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.
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.
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.
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.
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.
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.