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GIF(4)		       FreeBSD Kernel Interfaces Manual			GIF(4)

     gif -- generic tunnel interface

     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 NetBSD, 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 config-
     ured 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 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 break tunnel
     operation in an asymmetrically routed network.  It	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.

     gre(4), inet(4), inet6(4),	ifconfig(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 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
     address) configured in the	gif interface makes sense.  Make sure to spec-
     ify 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 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.

FreeBSD	11.0		      September	10, 2015		  FreeBSD 11.0


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