280 lines
9.0 KiB
Groff
280 lines
9.0 KiB
Groff
.\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $
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.\" $FreeBSD$
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.\"
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.\" Copyright 1998 (c) The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Heiko W.Rupp <hwr@pilhuhn.de>
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of the The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd June 9, 2002
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.Dt GRE 4
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.Os
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.Sh NAME
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.Nm gre
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.Nd encapsulating network device
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.Sh SYNOPSIS
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.Cd pseudo-device gre
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.Sh DESCRIPTION
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The
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.Nm gre
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network interface pseudo device encapsulates datagrams
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into IP. These encapsulated datagrams are routed to a destination host,
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where they are decapsulated and further routed to their final destination.
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The
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.Dq tunnel
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appears to the inner datagrams as one hop.
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.Pp
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.Nm
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interfaces are dynamically created and destroyed with the
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.Xr ifconfig 8
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.Cm create
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and
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.Cm destroy
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subcommands.
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.Pp
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This driver currently supports the following modes of operation:
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.Bl -tag -width abc
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.It GRE encapsulation (IP protocol number 47)
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Encapsulated datagrams are
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prepended an outer datagram and a GRE header. The GRE header specifies
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the type of the encapsulated datagram and thus allows for tunneling other
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protocols than IP like e.g. AppleTalk. GRE mode is also the default tunnel
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mode on Cisco routers. This is also the default mode of operation of the
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.Sy gre Ns Ar X
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interfaces.
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.It MOBILE encapsulation (IP protocol number 55)
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Datagrams are
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encapsulated into IP, but with a shorter encapsulation. The original
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IP header is modified and the modifications are inserted between the
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so modified header and the original payload. Like
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.Xr gif 4 ,
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only for IP in IP encapsulation.
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.El
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.Pp
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The
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.Sy gre Ns Ar X
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interfaces support a number of
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.Xr ioctl 2 Ns s ,
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such as:
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.Bl -tag -width aaa
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.It GRESADDRS :
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Set the IP address of the local tunnel end. This is the source address
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set by or displayed by ifconfig for the
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.Sy gre Ns Ar X
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interface.
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.It GRESADDRD :
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Set the IP address of the remote tunnel end. This is the destination address
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set by or displayed by ifconfig for the
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.Sy gre Ns Ar X
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interface.
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.It GREGADDRS :
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Query the IP address that is set for the local tunnel end. This is the
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address the encapsulation header carries as local address (i.e. the real
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address of the tunnel start point.)
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.It GREGADDRD :
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Query the IP address that is set for the remote tunnel end. This is the
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address the encapsulated packets are sent to (i.e. the real address of
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the remote tunnel endpoint.)
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.It GRESPROTO :
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Set the operation mode to the specified IP protocol value. The
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protocol is passed to the interface in (struct ifreq)-\*[Gt]ifr_flags.
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The operation mode can also be given as
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.Bl -tag -width link0xxx
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.It link0
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IPPROTO_GRE
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.It -link0
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IPPROTO_MOBILE
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.El
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.Pp
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to
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.Xr ifconfig 8 .
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.Pp
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The link1 flag is not used to choose encapsulation, but to modify the
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internal route search for the remote tunnel endpoint, see the
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.Sx BUGS
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section below.
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.It GREGPROTO :
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Query operation mode.
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.El
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.Pp
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Note that the IP addresses of the tunnel endpoints may be the same as the
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ones defined with
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.Xr ifconfig 8
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for the interface (as if IP is encapsulated), but need not be, as e.g. when
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encapsulating AppleTalk.
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.Sh EXAMPLES
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Configuration example:
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.Bd -literal
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Host X-- Host A ----------------tunnel---------- cisco D------Host E
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\\ |
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\\ /
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+------Host B----------Host C----------+
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.Ed
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On host A
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.Ns ( Nx ) :
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.Bd -literal
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# route add default B
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# ifconfig greN create
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# ifconfig greN A D netmask 0xffffffff linkX up
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# ifconfig greN tunnel A D
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# route add E D
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.Ed
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On Host D (Cisco):
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.Bd -literal
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Interface TunnelX
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ip unnumbered D ! e.g. address from Ethernet interface
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tunnel source D ! e.g. address from Ethernet interface
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tunnel destination A
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ip route C \*[Lt]some interface and mask\*[Gt]
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ip route A mask C
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ip route X mask tunnelX
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.Ed
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OR
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On Host D
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.Ns ( Nx ) :
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.Bd -literal
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# route add default C
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# ifconfig greN create
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# ifconfig greN D A
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# ifconfig tunnel greN D A
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.Ed
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.Pp
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If all goes well, you should see packets flowing ;-)
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.Pp
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If you want to reach Host A over the tunnel (from Host D (Cisco)), then
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you have to have an alias on Host A for e.g. the Ethernet interface like:
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.Bd -literal
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ifconfig \*[Lt]etherif\*[Gt] alias Y
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.Ed
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and on the cisco
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.Bd -literal
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ip route Y mask tunnelX
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.Ed
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.Pp
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A similar setup can be used to create a link between two private networks
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(for example in the 192.168 subnet) over the Internet:
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.Bd -literal
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192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.*
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\\ /
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\\ /
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+----- the Internet ------+
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.Ed
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Assuming router A has the (external) IP address A and the internal address
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192.168.1.1, while router B has external address B and internal address
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192.168.2.1, the following commands will configure the tunnel:
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.Pp
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On router A:
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.Bd -literal
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# ifconfig greN create
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# ifconfig greN 192.168.1.1 192.168.2.1 link1
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# ifconfig greN tunnel A B
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# route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1
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.Ed
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.Pp
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On router B:
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.Bd -literal
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# ifconfig greN create
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# ifconfig greN 192.168.2.1 192.168.1.1 link1
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# ifconfig greN tunnel B A
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# route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1
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.Ed
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.Pp
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Note that this is a safe situation where the link1 flag (as discussed in the
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.Sx BUGS
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section below) may (and probably should) be set.
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.Sh NOTES
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The MTU of
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.Sy gre Ns Ar X
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interfaces is set to 1476 by default to match the value used by Cisco routers.
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This may not be an optimal value, depending on the link between the two tunnel
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endpoints. It can be adjusted via
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.Xr ifconfig 8 .
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.Pp
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For correct operation, the
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.Nm
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device needs a route to the destination that is less specific than the
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one over the tunnel.
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(Basically, there needs to be a route to the decapsulating host that
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does not run over the tunnel, as this would be a loop.)
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If the addresses are ambiguous, doing the
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.Xr ifconfig 8
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.Li tunnel
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step before the
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.Xr ifconfig 8
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call to set the
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.Sy gre Ns Ar X
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IP addresses will help to find a route outside the tunnel.
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.Pp
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In order to tell
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.Xr ifconfig 8
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to actually mark the interface as up, the keyword
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.Dq up
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must be given last on its command line.
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.Pp
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The kernel must be set to forward datagrams by either option
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.Em GATEWAY
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in the kernel config file or by issuing the appropriate option to
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.Xr sysctl 8 .
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.Sh SEE ALSO
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.Xr atalk 4 ,
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.Xr gif 4 ,
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.Xr inet 4 ,
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.Xr ip 4 ,
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.Xr netintro 4 ,
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.Xr options 4 ,
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.Xr protocols 5 ,
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.Xr ifconfig 8 ,
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.Xr sysctl 8
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.Pp
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A description of GRE encapsulation can be found in RFC 1701 and RFC 1702.
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.Pp
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A description of MOBILE encapsulation can be found in RFC 2004.
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.Sh AUTHORS
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.An Heiko W.Rupp Aq hwr@pilhuhn.de
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.Sh BUGS
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The compute_route() code in if_gre.c toggles the last bit of the
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IP-address to provoke the search for a less specific route than the
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one directly over the tunnel to prevent loops. This is possibly not
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the best solution.
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.Pp
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To avoid the address munging described above, turn on the link1 flag
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on the
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.Xr ifconfig 8
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command line.
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This implies that the GRE packet destination and the ifconfig remote host
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are not the same IP addresses, and that the GRE destination does not route
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over the
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.Sy gre Ns Ar X
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interface itself.
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.Pp
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The GRE RFCs are not yet fully implemented (no GRE options).
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