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.Dd April 10, 1999
.Dt GIF 4
.Os
.Sh NAME
.Nm gif
.Nd
.Tn Generic tunnel interface
.Sh SYNOPSIS
.Cd "pseudo-device gif" Op Ar count
.Sh DESCRIPTION
The
.Nm
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
.Nm
is mainly based on RFC1933 IPv6-over-IPv4 configured tunnel.
.Pp
To use
.Nm ,
administrator needs to configure protocol and addresses used for the outer
header.
This can be done by using
.Xr gifconfig 8 ,
or
.Dv SIOCSIFPHYADDR
ioctl.
Also, administrator needs to configure protocol and addresses used for the
inner header, by using
.Xr ifconfig 8 .
Note that IPv6 link-local address
.Pq those start with Li fe80::
will be automatically configured whenever possible.
You may need to remove IPv6 link-local address manually using
.Xr ifconfig 8 ,
when you would like to disable the use of IPv6 as inner header
.Pq like when you need pure IPv4-over-IPv6 tunnel .
Finally, use routing table to route the packets toward
.Nm
interface.
.Pp
.Nm
interface can be configued to perform bidirectional tunnel, or
multi-destination tunnel.
This is controlled by
.Dv IFF_LINK0
interface flag.
Also,
.Nm
can be configured to be ECN friendly.
This can be configured by
.Dv IFF_LINK1 .
.\"
.Ss Bidirectional and multi-destination mode
Usually,
.Nm
implements bidirectional tunnel.
.Xr gifconfig 8
should configure a tunnel ingress point
.Pq this node
and an egress point
.Pq tunnel endpoint ,
and
one
.Nm
interface will tunnel to only a single tunnel endpoint,
and accept from only a single tunnel endpoint.
Source and destination address for outer IP header is always the
ingress and the egress point configued by
.Xr gifconfig 8 .
.Pp
With
.Dv IFF_LINK0
interface flag,
.Nm
can be configured to implement multi-destination tunnel.
With
.Dv IFF_LINK0 ,
it is able to configure egress point to IPv4 wildcard address
.Pq Li 0.0.0.0
or IPv6 unspecified address
.Pq Li 0::0 .
In this case, destination address for the outer IP header is
determined based on the routing table setup.
Therefore, one
.Nm
interface can tunnel to multiple destinations.
Also,
.Nm
will accept tunneled traffic from any outer source address.
.Pp
When finding a
.Nm
interface from the inbound tunneled traffic,
bidirectional mode interface is preferred than multi-destination mode interface.
For example, if you have the following three
.Nm
interfaces on node A, tunneled traffic from C to A will match the second
.Nm
interface, not the third one.
.Bl -bullet -compact -offset indent
.It
bidirectional, A to B
.It
bidirectional, A to C
.It
multi-destination, A to any
.El
.Pp
Please note that multi-destination mode is far less secure
than bidirectional mode.
Multi-destination mode
.Nm
can accept tunneled packet from anybody,
and can be attacked from a malicious node.
.Pp
.Ss ECN friendly behavior
.Nm
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
.Dv IFF_LINK1
interface flag.
.Pp
Without
.Dv IFF_LINK1 ,
.Nm
will show a normal behavior, like described in RFC1933.
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
.Po
.Dv 0x02
and
.Dv 0x01
on IPv4 TOS byte or IPv6 traffic class byte
.Pc
on egress and ingress, as follows:
.Bl -tag -width "Ingress" -offset indent
.It Ingress
Copy TOS bits except for ECN CE
.Po
masked with
.Dv 0xfe
.Pc
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 RFC1933.
This should be used in mutual agreement with the peer.
.Pp
.Ss Security
Malicious party may try to circumvent security filters by using
tunnelled packets.
For better protection,
.Nm
performs martian filter and ingress filter against outer source address,
on egress.
Note that martian/ingress filters are no way complete.
You may want to secure your node by using packet filters.
.Pp
As mentioned above, multi-destination mode
.Pq Dv IFF_LINK0
is far less secure than bidirectional mode.
.\"
.Sh SEE ALSO
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr gifconfig 8
.Rs
.%A	R. Gilligan
.%A	E. Nordmark
.%B	RFC1933
.%T	Transition Mechanisms for IPv6 Hosts and Routers
.%D	April 1996
.%O	ftp://ftp.isi.edu/in-notes/rfc1933.txt
.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 WIDE hydrangea IPv6 kit.
.\"
.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
.Xr gif 4
is an
.Dv IFF_POINTOPOINT
device, however, it supports NBMA behavior in multi-destination mode.