NAME

       gif — generic tunnel interface

SYNOPSIS

       device gif

DESCRIPTION

       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.

   Security
       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.

   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.

SEE ALSO

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

       R.   Gilligan   and   E.   Nordmark,  “Transition  Mechanisms  for  IPv6  Hosts  and  Routers”,  RFC2893,
       http://tools.ietf.org/html/rfc2893, August 2000.

       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 the WIDE hydrangea IPv6 kit.

BUGS

       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.

       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.

Debian                                          October 21, 2018                                          GIF(4)