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GIF(4)                 FreeBSD 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
     addresses 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
     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 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.

FreeBSD 4.10                    April 10, 1999                    FreeBSD 4.10


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