IFF_LINK1, gif will show a normal behavior, like 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.
Malicious party may try to circumvent security filters by using tunnelled packets. For better protection, gif performs martian filter and
ingress filter against outer source address, on egress. Note that martian/ingress filters are no way complete. You may want to secure your
node by using packet filters. Ingress filter can be turned off by IFF_LINK2 bit.
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.
inet(4), inet6(4), gifconfig(8)
R. Gilligan and E. Nordmark, "Transition Mechanisms for IPv6 Hosts and Routers", RFC2893, August 2000, ftp://ftp.isi.edu/in-
Sally Floyd, David L. Black, and K. K. Ramakrishnan, IPsec Interactions with ECN, December 1999, draft-ietf-ipsec-ecn-02.txt.
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 IPv4, gif does not try to perform path MTU discovery for the encapsulated packet (DF bit is set to 0).
If the outer protocol is IPv6, path MTU discovery for encapsulated packet may affect communication over the interface. The first bigger-
than-pmtu packet may be lost. To avoid the problem, you may want to set the interface MTU for gif to 1240 or smaller, when outer header is
IPv6 and inner header is IPv4.
gif does not translate ICMP messages for outer header into inner header.
In the past, gif had a multi-destination behavior, configurable via IFF_LINK0 flag. The behavior was obsoleted and is no longer supported.
It is thought that this is not actually a bug in gif, but rather lies somewhere around a manipulation of an IPv6 routing table.
April 10, 1999 BSD