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.\" 39.Dd November 11, 2007 40.Dt GRE 4 41.Os 42.Sh NAME 43.Nm gre 44.Nd encapsulating network device 45.Sh SYNOPSIS 46To compile this driver into the kernel, place the following line 47in the kernel configuration file: 48.Bd -ragged -offset indent 49.Cd "pseudo-device gre" 50.Ed 51.Pp 52Alternatively, to load this driver as a module at boot time, 53place the following line in 54.Xr loader.conf 5 : 55.Bd -literal -offset indent 56if_gre_load="YES" 57.Ed 58.Sh DESCRIPTION 59The 60.Nm 61network interface pseudo device encapsulates datagrams 62into IP. 63These encapsulated datagrams are routed to a destination host, 64where they are decapsulated and further routed to their final destination. 65The 66.Dq tunnel 67appears to the inner datagrams as one hop. 68.Pp 69.Nm 70interfaces are dynamically created and destroyed with the 71.Xr ifconfig 8 72.Cm create 73and 74.Cm destroy 75subcommands. 76.Pp 77This driver currently supports the following modes of operation: 78.Bl -tag -width indent 79.It "GRE encapsulation (IP protocol number 47)" 80Encapsulated datagrams are 81prepended an outer datagram and a GRE header. 82The GRE header specifies 83the type of the encapsulated datagram and thus allows for tunneling other 84protocols than IP. 85GRE mode is also the default tunnel mode on Cisco routers. 86This is also the default mode of operation of the 87.Nm 88interfaces. 89.\"As part of the GRE mode, 90.\".Nm 91.\"also supports Cisco WCCP protocol, both version 1 and version 2. 92.\"Since there is no reliable way to distinguish between WCCP versions, it 93.\"should be configured manually using the 94.\".Cm link2 95.\"flag. 96.\"If the 97.\".Cm link2 98.\"flag is not set (default), then WCCP version 1 is selected. 99.It "MOBILE encapsulation (IP protocol number 55)" 100Datagrams are 101encapsulated into IP, but with a shorter encapsulation. 102The original 103IP header is modified and the modifications are inserted between the 104so modified header and the original payload. 105Like 106.Xr gif 4 , 107only for IP-in-IP encapsulation. 108.El 109.Pp 110The 111.Nm 112interfaces support a number of 113.Xr ioctl 2 Ns s , 114such as: 115.Bl -tag -width ".Dv GRESADDRS" 116.It Dv GRESADDRS 117Set the IP address of the local tunnel end. 118This is the source address 119set by or displayed by 120.Xr ifconfig 8 121for the 122.Nm 123interface. 124.It Dv GRESADDRD 125Set the IP address of the remote tunnel end. 126This is the destination address 127set by or displayed by 128.Xr ifconfig 8 129for the 130.Nm 131interface. 132.It Dv GREGADDRS 133Query the IP address that is set for the local tunnel end. 134This is the 135address the encapsulation header carries as local address (i.e., the real 136address of the tunnel start point). 137.It Dv GREGADDRD 138Query the IP address that is set for the remote tunnel end. 139This is the 140address the encapsulated packets are sent to (i.e., the real address of 141the remote tunnel endpoint). 142.It Dv GRESPROTO 143Set the operation mode to the specified IP protocol value. 144The 145protocol is passed to the interface in 146.Po Vt "struct ifreq" Pc Ns Li -> Ns Va ifr_flags . 147The operation mode can also be given as 148.Pp 149.Bl -tag -width ".Cm -link0" -compact 150.It Cm link0 151.Dv IPPROTO_GRE 152.It Cm -link0 153.Dv IPPROTO_MOBILE 154.El 155.Pp 156to 157.Xr ifconfig 8 . 158.Pp 159The 160.Cm link1 161flag is not used to choose encapsulation, but to modify the 162internal route search for the remote tunnel endpoint, see the 163.Sx BUGS 164section below. 165.It Dv GREGPROTO 166Query operation mode. 167.El 168.Pp 169Note that the IP addresses of the tunnel endpoints may be the same as the 170ones defined with 171.Xr ifconfig 8 172for the interface (as if IP is encapsulated), but need not be. 173.Sh EXAMPLES 174Configuration example: 175.Bd -literal 176Host X-- Host A ----------------tunnel---------- Cisco D------Host E 177 \\ | 178 \\ / 179 +------Host B----------Host C----------+ 180.Ed 181.Pp 182On host A 183.Pq Dx : 184.Bd -literal -offset indent 185route add default B 186ifconfig greN create 187ifconfig greN A D netmask 0xffffffff linkX up 188ifconfig greN tunnel A D 189route add E D 190.Ed 191.Pp 192On Host D (Cisco): 193.Bd -literal -offset indent 194Interface TunnelX 195 ip unnumbered D ! e.g. address from Ethernet interface 196 tunnel source D ! e.g. address from Ethernet interface 197 tunnel destination A 198ip route C <some interface and mask> 199ip route A mask C 200ip route X mask tunnelX 201.Ed 202.Pp 203OR 204.Pp 205On Host D 206.Pq Dx : 207.Bd -literal -offset indent 208route add default C 209ifconfig greN create 210ifconfig greN D A 211ifconfig greN tunnel D A 212.Ed 213.Pp 214If all goes well, you should see packets flowing ;-) 215.Pp 216If you want to reach Host A over the tunnel (from Host D (Cisco)), then 217you have to have an alias on Host A for e.g.\& the Ethernet interface like: 218.Pp 219.Dl "ifconfig <etherif> alias Y" 220.Pp 221and on the Cisco: 222.Pp 223.Dl "ip route Y mask tunnelX" 224.Pp 225A similar setup can be used to create a link between two private networks 226(for example in the 192.168 subnet) over the Internet: 227.Bd -literal 228192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.* 229 \\ / 230 \\ / 231 +------ the Internet ------+ 232.Ed 233.Pp 234Assuming router A has the (external) IP address A and the internal address 235192.168.1.1, while router B has external address B and internal address 236192.168.2.1, the following commands will configure the tunnel: 237.Pp 238On router A: 239.Bd -literal -offset indent 240ifconfig greN create 241ifconfig greN 192.168.1.1 192.168.2.1 link1 242ifconfig greN tunnel A B 243route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1 244.Ed 245.Pp 246On router B: 247.Bd -literal -offset indent 248ifconfig greN create 249ifconfig greN 192.168.2.1 192.168.1.1 link1 250ifconfig greN tunnel B A 251route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1 252.Ed 253.Pp 254Note that this is a safe situation where the 255.Cm link1 256flag (as discussed in the 257.Sx BUGS 258section below) may (and probably should) be set. 259.Sh NOTES 260The MTU of 261.Nm 262interfaces is set to 1476 by default, to match the value used by Cisco routers. 263This may not be an optimal value, depending on the link between the two tunnel 264endpoints. 265It can be adjusted via 266.Xr ifconfig 8 . 267.Pp 268For correct operation, the 269.Nm 270device needs a route to the destination that is less specific than the 271one over the tunnel. 272(Basically, there needs to be a route to the decapsulating host that 273does not run over the tunnel, as this would be a loop.) 274If the addresses are ambiguous, doing the 275.Nm ifconfig Cm tunnel 276step before the 277.Xr ifconfig 8 278call to set the 279.Nm 280IP addresses will help to find a route outside the tunnel. 281.Pp 282In order to tell 283.Xr ifconfig 8 284to actually mark the interface as 285.Dq up , 286the keyword 287.Cm up 288must be given last on its command line. 289.Pp 290The kernel must be set to forward datagrams by setting the 291.Va net.inet.ip.forwarding 292.Xr sysctl 8 293variable to non-zero. 294.Sh SEE ALSO 295.Xr gif 4 , 296.Xr inet 4 , 297.Xr ip 4 , 298.Xr netintro 4 , 299.Xr protocols 5 , 300.Xr ifconfig 8 , 301.Xr sysctl 8 302.Pp 303A description of GRE encapsulation can be found in RFC 1701 and RFC 1702. 304.Pp 305A description of MOBILE encapsulation can be found in RFC 2004. 306.Sh AUTHORS 307.An Heiko W. Rupp Aq Mt hwr@pilhuhn.de 308.Sh BUGS 309The 310.Fn gre_compute_route 311code in 312.Pa if_gre.c 313toggles the last bit of the 314IP-address to provoke the search for a less specific route than the 315one directly over the tunnel to prevent loops. 316This is possibly not the best solution. 317.Pp 318To avoid the address munging described above, turn on the 319.Cm link1 320flag on the 321.Xr ifconfig 8 322command line. 323This implies that the GRE packet destination and the ifconfig remote host 324are not the same IP addresses, and that the GRE destination does not route 325over the 326.Nm 327interface itself. 328.Pp 329The GRE RFCs are not yet fully implemented (no GRE options). 330