1.\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $ 2.\" 3.\" Copyright 1998 (c) The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by Heiko W.Rupp <hwr@pilhuhn.de> 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. All advertising materials mentioning features or use of this software 18.\" must display the following acknowledgement: 19.\" This product includes software developed by the NetBSD 20.\" Foundation, Inc. and its contributors. 21.\" 4. Neither the name of the The NetBSD Foundation nor the names of its 22.\" contributors may be used to endorse or promote products derived 23.\" from this software without specific prior written permission. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 26.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 27.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 28.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 29.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 30.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 31.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 32.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 33.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 34.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 35.\" POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $FreeBSD: src/share/man/man4/gre.4,v 1.7 2006/10/19 07:41:47 danger Exp $ 38.\" $DragonFly: src/share/man/man4/gre.4,v 1.6 2007/11/12 19:07:40 swildner Exp $ 39.\" 40.Dd November 11, 2007 41.Dt GRE 4 42.Os 43.Sh NAME 44.Nm gre 45.Nd encapsulating network device 46.Sh SYNOPSIS 47To compile the 48.Ns Nm 49device into the kernel, place the following line in the kernel 50configuration file: 51.Bd -ragged -offset indent 52.Cd "pseudo-device gre" 53.Ed 54.Pp 55Alternatively, to load the 56.Ns Nm 57device as a module at boot time, place the following line in 58.Xr loader.conf 5 : 59.Bd -literal -offset indent 60if_gre_load="YES" 61.Ed 62.Sh DESCRIPTION 63The 64.Nm 65network interface pseudo device encapsulates datagrams 66into IP. 67These encapsulated datagrams are routed to a destination host, 68where they are decapsulated and further routed to their final destination. 69The 70.Dq tunnel 71appears to the inner datagrams as one hop. 72.Pp 73.Nm 74interfaces are dynamically created and destroyed with the 75.Xr ifconfig 8 76.Cm create 77and 78.Cm destroy 79subcommands. 80.Pp 81This driver currently supports the following modes of operation: 82.Bl -tag -width indent 83.It "GRE encapsulation (IP protocol number 47)" 84Encapsulated datagrams are 85prepended an outer datagram and a GRE header. 86The GRE header specifies 87the type of the encapsulated datagram and thus allows for tunneling other 88protocols than IP like e.g.\& AppleTalk. 89GRE mode is also the default tunnel mode on Cisco routers. 90This is also the default mode of operation of the 91.Nm 92interfaces. 93.\"As part of the GRE mode, 94.\".Nm 95.\"also supports Cisco WCCP protocol, both version 1 and version 2. 96.\"Since there is no reliable way to distinguish between WCCP versions, it 97.\"should be configured manually using the 98.\".Cm link2 99.\"flag. 100.\"If the 101.\".Cm link2 102.\"flag is not set (default), then WCCP version 1 is selected. 103.It "MOBILE encapsulation (IP protocol number 55)" 104Datagrams are 105encapsulated into IP, but with a shorter encapsulation. 106The original 107IP header is modified and the modifications are inserted between the 108so modified header and the original payload. 109Like 110.Xr gif 4 , 111only for IP-in-IP encapsulation. 112.El 113.Pp 114The 115.Nm 116interfaces support a number of 117.Xr ioctl 2 Ns s , 118such as: 119.Bl -tag -width ".Dv GRESADDRS" 120.It Dv GRESADDRS 121Set the IP address of the local tunnel end. 122This is the source address 123set by or displayed by 124.Xr ifconfig 8 125for the 126.Nm 127interface. 128.It Dv GRESADDRD 129Set the IP address of the remote tunnel end. 130This is the destination address 131set by or displayed by 132.Xr ifconfig 8 133for the 134.Nm 135interface. 136.It Dv GREGADDRS 137Query the IP address that is set for the local tunnel end. 138This is the 139address the encapsulation header carries as local address (i.e., the real 140address of the tunnel start point). 141.It Dv GREGADDRD 142Query the IP address that is set for the remote tunnel end. 143This is the 144address the encapsulated packets are sent to (i.e., the real address of 145the remote tunnel endpoint). 146.It Dv GRESPROTO 147Set the operation mode to the specified IP protocol value. 148The 149protocol is passed to the interface in 150.Po Vt "struct ifreq" Pc Ns Li -> Ns Va ifr_flags . 151The operation mode can also be given as 152.Pp 153.Bl -tag -width ".Cm -link0" -compact 154.It Cm link0 155.Dv IPPROTO_GRE 156.It Cm -link0 157.Dv IPPROTO_MOBILE 158.El 159.Pp 160to 161.Xr ifconfig 8 . 162.Pp 163The 164.Cm link1 165flag is not used to choose encapsulation, but to modify the 166internal route search for the remote tunnel endpoint, see the 167.Sx BUGS 168section below. 169.It Dv GREGPROTO 170Query operation mode. 171.El 172.Pp 173Note that the IP addresses of the tunnel endpoints may be the same as the 174ones defined with 175.Xr ifconfig 8 176for the interface (as if IP is encapsulated), but need not be, as e.g.\& when 177encapsulating AppleTalk. 178.Sh EXAMPLES 179Configuration example: 180.Bd -literal 181Host X-- Host A ----------------tunnel---------- Cisco D------Host E 182 \\ | 183 \\ / 184 +------Host B----------Host C----------+ 185.Ed 186.Pp 187On host A 188.Pq Dx : 189.Bd -literal -offset indent 190route add default B 191ifconfig greN create 192ifconfig greN A D netmask 0xffffffff linkX up 193ifconfig greN tunnel A D 194route add E D 195.Ed 196.Pp 197On Host D (Cisco): 198.Bd -literal -offset indent 199Interface TunnelX 200 ip unnumbered D ! e.g. address from Ethernet interface 201 tunnel source D ! e.g. address from Ethernet interface 202 tunnel destination A 203ip route C <some interface and mask> 204ip route A mask C 205ip route X mask tunnelX 206.Ed 207.Pp 208OR 209.Pp 210On Host D 211.Pq Dx : 212.Bd -literal -offset indent 213route add default C 214ifconfig greN create 215ifconfig greN D A 216ifconfig greN tunnel D A 217.Ed 218.Pp 219If all goes well, you should see packets flowing ;-) 220.Pp 221If you want to reach Host A over the tunnel (from Host D (Cisco)), then 222you have to have an alias on Host A for e.g.\& the Ethernet interface like: 223.Pp 224.Dl "ifconfig <etherif> alias Y" 225.Pp 226and on the Cisco: 227.Pp 228.Dl "ip route Y mask tunnelX" 229.Pp 230A similar setup can be used to create a link between two private networks 231(for example in the 192.168 subnet) over the Internet: 232.Bd -literal 233192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.* 234 \\ / 235 \\ / 236 +------ the Internet ------+ 237.Ed 238.Pp 239Assuming router A has the (external) IP address A and the internal address 240192.168.1.1, while router B has external address B and internal address 241192.168.2.1, the following commands will configure the tunnel: 242.Pp 243On router A: 244.Bd -literal -offset indent 245ifconfig greN create 246ifconfig greN 192.168.1.1 192.168.2.1 link1 247ifconfig greN tunnel A B 248route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1 249.Ed 250.Pp 251On router B: 252.Bd -literal -offset indent 253ifconfig greN create 254ifconfig greN 192.168.2.1 192.168.1.1 link1 255ifconfig greN tunnel B A 256route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1 257.Ed 258.Pp 259Note that this is a safe situation where the 260.Cm link1 261flag (as discussed in the 262.Sx BUGS 263section below) may (and probably should) be set. 264.Sh NOTES 265The MTU of 266.Nm 267interfaces is set to 1476 by default, to match the value used by Cisco routers. 268This may not be an optimal value, depending on the link between the two tunnel 269endpoints. 270It can be adjusted via 271.Xr ifconfig 8 . 272.Pp 273For correct operation, the 274.Nm 275device needs a route to the destination that is less specific than the 276one over the tunnel. 277(Basically, there needs to be a route to the decapsulating host that 278does not run over the tunnel, as this would be a loop.) 279If the addresses are ambiguous, doing the 280.Nm ifconfig Cm tunnel 281step before the 282.Xr ifconfig 8 283call to set the 284.Nm 285IP addresses will help to find a route outside the tunnel. 286.Pp 287In order to tell 288.Xr ifconfig 8 289to actually mark the interface as 290.Dq up , 291the keyword 292.Cm up 293must be given last on its command line. 294.Pp 295The kernel must be set to forward datagrams by setting the 296.Va net.inet.ip.forwarding 297.Xr sysctl 8 298variable to non-zero. 299.Sh SEE ALSO 300.\".Xr atalk 4 , 301.Xr gif 4 , 302.Xr inet 4 , 303.Xr ip 4 , 304.Xr netintro 4 , 305.Xr protocols 5 , 306.Xr ifconfig 8 , 307.Xr sysctl 8 308.Pp 309A description of GRE encapsulation can be found in RFC 1701 and RFC 1702. 310.Pp 311A description of MOBILE encapsulation can be found in RFC 2004. 312.Sh AUTHORS 313.An Heiko W.Rupp Aq hwr@pilhuhn.de 314.Sh BUGS 315The 316.Fn gre_compute_route 317code in 318.Pa if_gre.c 319toggles the last bit of the 320IP-address to provoke the search for a less specific route than the 321one directly over the tunnel to prevent loops. 322This is possibly not the best solution. 323.Pp 324To avoid the address munging described above, turn on the 325.Cm link1 326flag on the 327.Xr ifconfig 8 328command line. 329This implies that the GRE packet destination and the ifconfig remote host 330are not the same IP addresses, and that the GRE destination does not route 331over the 332.Nm 333interface itself. 334.Pp 335The GRE RFCs are not yet fully implemented (no GRE options). 336