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 like e.g.\& AppleTalk. 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, as e.g.\& when 173encapsulating AppleTalk. 174.Sh EXAMPLES 175Configuration example: 176.Bd -literal 177Host X-- Host A ----------------tunnel---------- Cisco D------Host E 178 \\ | 179 \\ / 180 +------Host B----------Host C----------+ 181.Ed 182.Pp 183On host A 184.Pq Fx : 185.Bd -literal -offset indent 186route add default B 187ifconfig greN create 188ifconfig greN A D netmask 0xffffffff linkX up 189ifconfig greN tunnel A D 190route add E D 191.Ed 192.Pp 193On Host D (Cisco): 194.Bd -literal -offset indent 195Interface TunnelX 196 ip unnumbered D ! e.g. address from Ethernet interface 197 tunnel source D ! e.g. address from Ethernet interface 198 tunnel destination A 199ip route C <some interface and mask> 200ip route A mask C 201ip route X mask tunnelX 202.Ed 203.Pp 204OR 205.Pp 206On Host D 207.Pq Fx : 208.Bd -literal -offset indent 209route add default C 210ifconfig greN create 211ifconfig greN D A 212ifconfig greN tunnel D A 213.Ed 214.Pp 215If all goes well, you should see packets flowing ;-) 216.Pp 217If you want to reach Host A over the tunnel (from Host D (Cisco)), then 218you have to have an alias on Host A for e.g.\& the Ethernet interface like: 219.Pp 220.Dl "ifconfig <etherif> alias Y" 221.Pp 222and on the Cisco: 223.Pp 224.Dl "ip route Y mask tunnelX" 225.Pp 226A similar setup can be used to create a link between two private networks 227(for example in the 192.168 subnet) over the Internet: 228.Bd -literal 229192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.* 230 \\ / 231 \\ / 232 +------ the Internet ------+ 233.Ed 234.Pp 235Assuming router A has the (external) IP address A and the internal address 236192.168.1.1, while router B has external address B and internal address 237192.168.2.1, the following commands will configure the tunnel: 238.Pp 239On router A: 240.Bd -literal -offset indent 241ifconfig greN create 242ifconfig greN 192.168.1.1 192.168.2.1 link1 243ifconfig greN tunnel A B 244route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1 245.Ed 246.Pp 247On router B: 248.Bd -literal -offset indent 249ifconfig greN create 250ifconfig greN 192.168.2.1 192.168.1.1 link1 251ifconfig greN tunnel B A 252route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1 253.Ed 254.Pp 255Note that this is a safe situation where the 256.Cm link1 257flag (as discussed in the 258.Sx BUGS 259section below) may (and probably should) be set. 260.Sh NOTES 261The MTU of 262.Nm 263interfaces is set to 1476 by default, to match the value used by Cisco routers. 264If grekey is set this is lowered to 1472. 265This may not be an optimal value, depending on the link between the two tunnel 266endpoints. 267It can be adjusted via 268.Xr ifconfig 8 . 269.Pp 270For correct operation, the 271.Nm 272device needs a route to the destination that is less specific than the 273one over the tunnel. 274(Basically, there needs to be a route to the decapsulating host that 275does not run over the tunnel, as this would be a loop.) 276If the addresses are ambiguous, doing the 277.Nm ifconfig Cm tunnel 278step before the 279.Xr ifconfig 8 280call to set the 281.Nm 282IP addresses will help to find a route outside the tunnel. 283.Pp 284In order to tell 285.Xr ifconfig 8 286to actually mark the interface as 287.Dq up , 288the keyword 289.Cm up 290must be given last on its command line. 291.Pp 292The kernel must be set to forward datagrams by setting the 293.Va net.inet.ip.forwarding 294.Xr sysctl 8 295variable to non-zero. 296.Sh SEE ALSO 297.\" Xr atalk 4 , 298.Xr gif 4 , 299.Xr inet 4 , 300.Xr ip 4 , 301.Xr netintro 4 , 302.\" Xr options 4 , 303.Xr protocols 5 , 304.Xr ifconfig 8 , 305.Xr sysctl 8 306.Pp 307A description of GRE encapsulation can be found in RFC 1701 and RFC 1702. 308.Pp 309A description of MOBILE encapsulation can be found in RFC 2004. 310.Sh AUTHORS 311.An Heiko W.Rupp Aq hwr@pilhuhn.de 312.Sh BUGS 313The 314.Fn compute_route 315code in 316.Pa if_gre.c 317toggles the last bit of the 318IP-address to provoke the search for a less specific route than the 319one directly over the tunnel to prevent loops. 320This is possibly not the best solution. 321.Pp 322To avoid the address munging described above, turn on the 323.Cm link1 324flag on the 325.Xr ifconfig 8 326command line. 327This implies that the GRE packet destination and the ifconfig remote host 328are not the same IP addresses, and that the GRE destination does not route 329over the 330.Nm 331interface itself. 332.Pp 333The current implementation uses the key only for outgoing packets. 334Incoming packets with a different key or without a key will be treated as if they 335would belong to this interface. 336.Pp 337RFC1701 is not fully supported, however all unsupported features have been 338deprecated in RFC2784. 339