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.Dd $Mdocdate: July 11 2018 $
.Dt GIF 4
.Os
.Sh NAME
.Nm gif
.Nd generic tunnel interface
.Sh SYNOPSIS
.Cd "pseudo-device gif"
.Sh DESCRIPTION
The
.Nm
interface is a generic tunnelling pseudo-device.
It can tunnel IPv4, IPv6, and MPLS over IPv4 and IPv6,
with behavior mainly based on RFC 4213 IPv6-over-IPv4.
.Pp
A
.Nm
interface can be created at runtime using the
.Ic ifconfig gif Ns Ar N Ic create
command or by setting up a
.Xr hostname.if 5
configuration file for
.Xr netstart 8 .
.Pp
The
.Nm
interface must be configured with the
addresses used for the outer header.
This can be done by using
.Xr ifconfig 8 Ns 's
.Ic tunnel
command (which uses the
.Dv SIOCSLIFPHYADDR
ioctl).
.Pp
The addresses of the inner header must be configured by using
.Xr ifconfig 8
in the normal way.
The routing table can be used to direct packets toward the
.Nm
interface.
.Sh SEE ALSO
.Xr sysctl 2 ,
.Xr etherip 4 ,
.Xr gre 4 ,
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr ipsec 4 ,
.Xr hostname.if 5 ,
.Xr ifconfig 8 ,
.Xr netstart 8
.Sh STANDARDS
.Rs
.%A E. Nordmark
.%A R. Gilligan
.%D October 2005
.%R RFC 4213
.%T Basic Transition Mechanisms for IPv6 Hosts and Routers
.Re
.Pp
.Rs
.%A T. Worster
.%A Y. Rekhter
.%A E. Rosen
.%D March 2005
.%R RFC 4023
.%T Encapsulating MPLS in IP or Generic Routing Encapsulation (GRE)
.Re
.Sh HISTORY
The
.Nm
device first appeared in WIDE hydrangea IPv6 kit.
.Pp
Previously,
.Nm
supported RFC 3378 EtherIP tunnels over
.Xr bridge 4
interfaces.
This is now handled by
.Xr etherip 4 .
.Sh BUGS
There are many tunnelling protocol specifications,
defined differently from each other.
.Nm
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
The current code does not check if the ingress address
.Pq outer source address
configured to
.Nm
makes sense.
Make sure to configure an address which belongs to your node.
Otherwise, your node will not be able to receive packets from the peer,
and your node will generate packets with a spoofed source address.
.Pp
If the outer protocol is IPv6, path MTU discovery for encapsulated packet
may affect communication over the interface.