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+.\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $
+.\"
+.\" Copyright 1998 (c) The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" This code is derived from software contributed to The NetBSD Foundation
+.\" by Heiko W.Rupp <hwr@pilhuhn.de>
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\" notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\" notice, this list of conditions and the following disclaimer in the
+.\" documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\" must display the following acknowledgement:
+.\" This product includes software developed by the NetBSD
+.\" Foundation, Inc. and its contributors.
+.\" 4. Neither the name of the The NetBSD Foundation nor the names of its
+.\" contributors may be used to endorse or promote products derived
+.\" from this software without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd June 20, 2008
+.Dt GRE 4
+.Os
+.Sh NAME
+.Nm gre
+.Nd encapsulating network device
+.Sh SYNOPSIS
+To compile the
+.Ns Nm
+device into the kernel, place the following line in the kernel
+configuration file:
+.Bd -ragged -offset indent
+.Cd "device gre"
+.Ed
+.Pp
+Alternatively, to load the
+.Ns Nm
+device as a module at boot time, place the following line in
+.Xr loader.conf 5 :
+.Bd -literal -offset indent
+if_gre_load="YES"
+.Ed
+.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 indent
+.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
+.Nm
+interfaces.
+As part of the GRE mode,
+.Nm
+also supports Cisco WCCP protocol, both version 1 and version 2.
+Since there is no reliable way to distinguish between WCCP versions, it
+should be configured manually using the
+.Cm link2
+flag.
+If the
+.Cm link2
+flag is not set (default), then WCCP version 1 is selected.
+.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
+.Nm
+interfaces support a number of
+.Xr ioctl 2 Ns s ,
+such as:
+.Bl -tag -width ".Dv GRESADDRS"
+.It Dv GRESADDRS
+Set the IP address of the local tunnel end.
+This is the source address
+set by or displayed by
+.Xr ifconfig 8
+for the
+.Nm
+interface.
+.It Dv GRESADDRD
+Set the IP address of the remote tunnel end.
+This is the destination address
+set by or displayed by
+.Xr ifconfig 8
+for the
+.Nm
+interface.
+.It Dv 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 Dv 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 Dv GRESPROTO
+Set the operation mode to the specified IP protocol value.
+The
+protocol is passed to the interface in
+.Po Vt "struct ifreq" Pc Ns Li -> Ns Va ifr_flags .
+The operation mode can also be given as
+.Pp
+.Bl -tag -width ".Cm -link0" -compact
+.It Cm link0
+.Dv IPPROTO_GRE
+.It Cm -link0
+.Dv IPPROTO_MOBILE
+.El
+.Pp
+to
+.Xr ifconfig 8 .
+.Pp
+The
+.Cm 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 Dv GREGPROTO
+Query operation mode.
+.It Dv GRESKEY
+Set the GRE key used for outgoing packets.
+A value of 0 disables the key option.
+.It Dv GREGKEY
+Get the GRE key currently used for outgoing packets.
+0 means no outgoing key.
+.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
+.Pp
+On host A
+.Pq Fx :
+.Bd -literal -offset indent
+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
+.Pp
+On Host D (Cisco):
+.Bd -literal -offset indent
+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
+.Pp
+On Host D
+.Pq Fx :
+.Bd -literal -offset indent
+route add default C
+ifconfig greN create
+ifconfig greN D A
+ifconfig greN tunnel 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:
+.Pp
+.Dl "ifconfig <etherif> alias Y"
+.Pp
+and on the Cisco:
+.Pp
+.Dl "ip route Y mask tunnelX"
+.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
+.Pp
+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 -offset indent
+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 -offset indent
+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
+.Cm link1
+flag (as discussed in the
+.Sx BUGS
+section below) may (and probably should) be set.
+.Sh NOTES
+The MTU of
+.Nm
+interfaces is set to 1476 by default, to match the value used by Cisco routers.
+If grekey is set this is lowered to 1472.
+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
+.Nm ifconfig Cm tunnel
+step before the
+.Xr ifconfig 8
+call to set the
+.Nm
+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
+.Dq up ,
+the keyword
+.Cm up
+must be given last on its command line.
+.Pp
+The kernel must be set to forward datagrams by setting the
+.Va net.inet.ip.forwarding
+.Xr sysctl 8
+variable to non-zero.
+.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
+.Fn compute_route
+code in
+.Pa 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
+.Cm 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
+.Nm
+interface itself.
+.Pp
+The current implementation uses the key only for outgoing packets.
+Incomming packets with a different key or without a key will be treated as if they
+would belong to this interface.
+.Pp
+RFC1701 is not fully supported, however all unsupported features have been
+deprecated in RFC2784.
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