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     gif — generic tunnel interface


     device gif


     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 addresses used for the
     outer header.  This can be done by using ifconfig(8) tunnel, 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 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 device can be configured to be ECN friendly.  This can be configured 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.

     A malicious party may try to circumvent security filters by using tunnelled 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 table updates.  Route caching can be enabled with the IFF_LINK0 flag.

     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 setting the sysctl(8) variable to 1.


     gre(4), inet(4), inet6(4), ifconfig(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 differently 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 address) 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 discovery for the
     encapsulated packet (DF bit is set to 0).

     If the outer protocol is IPv6, path MTU discovery for encapsulated packets 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 disabled, 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.