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.Dd October 21, 2018
.Dt GIF 4
.Os
.Sh NAME
.Nm gif
.Nd generic tunnel interface
.Sh SYNOPSIS
.Cd "device gif"
.Sh DESCRIPTION
The
.Nm
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
.Nm
is mainly based on RFC2893 IPv6-over-IPv4 configured tunnel.
On
.Nx ,
.Nm
can also tunnel ISO traffic over IPv[46] using EON encapsulation.
Note that
.Nm
does not perform GRE encapsulation; use
.Xr gre 4
for GRE encapsulation.
.Pp
Each
.Nm
interface is created at runtime using interface cloning.
This is
most easily done with the
.Dq Nm ifconfig Cm create
command or using the
.Va ifconfig_ Ns Aq Ar interface
variable in
.Xr rc.conf 5 .
.Pp
To use
.Nm ,
the administrator needs to configure the protocol and addresses used for the outer
header.
This can be done by using
.Xr ifconfig 8
.Cm tunnel ,
or
.Dv SIOCSIFPHYADDR
ioctl.
The administrator also needs to configure the protocol and addresses for the
inner header, with
.Xr ifconfig 8 .
Note that IPv6 link-local addresses
(those that start with
.Li fe80:: )
will be automatically configured whenever possible.
You may need to remove IPv6 link-local addresses manually using
.Xr 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
.Nm
interface.
.Pp
The
.Nm
device can be configured to be ECN friendly.
This can be configured by
.Dv IFF_LINK1 .
.Ss ECN friendly behavior
The
.Nm
device can be configured to be ECN friendly, as described in
.Dv draft-ietf-ipsec-ecn-02.txt .
This is turned off by default, and can be turned on by the
.Dv IFF_LINK1
interface flag.
.Pp
Without
.Dv IFF_LINK1 ,
.Nm
will show normal behavior, as described in RFC2893.
This can be summarized as follows:
.Bl -tag -width "Ingress" -offset indent
.It Ingress
Set outer TOS bit to
.Dv 0 .
.It Egress
Drop outer TOS bit.
.El
.Pp
With
.Dv IFF_LINK1 ,
.Nm
will copy ECN bits
.Dv ( 0x02
and
.Dv 0x01
on IPv4 TOS byte or IPv6 traffic class byte)
on egress and ingress, as follows:
.Bl -tag -width "Ingress" -offset indent
.It Ingress
Copy TOS bits except for ECN CE
(masked with
.Dv 0xfe )
from
inner to outer.
Set ECN CE bit to
.Dv 0 .
.It Egress
Use inner TOS bits with some change.
If outer ECN CE bit is
.Dv 1 ,
enable ECN CE bit on the inner.
.El
.Pp
Note that the ECN friendly behavior violates RFC2893.
This should be used in mutual agreement with the peer.
.Ss Security
A malicious party may try to circumvent security filters by using
tunnelled packets.
For better protection,
.Nm
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
.Dv IFF_LINK2
bit.
.Ss Miscellaneous
By default,
.Nm
tunnels may not be nested.
This behavior may be modified at runtime by setting the
.Xr sysctl 8
variable
.Va net.link.gif.max_nesting
to the desired level of nesting.
.Sh SEE ALSO
.Xr gre 4 ,
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr ifconfig 8
.Rs
.%A R. Gilligan
.%A E. Nordmark
.%B RFC2893
.%T Transition Mechanisms for IPv6 Hosts and Routers
.%D August 2000
.%U http://tools.ietf.org/html/rfc2893
.Re
.Rs
.%A Sally Floyd
.%A David L. Black
.%A K. K. Ramakrishnan
.%T "IPsec Interactions with ECN"
.%D December 1999
.%O draft-ietf-ipsec-ecn-02.txt
.Re
.\"
.Sh HISTORY
The
.Nm
device first appeared in the WIDE hydrangea IPv6 kit.
.\"
.Sh BUGS
There are many tunnelling protocol specifications, all
defined differently from each other.
The
.Nm
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
.Nm
to talk with IPsec devices that use IPsec tunnel mode.
.Pp
If the outer protocol is IPv4,
.Nm
does not try to perform path MTU discovery for the encapsulated packet
(DF bit is set to 0).
.Pp
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
.Nm
to 1240 or smaller, when the outer header is IPv6 and the inner header is IPv4.
.Pp
The
.Nm
device does not translate ICMP messages for the outer header into the inner header.
.Pp
In the past,
.Nm
had a multi-destination behavior, configurable via
.Dv IFF_LINK0
flag.
The behavior is obsolete and is no longer supported.