1.\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $ 2.\" $FreeBSD$ 3.\" 4.\" Copyright 1998 (c) The NetBSD Foundation, Inc. 5.\" All rights reserved. 6.\" 7.\" This code is derived from software contributed to The NetBSD Foundation 8.\" by Heiko W.Rupp <hwr@pilhuhn.de> 9.\" 10.\" Redistribution and use in source and binary forms, with or without 11.\" modification, are permitted provided that the following conditions 12.\" are met: 13.\" 1. Redistributions of source code must retain the above copyright 14.\" notice, this list of conditions and the following disclaimer. 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in the 17.\" documentation and/or other materials provided with the distribution. 18.\" 3. All advertising materials mentioning features or use of this software 19.\" must display the following acknowledgement: 20.\" This product includes software developed by the NetBSD 21.\" Foundation, Inc. and its contributors. 22.\" 4. Neither the name of the The NetBSD Foundation nor the names of its 23.\" contributors may be used to endorse or promote products derived 24.\" from this software without specific prior written permission. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 27.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 28.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 29.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 30.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 31.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 32.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 33.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 34.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 35.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 36.\" POSSIBILITY OF SUCH DAMAGE. 37.\" 38.Dd June 9, 2002 39.Dt GRE 4 40.Os 41.Sh NAME 42.Nm gre 43.Nd encapsulating network device 44.Sh SYNOPSIS 45.Cd pseudo-device gre 46.Sh DESCRIPTION 47The 48.Nm gre 49network interface pseudo device encapsulates datagrams 50into IP. These encapsulated datagrams are routed to a destination host, 51where they are decapsulated and further routed to their final destination. 52The 53.Dq tunnel 54appears to the inner datagrams as one hop. 55.Pp 56.Nm 57interfaces are dynamically created and destroyed with the 58.Xr ifconfig 8 59.Cm create 60and 61.Cm destroy 62subcommands. 63.Pp 64This driver currently supports the following modes of operation: 65.Bl -tag -width abc 66.It GRE encapsulation (IP protocol number 47) 67Encapsulated datagrams are 68prepended an outer datagram and a GRE header. The GRE header specifies 69the type of the encapsulated datagram and thus allows for tunneling other 70protocols than IP like e.g. AppleTalk. GRE mode is also the default tunnel 71mode on Cisco routers. This is also the default mode of operation of the 72.Sy gre Ns Ar X 73interfaces. 74.It MOBILE encapsulation (IP protocol number 55) 75Datagrams are 76encapsulated into IP, but with a shorter encapsulation. The original 77IP header is modified and the modifications are inserted between the 78so modified header and the original payload. Like 79.Xr gif 4 , 80only for IP in IP encapsulation. 81.El 82.Pp 83The 84.Sy gre Ns Ar X 85interfaces support a number of 86.Xr ioctl 2 Ns s , 87such as: 88.Bl -tag -width aaa 89.It GRESADDRS : 90Set the IP address of the local tunnel end. This is the source address 91set by or displayed by ifconfig for the 92.Sy gre Ns Ar X 93interface. 94.It GRESADDRD : 95Set the IP address of the remote tunnel end. This is the destination address 96set by or displayed by ifconfig for the 97.Sy gre Ns Ar X 98interface. 99.It GREGADDRS : 100Query the IP address that is set for the local tunnel end. This is the 101address the encapsulation header carries as local address (i.e. the real 102address of the tunnel start point.) 103.It GREGADDRD : 104Query the IP address that is set for the remote tunnel end. This is the 105address the encapsulated packets are sent to (i.e. the real address of 106the remote tunnel endpoint.) 107.It GRESPROTO : 108Set the operation mode to the specified IP protocol value. The 109protocol is passed to the interface in (struct ifreq)-\*[Gt]ifr_flags. 110The operation mode can also be given as 111.Bl -tag -width link0xxx 112.It link0 113IPPROTO_GRE 114.It -link0 115IPPROTO_MOBILE 116.El 117.Pp 118to 119.Xr ifconfig 8 . 120.Pp 121The link1 flag is not used to choose encapsulation, but to modify the 122internal route search for the remote tunnel endpoint, see the 123.Sx BUGS 124section below. 125.It GREGPROTO : 126Query operation mode. 127.El 128.Pp 129Note that the IP addresses of the tunnel endpoints may be the same as the 130ones defined with 131.Xr ifconfig 8 132for the interface (as if IP is encapsulated), but need not be, as e.g. when 133encapsulating AppleTalk. 134.Sh EXAMPLES 135Configuration example: 136.Bd -literal 137Host X-- Host A ----------------tunnel---------- cisco D------Host E 138 \\ | 139 \\ / 140 +------Host B----------Host C----------+ 141.Ed 142On host A 143.Ns ( Nx ) : 144.Bd -literal 145 # route add default B 146 # ifconfig greN create 147 # ifconfig greN A D netmask 0xffffffff linkX up 148 # ifconfig greN tunnel A D 149 # route add E D 150.Ed 151On Host D (Cisco): 152.Bd -literal 153 Interface TunnelX 154 ip unnumbered D ! e.g. address from Ethernet interface 155 tunnel source D ! e.g. address from Ethernet interface 156 tunnel destination A 157 ip route C \*[Lt]some interface and mask\*[Gt] 158 ip route A mask C 159 ip route X mask tunnelX 160.Ed 161OR 162On Host D 163.Ns ( Nx ) : 164.Bd -literal 165 # route add default C 166 # ifconfig greN create 167 # ifconfig greN D A 168 # ifconfig tunnel greN D A 169.Ed 170.Pp 171If all goes well, you should see packets flowing ;-) 172.Pp 173If you want to reach Host A over the tunnel (from Host D (Cisco)), then 174you have to have an alias on Host A for e.g. the Ethernet interface like: 175.Bd -literal 176 ifconfig \*[Lt]etherif\*[Gt] alias Y 177.Ed 178and on the cisco 179.Bd -literal 180 ip route Y mask tunnelX 181.Ed 182.Pp 183A similar setup can be used to create a link between two private networks 184(for example in the 192.168 subnet) over the Internet: 185.Bd -literal 186192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.* 187 \\ / 188 \\ / 189 +----- the Internet ------+ 190.Ed 191Assuming router A has the (external) IP address A and the internal address 192192.168.1.1, while router B has external address B and internal address 193192.168.2.1, the following commands will configure the tunnel: 194.Pp 195On router A: 196.Bd -literal 197 # ifconfig greN create 198 # ifconfig greN 192.168.1.1 192.168.2.1 link1 199 # ifconfig greN tunnel A B 200 # route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1 201.Ed 202.Pp 203On router B: 204.Bd -literal 205 # ifconfig greN create 206 # ifconfig greN 192.168.2.1 192.168.1.1 link1 207 # ifconfig greN tunnel B A 208 # route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1 209.Ed 210.Pp 211Note that this is a safe situation where the link1 flag (as discussed in the 212.Sx BUGS 213section below) may (and probably should) be set. 214.Sh NOTES 215The MTU of 216.Sy gre Ns Ar X 217interfaces is set to 1476 by default to match the value used by Cisco routers. 218This may not be an optimal value, depending on the link between the two tunnel 219endpoints. It can be adjusted via 220.Xr ifconfig 8 . 221.Pp 222For correct operation, the 223.Nm 224device needs a route to the destination that is less specific than the 225one over the tunnel. 226(Basically, there needs to be a route to the decapsulating host that 227does not run over the tunnel, as this would be a loop.) 228If the addresses are ambiguous, doing the 229.Xr ifconfig 8 230.Li tunnel 231step before the 232.Xr ifconfig 8 233call to set the 234.Sy gre Ns Ar X 235IP addresses will help to find a route outside the tunnel. 236.Pp 237In order to tell 238.Xr ifconfig 8 239to actually mark the interface as up, the keyword 240.Dq up 241must be given last on its command line. 242.Pp 243The kernel must be set to forward datagrams by either option 244.Em GATEWAY 245in the kernel config file or by issuing the appropriate option to 246.Xr sysctl 8 . 247.Sh SEE ALSO 248.Xr atalk 4 , 249.Xr gif 4 , 250.Xr inet 4 , 251.Xr ip 4 , 252.Xr netintro 4 , 253.Xr options 4 , 254.Xr protocols 5 , 255.Xr ifconfig 8 , 256.Xr sysctl 8 257.Pp 258A description of GRE encapsulation can be found in RFC 1701 and RFC 1702. 259.Pp 260A description of MOBILE encapsulation can be found in RFC 2004. 261.Sh AUTHORS 262.An Heiko W.Rupp Aq hwr@pilhuhn.de 263.Sh BUGS 264The compute_route() code in if_gre.c toggles the last bit of the 265IP-address to provoke the search for a less specific route than the 266one directly over the tunnel to prevent loops. This is possibly not 267the best solution. 268.Pp 269To avoid the address munging described above, turn on the link1 flag 270on the 271.Xr ifconfig 8 272command line. 273This implies that the GRE packet destination and the ifconfig remote host 274are not the same IP addresses, and that the GRE destination does not route 275over the 276.Sy gre Ns Ar X 277interface itself. 278.Pp 279The GRE RFCs are not yet fully implemented (no GRE options). 280