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GIF(4)			 BSD Kernel Interfaces Manual			GIF(4)

NAME
     gif -- generic tunnel interface

SYNOPSIS
     device gif

DESCRIPTION
     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 ad-
     dresses used for the outer	header.	 This can be done by using ifconfig(8)
     tunnel, or	SIOCSIFPHYADDR ioctl.  The administrator also needs to config-
     ure 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 dis-
     able 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.

   Security
     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.

   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 ta-
     ble updates.  Route caching can be	enabled	with the IFF_LINK0 flag.

   Miscellaneous
     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	set-
     ting the sysctl(8)	variable net.link.gif.parallel_tunnels to 1.

SEE ALSO
     gre(4), inet(4), inet6(4),	ifconfig(8)

     R.	Gilligan and E.	Nordmark, "Transition Mechanisms for IPv6 Hosts	and
     Routers", RFC2893,	August 2000, ftp://ftp.isi.edu/in-notes/rfc2893.txt.

     Sally Floyd, David	L. Black, and K. K. Ramakrishnan, IPsec	Interactions
     with ECN, December	1999, draft-ietf-ipsec-ecn-02.txt.

HISTORY
     The gif device first appeared in the WIDE hydrangea IPv6 kit.

BUGS
     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 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 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.

     On	FreeBSD	6.1, 6.2, 6.3, 7.0, 7.1, and 7.2 the gif sends and receives
     incorrect EtherIP packets with reversed version field when	if_bridge(4)
     is	used together.	As a workaround	on this	interoperability issue,	the
     following two ifconfig(8) flags can be used:

	   accept_rev_ethip_ver	 accepts both correct EtherIP packets and ones
				 with reversed version field, if enabled.  If
				 disabled, the gif accepts the correct packets
				 only.	This flag is enabled by	default.

	   send_rev_ethip_ver	 sends EtherIP packets with reversed version
				 field intentionally, if enabled.  If dis-
				 abled,	the gif	sends the correct packets
				 only.	This flag is disabled by default.

     If	interoperability with the older	FreeBSD	machines is needed, both of
     these two flags must be enabled.

BSD				 June 8, 2009				   BSD

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | HISTORY | BUGS

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