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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 create" command or using the
     gifconfig_<interface> variable in rc.conf(5).

     To	use gif, the administrator needs to configure the protocol and ad-
     dresses used for the outer	header.	 This can be done by using
     gifconfig(8), 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 be 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 pseudo-device can be configured to	be ECN friendly.  This can be
     configured	by IFF_LINK1.

   ECN friendly	behavior
     The gif pseudo-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 tun-
     nelled 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 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 the WIDE hydrangea IPv6 kit.

     There are many tunnelling protocol	specifications,	all defined differ-
     ently from	each other. The	gif pseudo-device 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 in the gif interface makes sense.  Make sure to specify
     an	address	which belongs to your node.  Otherwise,	your node will not be
     able to receive packets from the peer, and	it 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	pack-
     ets 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 the	outer header is	IPv6
     and the inner header is IPv4.

     The gif pseudo-device does	not translate ICMP messages for	the outer
     header into the inner header.

     In	the past, gif had a multi-destination behavior,	configurable via
     IFF_LINK0 flag.  The behavior is obsolete 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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