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GIF(4)                       Device Drivers Manual                      GIF(4)

     gif - generic tunnel interface

     pseudo-device gif

     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 RFC 2893
     IPv6-over-IPv4 configured tunnel.

     To use gif, the administrator must first create the interface and then
     configure protocol and addresses used for the outer header.  This can be
     done by using ifconfig(8) create and tunnel subcommands, or SIOCIFCREATE
     and SIOCSIFPHYADDR ioctls.  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 tunnel).  Finally, use routing table to route the packets
     toward gif interface.

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

   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 RFC
     2893.  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 RFC 2893.  This should be
     used in mutual agreement with the peer.

   Packet format
     Every inner packet is encapsulated in an outer packet.  The inner packet
     may be IPv4 or IPv6.  The outer packet may be IPv4 or IPv6, and has all
     the usual IP headers, including a protocol field that identifies the type
     of inner packet.

     When the inner packet is IPv4, the protocol field of the outer packet is
     4 (IPPROTO_IPV4).  When the inner packet is IPv6, the protocol field of
     the outer packet is 41 (IPPROTO_IPV6).

     Malicious party may try to circumvent security filters by using tunneled
     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.  Ingress filter can be turned off by
     IFF_LINK2 bit.

     Configuration example:

     Host X--NetBSD A  ----------------tunnel---------- cisco D------Host E
                \                                          |
                 \                                        /
                  +-----Router B--------Router C---------+

     On NetBSD system A (NetBSD):

        # route add default B
        # ifconfig gifN create
        # ifconfig gifN A netmask 0xffffffff tunnel A D up
        # route add E 0
        # route change E -ifp gif0

     On Host D (Cisco):

        Interface TunnelX
         ip unnumbered D   ! e.g. address from Ethernet interface
         tunnel source D   ! e.g. address from Ethernet interface
         tunnel destination A
         tunnel mode ipip
        ip route C <some interface and mask>
        ip route A mask C
        ip route X mask tunnelX

     or on Host D (NetBSD):

        # route add default C
        # ifconfig gifN D A

     If all goes well, you should see packets flowing.

     If you want to reach Host A over the tunnel (from the Cisco D), then you
     have to have an alias on Host A for e.g. the Ethernet interface like:
     ifconfig <etherif> alias Y and on the cisco ip route Y mask tunnelX.

     inet(4), inet6(4), l2tp(4), ifconfig(8)

     C. Perkins, "IP Encapsulation within IP", RFC 2003,
     ftp://ftp.isi.edu/in-notes/rfc2003.txt, October 1996.

     R. Gilligan and E. Nordmark, "Transition Mechanisms for IPv6 Hosts and
     Routers", RFC 2893, ftp://ftp.isi.edu/in-notes/rfc2893.txt, August 2000.

     Sally Floyd, David L. Black, and K. K. Ramakrishnan, IPsec Interactions
     with ECN,
     December 1999.

     F. Baker and P. Savola, "Ingress Filtering for Multihomed Networks", RFC
     3704, ftp://ftp.isi.edu/in-notes/rfc3704.txt, March 2004.

     IPv4 over IPv4 encapsulation is compatible with RFC 2003.  IPv6 over IPv4
     encapsulation is compatible with RFC 2893.

     The gif device first appeared in WIDE hydrangea IPv6 kit.

     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
     packets with a spoofed source address.

     If the outer protocol is IPv6, path MTU discovery for encapsulated packet
     may affect communication over the interface.

     In the past, gif had a multi-destination behavior, configurable via
     IFF_LINK0 flag.  The behavior was obsoleted and is no longer supported.

NetBSD 10.99                    August 14, 2018                   NetBSD 10.99