gif(4)

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 proto

col and
addresses used for the outer header. This can be done by

using
ifconfig(8) tunnel, or SIOCSIFPHYADDR ioctl. The adminis

trator 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 ifcon

fig(8), if you
want to disable the use of IPv6 as the inner header (for ex

ample, if you
need a pure IPv4-over-IPv6 tunnel). Finally, you must modi

fy 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 de

scribed 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 de

scribed 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 out

er 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.

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 dynam

ically routed
network, the tunnel will stick to the cached route, ignoring

routing
table updates. Route caching can be enabled with the

IFF_LINK0 flag.

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 en

abled by setting the sysctl(8) variable net.link.gif.parallel_tunnels to

1.

SEE ALSO

gre(4), inet(4), inet6(4), ifconfig(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 the WIDE hydrangea IPv6

kit.

BUGS

There are many tunnelling protocol specifications, all de

fined 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 (out

er 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 gener

ate 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 encap

sulated packets may affect communication over the interface. The first

bigger-thanpmtu 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 out

er header into
the inner header.

In the past, gif had a multi-destination behavior, config

urable via
IFF_LINK0 flag. The behavior is obsolete and is no longer

supported.

BSD April 10, 1999

BSD