gif(7P)

NAME
     gif - Generic tunnel interface


SYNOPSIS
     pseudo-device gif  [count]


DESCRIPTION
     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
     RFC1933 IPv6-over-IPv4 configured tunnel.

     To use gif, administrator needs to configure protocol and addresses used
     for the outer header.  This can be done by using gifconfig(1),  or
     SIOCSIFPHYADDR ioctl.  Also, the administrator needs to configure
     protocol and addresses used for the inner header by using ifconfig(1).
     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(1),  when you would like to
     disable the use of IPv6 as inner header (like when you need pure IPv4-
     over-IPv6 tunnel).  Finally, use routing table to route the packets
     toward gif interface.

     gif interface can be configued to perform bidirectional tunnel, or
     multi-destination tunnel.  This is controlled by IFF_LINK0 interface
     flag.


Bidirectional and multi-destination mode
     Usually, gif implements bidirectional tunnel.  gifconfig(1) should
     configure a tunnel ingress point (this node) and an egress point (tunnel
     endpoint), and one gif interface will tunnel to only a single tunnel
     endpoint, and accept from only a single tunnel endpoint.  Source and
     destination address for outer IP header is always the ingress and the
     egress point configued by gifconfig(1).

     With IFF_LINK0 interface flag, gif can be configured to implement multi-
     destination tunnel.  With IFF_LINK0, it is able to configure egress point
     to IPv4 wildcard address (0.0.0.0) or IPv6 unspecified address (0::0). In
     this case, destination address for the outer IP header is determined
     based on the routing table setup.  Therefore, one gif interface can
     tunnel to multiple destinations.  Also, gif will accept tunneled traffic
     from any outer source address.

     When finding a gif interface from the inbound tunneled traffic,
     bidirectional mode interface is preferred than multi-destination mode
     interface.  For example, if you have the following three gif interfaces
     on node A, tunneled traffic from C to A will match the second gif
     interface, not the third one.
           o   bidirectional, A to B
           o   bidirectional, A to C
           o   multi-destination, A to any

     Please note that multi-destination mode is far less secure than
     bidirectional mode.  Multi-destination mode gif can accept tunneled
     packet from anybody, and can be attacked from a malicious node.



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
     martian/ingress filters are no way complete.  You may want to secure your
     node by using packet filters.

     As mentioned above, multi-destination mode (IFF_LINK0) is far less secure
     than bidirectional mode.


SEE ALSO
     inet(7),  inet6(7),  gifconfig(1)

     R. Gilligan, and E. Nordmark, "Transition Mechanisms for IPv6 Hosts and
     Routers", RFC1933, April 1996, ftp://ftp.isi.edu/in-notes/rfc1933.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 tunnelling 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
     packets with a spoofed source address.

     gif(7) is an IFF_POINTOPOINT device, however, it supports NBMA behavior
     in multi-destination mode.