NetBSD/share/man/man4/gre.4

.\" $NetBSD: gre.4,v 1.14 2001/04/11 18:31:23 wiz Exp $
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Heiko W.Rupp <hwr@pilhuhn.de>
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.Dd September 13, 1998
.Dt GRE 4
.Os
.Sh NAME
.Nm gre
.Nd encapsulating network device
.Sh SYNOPSIS
.Cd pseudo-device gre
.Sh DESCRIPTION
The
.Nm gre
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 ``tunnel'' appears to the inner datagrams like one hop.
.Pp
.Nm
interfaces are dynamically created and destroyed with the
.Xr ifconfig 8
.Cm create
and
.Cm destroy
subcommands.
.Pp
This driver currently supports the following modes of operation:
.Bl -tag -width abc
.It GRE encapsulation (IP protocol number 47) .
Encapsulated datagrams are
prepended by a outer datagram and a GRE header. The GRE header specifies
the type of the encapsulated datagram and thus allows for tunneling other
protocols than IP like e.g. AppleTalk (not yet supported). GRE mode is
also the default tunnel mode on Cisco routers. This is also the default
mode of operation of the
.Sy gre Ns Ar X
interfaces.
.It MOBILE encapsulation (IP protocol number 55) .
Datagrams are
encapsulated into IP, but with a shorter encapsulation. The original
IP header is modified and the modifications are inserted between the
so modified header and the original payload. Like IPIP only for IP in IP
encapsulation.
.El
.Pp
The network interfaces are named
.Sy gre Ns Ar 0 ,
.Sy gre Ns Ar 1
and so on, as many as have given on the
.Sy pseudo-device
line in the system config file. Each interface supports a number of
.Xr ioctl 2 Ns s ,
such as:
.Bl -tag -width aaa
.It GRESADDRS :
Set the IP address of the local tunnel end.
.It GRESADDRD :
Set the IP address of the remote tunnel end.
.It GREGADDRS :
Query the IP address that is set for the local tunnel end.
.It GREGADDRD :
Query the IP address that is set for the remote tunnel end.
.It GRESPROTO :
Set the operation mode to the specified IP protocol value. The
protocol is passed to the interface in (struct ifreq)->ifr_flags.
The operation mode can also
be given as
.Bl -tag -width bbb
.It link0
IPPROTO_GRE
.It link2
IPPROTO_MOBILE
.El
to
.Ic ifconfig .
As the linkN flags are not mutually exclusive, modes must be set by applying
positive and negative flags as e.g.
.Ic ifconfig
link0 -link1 -link2
.It GREGPROTO :
Query operation mode.
.El
.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, as e.g. when
encapsulating AppleTalk.
.Pp
.Sh EXAMPLE
Configuration example:
.Bd -literal
Host X-- Host A  ----------------tunnel---------- cisco D------Host E
          \\                                          |
           \\                                        /
             +------Host B----------Host C----------+

.Ed
   On host A (NetBSD):

   # route add default B
   # ifconfig greN  A D netmask 0xffffffff linkX up
   # greconfig -i greN -s A -d D
   # route add E D

   On Host D (Cisco):

   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

   OR

   On Host D (NetBSD):

   # route add default C
   # ifconfig greN D A
.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:
     ifconfig <etherif> alias Y
 and on the cisco
     ip route Y mask tunnelX
.Sh NOTE
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
.Ic ifconfig
to actually mark the interface as up, the keyword ``up'' must be given
last on its command line.
.Pp
The kernel must be set to forward datagrams by either option
``GATEWAY'' in the kernel config file or by issuing the appropriate
option to
.Xr sysctl 8 .
.Sh SEE ALSO
.Xr atalk 4 ,
.Xr inet 4 ,
.Xr ip 4 ,
.Xr netintro 4 ,
.Xr options 4 ,
.Xr protocols 5 ,
.Xr greconfig 8 ,
.Xr ifconfig 8 ,
.Xr sysctl 8
.Pp
A description of GRE encapsulation can be found in RFC 1701, RFC 1702.
.Pp
A description of MOBILE encapsulation can be found in RFC 2004.
.Sh BUGS
The compute_route() code in if_gre.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
To avoid the address munging described above, turn on the link1 flag
on the ifconfig command line.  This implies that the GRE packet
destination (set via greconfig -d) and the ifconfig remote host are
not the same IP addresses, and that the GRE destination does not route
over the greX interface itself.
.Pp
GRE RFC not yet fully implemented (no GRE options), no other protocols
yet than IP over IP.
.Pp
Traceroute does not work yet over the tunnel :(
.Pp
BPF does probably not yet work (it might, but last time I looked,
it bombed, so I #if 0'd it out).
.Pp
.Sh AUTHOR
Heiko W.Rupp <hwr@pilhuhn.de>