GIF(4) FreeBSD Kernel Interfaces Manual 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.
The gif interface can also tunnel Ethernet traffic over IPv4 or IPv6 when
combined with a if_bridge(4) interface using EtherIP protocol. See
if_bridge(4) for detailed setup.
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.
MTU Configuration and Path MTU Discovery
The gif interface uses the fixed length, 1280, to determine whether the
outgoing IPv6 packets are split. This means the MTU value configured on
the interface will be ignored when the outer protocol is IPv6. When the
NOCLAMP interface flag is set, gif uses the same configured value as IPv4
communications. This behavior prevents potential issues when the path
MTU is smaller than the interface MTU. This section describes the reason
why the default behavior is different. The NOCLAMP interface flag can be
set using the following command:
ifconfig gif0 noclamp
and clear the flag using the following:
ifconfig gif0 -noclamp
where gif0 is the actual interface name.
A tunnel interface always has an implicit smaller MTU for the inner
protocol than the outer protocol because of the additional header. Note
that the interface MTU on a gif interface, the default value is 1280, is
used as MTU for the outer protocol. This means that the MTU for the
inner protocol varies depending on the outer protocol header length. If
an outgoing packet bigger than the inner protocol MTU arrives at a gif
interface for encapsulation, it will be split into fragments.
Specifically, if IPv4 is used as the outer protocol, the inner is 20
octets smaller than the interface MTU. In the case of the default
interface MTU, 1280, inner packets bigger than 1260 will be fragmented.
In the case of IPv6, the inner is 40 octets smaller than the outer.
This fragmentation is not harmful though it can degrade the performance.
Note that while an increased MTU on gif interface helps to mitigate this
reduced performance issue, it can also cause packet losses on the
intermediate narrowest path between the two communication endpoints in
IPv6. IPv6 allows fragmentation only on the sender, not on the routers
in the communication path. A big outgoing packet will be dropped on a
router with a smaller MTU.
In normal IPv6 communication, an ICMPv6 Packet Too Big error will be sent
back to the sender, who can adjust the packet length and re-send it.
This process is performed in the upper protocols than L3, such as TCP,
and makes the packet length shorter so that packets go through the path
without fragmentation. This behavior is known as path MTU discovery.
When using a gif interface, the Packet Too Big message is generated for
the outer protocol. Since the gif interface does not translate this
error to the inner protocol, the inner protocol sees it just as a packet
loss with no useful information to adjust the length of the next packets.
In this situation, path MTU discovery does not work, and communications
of the inner protocol become stalled.
In order to avoid this, a gif interface silently splits a packet of over
1240 octets into fragments to make the outer protocol packets equal or
shorter than 1280 octets, even when the interface MTU is configured as
larger than 1280. Note that this occurs only when the outer protocol is
IPv6. 1280 is the smallest MTU in IPv6 and guarantees no packet loss
occurs on intermediate routers.
As mentioned earlier, the performance is sub-optimal if the actual path
MTU is larger than 1280. A typical confusing scenario is as follows.
The gif interface can have Ethernet, whose MTU is usually 1500, as the
inner protocol. It is called an EtherIP tunnel, and can be configured by
adding the gif interface as a member of if_bridge(4) interface. The
if_bridge(4) interface forcibly changes the MTU of the gif interface with
those for the other member interfaces, which are likely 1500. In this
case, a situation in which the MTU of the gif interface is 1500 but
fragmentation in 1280 octets always occurs.
The default behavior is most conservative to prevent confusing packet
loss. Depending on the network configuration, enabling the NOCLAMP
interface flag might be helpful for better performance. It is crucial to
ensure that the path MTU is equal to or larger than the interface MTU
when enabling this flag.
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), if_bridge(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.
R. Housley and S. Hollenbeck, EtherIP: Tunneling Ethernet Frames in IP
Datagrams, RFC 3378, September 2002.
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
NOCLAMP flag. The behavior is obsolete and is no longer supported. This
flag is now used to determine whether performing fragmentation when the
outer protocol is IPv6.
FreeBSD 15.1-STABLE-HBSD August 27, 2025 GIF(4)