.\" $NetBSD: ipip.4,v 1.8 2001/09/22 00:57:41 wiz Exp $
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.\" by Heiko W.Rupp <hwr@pilhuhn.de>
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.Dd March 28, 1999
.Dt IPIP 4
.Os
.Sh NAME
.Nm ipip
.Nd encapsulating network device
.Sh SYNOPSIS
.Cd pseudo-device ipip Op Ar count
.Sh DESCRIPTION
The
.Nm ipip
network interface is a pseudo device that allows to encapsulate datagrams
into IP. These encapsulated datagrams are routed to a destination host,
where they are decapsulated and further routed to their final destination.
The so called
.Dq tunnel
appears to the inner datagrams like one hop.
Encapsulation is according to RFC 2003. The outer datagram header is of
IP type 4 (IPIP).
.Pp
The network interfaces are named
.Sy ipip Ns Ar 0 ,
.Sy ipip Ns Ar 1
and so on, as many as have given on the
.Sy pseudo-device
line in the system config file.
.Pp
Note that the IP addresses of the tunnel endpoints may be the same as the
ones defined with
.Ic ifconfig
for the interface (as if IP is encapsulated), but need not be.
.Sh EXAMPLES
Configuration example:
.Bd -literal
Host X-- Host A  ----------------tunnel---------- cisco D------Host E
          \\                                          |
           \\                                        /
             +------Host B----------Host C----------+
.Ed
On host A
.Ns ( Nx ) :
.Bd -literal
   # route add default B
   # ifconfig ipipN  A D netmask 0xffffffff  up
   # route add E D
.Ed
.Pp
On Host D (Cisco):
.Bd -literal
   Interface TunnelX
    ip unnumbered D   ! e.g. address from Ethernet interface
    tunnel source D   ! e.g. address from Ethernet interface
    tunnel destination A
   ip route C <some interface and mask>
   ip route A mask C
   ip route X mask tunnelX
.Ed
.Pp
or on Host D
.Ns ( Nx ) :
.Bd -literal
   # route add default C
   # ifconfig ipipN D A
.Ed
.Pp
If all goes well, you should see packets flowing ;-)
.Pp
If you want to reach Host A over the tunnel (from the Cisco D), then
you have to have an alias on Host A for e.g. the Ethernet interface like:
.Ic ifconfig Ar <etherif> alias Y
and on the cisco
.Ic ip Ar route Y mask tunnelX .
.Sh NOTES
For correct operation, the
.Nm
device needs a route to the destination, that is less specific than the
one over the tunnel.
(Basically, there needs to be a route to the decapsulating host that
does not run over the tunnel, as this would be a loop ..)
.Pp
In order to get
.Ic ifconfig
to actually mark the interface as up, the keyword
.Ar up
must be given last on its command line.
.Pp
The kernel must be set to forward datagrams by either option
.Dq GATEWAY
in the kernel config file or by issuing the appropriate option to
.Xr sysctl 8 .
.Sh SEE ALSO
.Xr atalk 4 ,
.Xr gre 4 ,
.Xr inet 4 ,
.Xr ip 4 ,
.Xr netintro 4 ,
.Xr options 4 ,
.Xr protocols 5 ,
.Xr ifconfig 8 ,
.Xr sysctl 8
.Pp
A description of IPIP encapsulation can be found in RFC 2003.
.Sh BUGS
The ipip_compute_route() code in ip_ipip.c toggles the last bit of the
IP-address to provoke the search for a less specific route than the
one directly over the tunnel to prevent loops. This is possibly not
the best solution.
.Pp
Traceroute does not work yet over the tunnel.