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.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.\" $FreeBSD$ 31.\" 32.Dd June 20, 2008 33.Dt GRE 4 34.Os 35.Sh NAME 36.Nm gre 37.Nd encapsulating network device 38.Sh SYNOPSIS 39To compile the 40driver into the kernel, place the following line in the kernel 41configuration file: 42.Bd -ragged -offset indent 43.Cd "device gre" 44.Ed 45.Pp 46Alternatively, to load the 47driver as a module at boot time, place the following line in 48.Xr loader.conf 5 : 49.Bd -literal -offset indent 50if_gre_load="YES" 51.Ed 52.Sh DESCRIPTION 53The 54.Nm 55network interface pseudo device encapsulates datagrams 56into IP. 57These encapsulated datagrams are routed to a destination host, 58where they are decapsulated and further routed to their final destination. 59The 60.Dq tunnel 61appears to the inner datagrams as one hop. 62.Pp 63.Nm 64interfaces are dynamically created and destroyed with the 65.Xr ifconfig 8 66.Cm create 67and 68.Cm destroy 69subcommands. 70.Pp 71This driver currently supports the following modes of operation: 72.Bl -tag -width indent 73.It "GRE encapsulation (IP protocol number 47)" 74Encapsulated datagrams are 75prepended an outer datagram and a GRE header. 76The GRE header specifies 77the type of the encapsulated datagram and thus allows for tunneling other 78protocols than IP. 79GRE mode is also the default tunnel mode on Cisco routers. 80This is also the default mode of operation of the 81.Nm 82interfaces. 83As part of the GRE mode, 84.Nm 85also supports Cisco WCCP protocol, both version 1 and version 2. 86Since there is no reliable way to distinguish between WCCP versions, it 87should be configured manually using the 88.Cm link2 89flag. 90If the 91.Cm link2 92flag is not set (default), then WCCP version 1 is selected. 93.It "MOBILE encapsulation (IP protocol number 55)" 94Datagrams are 95encapsulated into IP, but with a shorter encapsulation. 96The original 97IP header is modified and the modifications are inserted between the 98so modified header and the original payload. 99Like 100.Xr gif 4 , 101only for IP-in-IP encapsulation. 102.El 103.Pp 104The 105.Nm 106interfaces support a number of 107.Xr ioctl 2 Ns s , 108such as: 109.Bl -tag -width ".Dv GRESADDRS" 110.It Dv GRESADDRS 111Set the IP address of the local tunnel end. 112This is the source address 113set by or displayed by 114.Xr ifconfig 8 115for the 116.Nm 117interface. 118.It Dv GRESADDRD 119Set the IP address of the remote tunnel end. 120This is the destination address 121set by or displayed by 122.Xr ifconfig 8 123for the 124.Nm 125interface. 126.It Dv GREGADDRS 127Query the IP address that is set for the local tunnel end. 128This is the 129address the encapsulation header carries as local address (i.e., the real 130address of the tunnel start point). 131.It Dv GREGADDRD 132Query the IP address that is set for the remote tunnel end. 133This is the 134address the encapsulated packets are sent to (i.e., the real address of 135the remote tunnel endpoint). 136.It Dv GRESPROTO 137Set the operation mode to the specified IP protocol value. 138The 139protocol is passed to the interface in 140.Po Vt "struct ifreq" Pc Ns Li -> Ns Va ifr_flags . 141The operation mode can also be given as 142.Pp 143.Bl -tag -width ".Cm -link0" -compact 144.It Cm link0 145.Dv IPPROTO_GRE 146.It Cm -link0 147.Dv IPPROTO_MOBILE 148.El 149.Pp 150to 151.Xr ifconfig 8 . 152.Pp 153The 154.Cm link1 155flag is not used to choose encapsulation, but to modify the 156internal route search for the remote tunnel endpoint, see the 157.Sx BUGS 158section below. 159.It Dv GREGPROTO 160Query operation mode. 161.It Dv GRESKEY 162Set the GRE key used for outgoing packets. 163A value of 0 disables the key option. 164.It Dv GREGKEY 165Get the GRE key currently used for outgoing packets. 1660 means no outgoing key. 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 Fx : 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 Fx : 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. 263If grekey is set this is lowered to 1472. 264This may not be an optimal value, depending on the link between the two tunnel 265endpoints. 266It can be adjusted via 267.Xr ifconfig 8 . 268.Pp 269For correct operation, the 270.Nm 271device needs a route to the destination that is less specific than the 272one over the tunnel. 273(Basically, there needs to be a route to the decapsulating host that 274does not run over the tunnel, as this would be a loop.) 275If the addresses are ambiguous, doing the 276.Nm ifconfig Cm tunnel 277step before the 278.Xr ifconfig 8 279call to set the 280.Nm 281IP addresses will help to find a route outside the tunnel. 282.Pp 283In order to tell 284.Xr ifconfig 8 285to actually mark the interface as 286.Dq up , 287the keyword 288.Cm up 289must be given last on its command line. 290.Pp 291The kernel must be set to forward datagrams by setting the 292.Va net.inet.ip.forwarding 293.Xr sysctl 8 294variable to non-zero. 295.Sh SEE ALSO 296.Xr gif 4 , 297.Xr inet 4 , 298.Xr ip 4 , 299.Xr netintro 4 , 300.\" Xr options 4 , 301.Xr protocols 5 , 302.Xr ifconfig 8 , 303.Xr sysctl 8 304.Pp 305A description of GRE encapsulation can be found in RFC 1701 and RFC 1702. 306.Pp 307A description of MOBILE encapsulation can be found in RFC 2004. 308.Sh AUTHORS 309.An Heiko W.Rupp Aq Mt hwr@pilhuhn.de 310.Sh BUGS 311The 312.Fn compute_route 313code in 314.Pa if_gre.c 315toggles the last bit of the 316IP-address to provoke the search for a less specific route than the 317one directly over the tunnel to prevent loops. 318This is possibly not the best solution. 319.Pp 320To avoid the address munging described above, turn on the 321.Cm link1 322flag on the 323.Xr ifconfig 8 324command line. 325This implies that the GRE packet destination and the ifconfig remote host 326are not the same IP addresses, and that the GRE destination does not route 327over the 328.Nm 329interface itself. 330.Pp 331The current implementation uses the key only for outgoing packets. 332Incoming packets with a different key or without a key will be treated as if they 333would belong to this interface. 334.Pp 335RFC1701 is not fully supported, however all unsupported features have been 336deprecated in RFC2784. 337