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.\" $FreeBSD$ 38.\" 39.Dd June 9, 2002 40.Dt GRE 4 41.Os 42.Sh NAME 43.Nm gre 44.Nd encapsulating network device 45.Sh SYNOPSIS 46To compile the 47.Ns Nm 48device into the kernel, place the following line in the kernel 49configuration file: 50.Bd -ragged -offset indent 51.Cd "device gre" 52.Ed 53.Pp 54Alternatively, to load the 55.Ns Nm 56device as a module at boot time, place the following line in 57.Xr loader.conf 5 : 58.Bd -literal -offset indent 59if_gre_load="YES" 60.Ed 61.Sh DESCRIPTION 62The 63.Nm 64network interface pseudo device encapsulates datagrams 65into IP. 66These encapsulated datagrams are routed to a destination host, 67where they are decapsulated and further routed to their final destination. 68The 69.Dq tunnel 70appears to the inner datagrams as one hop. 71.Pp 72.Nm 73interfaces are dynamically created and destroyed with the 74.Xr ifconfig 8 75.Cm create 76and 77.Cm destroy 78subcommands. 79.Pp 80This driver currently supports the following modes of operation: 81.Bl -tag -width indent 82.It "GRE encapsulation (IP protocol number 47)" 83Encapsulated datagrams are 84prepended an outer datagram and a GRE header. 85The GRE header specifies 86the type of the encapsulated datagram and thus allows for tunneling other 87protocols than IP like e.g.\& AppleTalk. 88GRE mode is also the default tunnel mode on Cisco routers. 89This is also the default mode of operation of the 90.Nm 91interfaces. 92As part of the GRE mode, 93.Nm 94also supports Cisco WCCP protocol, both version 1 and version 2. 95Since there is no reliable way to distinguish between WCCP versions, it 96should be configured manually using the 97.Cm link2 98flag. 99If the 100.Cm link2 101flag is not set (default), then WCCP version 1 is selected. 102.It "MOBILE encapsulation (IP protocol number 55)" 103Datagrams are 104encapsulated into IP, but with a shorter encapsulation. 105The original 106IP header is modified and the modifications are inserted between the 107so modified header and the original payload. 108Like 109.Xr gif 4 , 110only for IP-in-IP encapsulation. 111.El 112.Pp 113The 114.Nm 115interfaces support a number of 116.Xr ioctl 2 Ns s , 117such as: 118.Bl -tag -width ".Dv GRESADDRS" 119.It Dv GRESADDRS 120Set the IP address of the local tunnel end. 121This is the source address 122set by or displayed by 123.Xr ifconfig 8 124for the 125.Nm 126interface. 127.It Dv GRESADDRD 128Set the IP address of the remote tunnel end. 129This is the destination address 130set by or displayed by 131.Xr ifconfig 8 132for the 133.Nm 134interface. 135.It Dv GREGADDRS 136Query the IP address that is set for the local tunnel end. 137This is the 138address the encapsulation header carries as local address (i.e., the real 139address of the tunnel start point). 140.It Dv GREGADDRD 141Query the IP address that is set for the remote tunnel end. 142This is the 143address the encapsulated packets are sent to (i.e., the real address of 144the remote tunnel endpoint). 145.It Dv GRESPROTO 146Set the operation mode to the specified IP protocol value. 147The 148protocol is passed to the interface in 149.Po Vt "struct ifreq" Pc Ns Li -> Ns Va ifr_flags . 150The operation mode can also be given as 151.Pp 152.Bl -tag -width ".Cm -link0" -compact 153.It Cm link0 154.Dv IPPROTO_GRE 155.It Cm -link0 156.Dv IPPROTO_MOBILE 157.El 158.Pp 159to 160.Xr ifconfig 8 . 161.Pp 162The 163.Cm link1 164flag is not used to choose encapsulation, but to modify the 165internal route search for the remote tunnel endpoint, see the 166.Sx BUGS 167section below. 168.It Dv GREGPROTO 169Query operation mode. 170.El 171.Pp 172Note that the IP addresses of the tunnel endpoints may be the same as the 173ones defined with 174.Xr ifconfig 8 175for the interface (as if IP is encapsulated), but need not be, as e.g.\& when 176encapsulating AppleTalk. 177.Sh EXAMPLES 178Configuration example: 179.Bd -literal 180Host X-- Host A ----------------tunnel---------- Cisco D------Host E 181 \\ | 182 \\ / 183 +------Host B----------Host C----------+ 184.Ed 185.Pp 186On host A 187.Pq Fx : 188.Bd -literal -offset indent 189route add default B 190ifconfig greN create 191ifconfig greN A D netmask 0xffffffff linkX up 192ifconfig greN tunnel A D 193route add E D 194.Ed 195.Pp 196On Host D (Cisco): 197.Bd -literal -offset indent 198Interface TunnelX 199 ip unnumbered D ! e.g. address from Ethernet interface 200 tunnel source D ! e.g. address from Ethernet interface 201 tunnel destination A 202ip route C <some interface and mask> 203ip route A mask C 204ip route X mask tunnelX 205.Ed 206.Pp 207OR 208.Pp 209On Host D 210.Pq Fx : 211.Bd -literal -offset indent 212route add default C 213ifconfig greN create 214ifconfig greN D A 215ifconfig greN tunnel D A 216.Ed 217.Pp 218If all goes well, you should see packets flowing ;-) 219.Pp 220If you want to reach Host A over the tunnel (from Host D (Cisco)), then 221you have to have an alias on Host A for e.g.\& the Ethernet interface like: 222.Pp 223.Dl "ifconfig <etherif> alias Y" 224.Pp 225and on the Cisco: 226.Pp 227.Dl "ip route Y mask tunnelX" 228.Pp 229A similar setup can be used to create a link between two private networks 230(for example in the 192.168 subnet) over the Internet: 231.Bd -literal 232192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.* 233 \\ / 234 \\ / 235 +------ the Internet ------+ 236.Ed 237.Pp 238Assuming router A has the (external) IP address A and the internal address 239192.168.1.1, while router B has external address B and internal address 240192.168.2.1, the following commands will configure the tunnel: 241.Pp 242On router A: 243.Bd -literal -offset indent 244ifconfig greN create 245ifconfig greN 192.168.1.1 192.168.2.1 link1 246ifconfig greN tunnel A B 247route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1 248.Ed 249.Pp 250On router B: 251.Bd -literal -offset indent 252ifconfig greN create 253ifconfig greN 192.168.2.1 192.168.1.1 link1 254ifconfig greN tunnel B A 255route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1 256.Ed 257.Pp 258Note that this is a safe situation where the 259.Cm link1 260flag (as discussed in the 261.Sx BUGS 262section below) may (and probably should) be set. 263.Sh NOTES 264The MTU of 265.Nm 266interfaces is set to 1476 by default, to match the value used by Cisco routers. 267This may not be an optimal value, depending on the link between the two tunnel 268endpoints. 269It can be adjusted via 270.Xr ifconfig 8 . 271.Pp 272For correct operation, the 273.Nm 274device needs a route to the destination that is less specific than the 275one over the tunnel. 276(Basically, there needs to be a route to the decapsulating host that 277does not run over the tunnel, as this would be a loop.) 278If the addresses are ambiguous, doing the 279.Nm ifconfig Cm tunnel 280step before the 281.Xr ifconfig 8 282call to set the 283.Nm 284IP addresses will help to find a route outside the tunnel. 285.Pp 286In order to tell 287.Xr ifconfig 8 288to actually mark the interface as 289.Dq up , 290the keyword 291.Cm up 292must be given last on its command line. 293.Pp 294The kernel must be set to forward datagrams by setting the 295.Va net.inet.ip.forwarding 296.Xr sysctl 8 297variable to non-zero. 298.Sh SEE ALSO 299.\" Xr atalk 4 , 300.Xr gif 4 , 301.Xr inet 4 , 302.Xr ip 4 , 303.Xr netintro 4 , 304.\" Xr options 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 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