NAME
gif —
generic tunnel interface
SYNOPSIS
pseudo-device gif
DESCRIPTION
Thegif interface is a generic tunnelling 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. On NetBSD,
gif can also tunnel ISO traffic over IPv[46] using EON
encapsulation.
Each gif interface is created at runtime
using interface cloning. This is most easily done with the
“ifconfig
create” command or using the
gifconfig_⟨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
gifconfig(8), 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 be
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 pseudo-device can be configured to
be ECN friendly. This can be configured by
IFF_LINK1.
ECN friendly behavior
The gif pseudo-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 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 to0. - 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.
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 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. Additionally, gif
tunnels are restricted to one per pair of end points. Parallel tunnels may
be enabled by setting the
sysctl(8) variable
net.link.gif.parallel_tunnels to 1.
SEE ALSO
inet(4), inet6(4), gifconfig(8)
R. Gilligan and E. Nordmark, Transition Mechanisms for IPv6 Hosts and Routers, RFC 2893, 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 defined
differently from each other. The gif pseudo-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 pseudo-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.
It is thought that this is not actually a bug in gif, but rather lies somewhere around a manipulation of an IPv6 routing table.