.\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $
.\"  $FreeBSD: src/share/man/man4/gre.4,v 1.1.2.1 2002/12/01 14:03:11 sobomax Exp $
.\"  $DragonFly: src/share/man/man4/gre.4,v 1.3 2007/07/29 17:27:45 swildner Exp $
.\"
.\" Copyright 1998 (c) The NetBSD Foundation, Inc.
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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 June 9, 2002
.Dt GRE 4
.Os
.Sh NAME
.Nm gre
.Nd encapsulating network device
.Sh SYNOPSIS
.Cd pseudo-device gre
.Sh DESCRIPTION
The
.Nm
network interface pseudo device encapsulates datagrams
into IP.  These encapsulated datagrams are routed to a destination host,
where they are decapsulated and further routed to their final destination.
The
.Dq tunnel
appears to the inner datagrams as 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 an 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.  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
.Xr gif 4 ,
only for IP in IP encapsulation.
.El
.Pp
The
.Sy gre Ns Ar X
interfaces support 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.  This is the source address
set by or displayed by ifconfig for the
.Sy gre Ns Ar X
interface.
.It GRESADDRD :
Set the IP address of the remote tunnel end.  This is the destination address
set by or displayed by ifconfig for the
.Sy gre Ns Ar X
interface.
.It GREGADDRS :
Query the IP address that is set for the local tunnel end.  This is the
address the encapsulation header carries as local address (i.e. the real
address of the tunnel start point.)
.It GREGADDRD :
Query the IP address that is set for the remote tunnel end.  This is the
address the encapsulated packets are sent to (i.e. the real address of
the remote tunnel endpoint.)
.It GRESPROTO :
Set the operation mode to the specified IP protocol value.  The
protocol is passed to the interface in (struct ifreq)-\*[Gt]ifr_flags.
The operation mode can also be given as
.Bl -tag -width link0xxx
.It link0
IPPROTO_GRE
.It -link0
IPPROTO_MOBILE
.El
.Pp
to
.Xr ifconfig 8 .
.Pp
The link1 flag is not used to choose encapsulation, but to modify the
internal route search for the remote tunnel endpoint, see the
.Sx BUGS
section below.
.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
.Xr ifconfig 8
for the interface (as if IP is encapsulated), but need not be, as e.g. when
encapsulating AppleTalk.
.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 greN create
   # ifconfig greN A D netmask 0xffffffff linkX up
   # ifconfig greN tunnel A D
   # route add E D
.Ed
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 \*[Lt]some interface and mask\*[Gt]
   ip route A mask C
   ip route X mask tunnelX
.Ed
OR
On Host D
.Ns ( Nx ) :
.Bd -literal
   # route add default C
   # ifconfig greN create
   # ifconfig greN D A
   # ifconfig tunnel greN 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 Host D (Cisco)), then
you have to have an alias on Host A for e.g. the Ethernet interface like:
.Bd -literal
     ifconfig \*[Lt]etherif\*[Gt] alias Y
.Ed
and on the cisco
.Bd -literal
     ip route Y mask tunnelX
.Ed
.Pp
A similar setup can be used to create a link between two private networks
(for example in the 192.168 subnet) over the Internet:
.Bd -literal
192.168.1.* --- Router A  -------tunnel-------- Router B --- 192.168.2.*
                   \\                              /
                    \\                            /
                      +----- the Internet ------+
.Ed
Assuming router A has the (external) IP address A and the internal address
192.168.1.1, while router B has external address B and internal address
192.168.2.1, the following commands will configure the tunnel:
.Pp
On router A:
.Bd -literal
   # ifconfig greN create
   # ifconfig greN 192.168.1.1 192.168.2.1 link1
   # ifconfig greN tunnel A B
   # route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1
.Ed
.Pp
On router B:
.Bd -literal
   # ifconfig greN create
   # ifconfig greN 192.168.2.1 192.168.1.1 link1
   # ifconfig greN tunnel B A
   # route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1
.Ed
.Pp
Note that this is a safe situation where the link1 flag (as discussed in the
.Sx BUGS
section below) may (and probably should) be set.
.Sh NOTES
The MTU of
.Sy gre Ns Ar X
interfaces is set to 1476 by default to match the value used by Cisco routers.
This may not be an optimal value, depending on the link between the two tunnel
endpoints.  It can be adjusted via
.Xr ifconfig 8 .
.Pp
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.)
If the addresses are ambiguous, doing the
.Xr ifconfig 8
.Li tunnel
step before the
.Xr ifconfig 8
call to set the
.Sy gre Ns Ar X
IP addresses will help to find a route outside the tunnel.
.Pp
In order to tell
.Xr ifconfig 8
to actually mark the interface as up, the keyword
.Dq up
must be given last on its command line.
.Pp
The kernel must be set to forward datagrams by either option
.Em GATEWAY
in the kernel config file or by issuing the appropriate option to
.Xr sysctl 8 .
.Sh SEE ALSO
.Xr atalk 4 ,
.Xr gif 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 GRE encapsulation can be found in RFC 1701 and RFC 1702.
.Pp
A description of MOBILE encapsulation can be found in RFC 2004.
.Sh AUTHORS
.An Heiko W.Rupp Aq hwr@pilhuhn.de
.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
.Xr ifconfig 8
command line.
This implies that the GRE packet destination and the ifconfig remote host
are not the same IP addresses, and that the GRE destination does not route
over the
.Sy gre Ns Ar X
interface itself.
.Pp
The GRE RFCs are not yet fully implemented (no GRE options).