.\" $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 .\" .\" 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. 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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 9, 2002 .Dt GRE 4 .Os .Sh NAME .Nm gre .Nd encapsulating network device .Sh SYNOPSIS .Cd "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 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. .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 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 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: .Pp .Dl "ifconfig 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. 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 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 GRE RFCs are not yet fully implemented (no GRE options).