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GIF(4)			 BSD Kernel Interfaces 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] 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 NetBSD, gif can also	tunnel
     ISO traffic over IPv[46] 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 ad-
     dress 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

   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	mar-
     tian/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 to
     the desired level of nesting.  Additionally, gif tunnels are restricted
     to	one per	pair of	end points.  Parallel tunnels may be enabled by	set-
     ting the sysctl(8)	variable to 1.

     inet(4), inet6(4),	gifconfig(8)

     R.	Gilligan and E.	Nordmark, "Transition Mechanisms for IPv6 Hosts	and
     Routers", RFC2893,	August 2000,

     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 ad-
     dress) configured to gif makes sense.  Make sure to configure an address
     which belongs to your node.  Otherwise, your node will not	be able	to re-
     ceive 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 dis-
     covery 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	inter-
     face 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.

BSD				April 10, 1999				   BSD


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