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

NAME
     gif -- generic tunnel interface

SYNOPSIS
     pseudo-device gif

DESCRIPTION
     The gif interface is a generic tunneling 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
     address 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 tun-
     nel).  Finally, use routing table to route the packets toward gif inter-
     face.

     gif can be configured to be ECN friendly.  This can be configured by
     IFF_LINK1.

   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.

   Security
     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.

   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
     inet(4), inet6(4), gifconfig(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 WIDE hydrangea IPv6 kit.

BUGS
     There are many tunneling 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
     address) configured to gif makes sense.  Make sure to configure an
     address which belongs to your node.  Otherwise, your node will not be
     able to receive packets from the peer, and your node will generate pack-
     ets 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