|GIF(4)||Device Drivers Manual||GIF(4)|
gif — generic
gif interface is a generic tunnelling
device for IPv4 and IPv6. It can tunnel IPv traffic over IPv.
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 using
EON encapsulation. Note that
gif does not perform
GRE encapsulation; use gre(4) for GRE encapsulation.
gif interface is created at runtime
using interface cloning. This is most easily done with the
create” command or using the
variable in rc.conf(5).
gif, the administrator needs to
configure the protocol and addresses used for the outer header. This can be
done by using ifconfig(8)
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 device can be configured to be ECN
friendly. This can be configured by
ECN friendly behavior¶
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
gif will show normal behavior, as described in
RFC2893. This can be summarized as follows:
- Set outer TOS bit to
- Drop outer TOS bit.
gif will copy ECN bits (
0x01 on IPv4 TOS byte or IPv6 traffic class
byte) on egress and ingress, as follows:
- Copy TOS bits except for ECN CE (masked with
0xfe) from inner to outer. Set ECN CE bit to
- 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,
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
gif tunnels may not be nested.
This behavior may be modified at runtime by setting the
net.link.gif.max_nesting to the desired level of
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.
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
If the outer protocol is IPv4,
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.
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.
|October 21, 2018||Debian|