1 .\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $
3 .\" Copyright 1998 (c) The NetBSD Foundation, Inc.
4 .\" All rights reserved.
6 .\" This code is derived from software contributed to The NetBSD Foundation
7 .\" by Heiko W.Rupp <hwr@pilhuhn.de>
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20 .\" Foundation, Inc. and its contributors.
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37 .\" $FreeBSD: src/share/man/man4/gre.4,v 1.7 2006/10/19 07:41:47 danger Exp $
44 .Nd encapsulating network device
48 device into the kernel, place the following line in the kernel
50 .Bd -ragged -offset indent
51 .Cd "pseudo-device gre"
54 Alternatively, to load the
56 device as a module at boot time, place the following line in
58 .Bd -literal -offset indent
64 network interface pseudo device encapsulates datagrams
66 These encapsulated datagrams are routed to a destination host,
67 where they are decapsulated and further routed to their final destination.
70 appears to the inner datagrams as one hop.
73 interfaces are dynamically created and destroyed with the
80 This driver currently supports the following modes of operation:
81 .Bl -tag -width indent
82 .It "GRE encapsulation (IP protocol number 47)"
83 Encapsulated datagrams are
84 prepended an outer datagram and a GRE header.
85 The GRE header specifies
86 the type of the encapsulated datagram and thus allows for tunneling other
88 GRE mode is also the default tunnel mode on Cisco routers.
89 This is also the default mode of operation of the
92 .\"As part of the GRE mode,
94 .\"also supports Cisco WCCP protocol, both version 1 and version 2.
95 .\"Since there is no reliable way to distinguish between WCCP versions, it
96 .\"should be configured manually using the
101 .\"flag is not set (default), then WCCP version 1 is selected.
102 .It "MOBILE encapsulation (IP protocol number 55)"
104 encapsulated into IP, but with a shorter encapsulation.
106 IP header is modified and the modifications are inserted between the
107 so modified header and the original payload.
110 only for IP-in-IP encapsulation.
115 interfaces support a number of
118 .Bl -tag -width ".Dv GRESADDRS"
120 Set the IP address of the local tunnel end.
121 This is the source address
122 set by or displayed by
128 Set the IP address of the remote tunnel end.
129 This is the destination address
130 set by or displayed by
136 Query the IP address that is set for the local tunnel end.
138 address the encapsulation header carries as local address (i.e., the real
139 address of the tunnel start point).
141 Query the IP address that is set for the remote tunnel end.
143 address the encapsulated packets are sent to (i.e., the real address of
144 the remote tunnel endpoint).
146 Set the operation mode to the specified IP protocol value.
148 protocol is passed to the interface in
149 .Po Vt "struct ifreq" Pc Ns Li -> Ns Va ifr_flags .
150 The operation mode can also be given as
152 .Bl -tag -width ".Cm -link0" -compact
164 flag is not used to choose encapsulation, but to modify the
165 internal route search for the remote tunnel endpoint, see the
169 Query operation mode.
172 Note that the IP addresses of the tunnel endpoints may be the same as the
175 for the interface (as if IP is encapsulated), but need not be.
177 Configuration example:
179 Host X-- Host A ----------------tunnel---------- Cisco D------Host E
182 +------Host B----------Host C----------+
187 .Bd -literal -offset indent
190 ifconfig greN A D netmask 0xffffffff linkX up
191 ifconfig greN tunnel A D
196 .Bd -literal -offset indent
198 ip unnumbered D ! e.g. address from Ethernet interface
199 tunnel source D ! e.g. address from Ethernet interface
201 ip route C <some interface and mask>
203 ip route X mask tunnelX
210 .Bd -literal -offset indent
214 ifconfig greN tunnel D A
217 If all goes well, you should see packets flowing ;-)
219 If you want to reach Host A over the tunnel (from Host D (Cisco)), then
220 you have to have an alias on Host A for e.g.\& the Ethernet interface like:
222 .Dl "ifconfig <etherif> alias Y"
226 .Dl "ip route Y mask tunnelX"
228 A similar setup can be used to create a link between two private networks
229 (for example in the 192.168 subnet) over the Internet:
231 192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.*
234 +------ the Internet ------+
237 Assuming router A has the (external) IP address A and the internal address
238 192.168.1.1, while router B has external address B and internal address
239 192.168.2.1, the following commands will configure the tunnel:
242 .Bd -literal -offset indent
244 ifconfig greN 192.168.1.1 192.168.2.1 link1
245 ifconfig greN tunnel A B
246 route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1
250 .Bd -literal -offset indent
252 ifconfig greN 192.168.2.1 192.168.1.1 link1
253 ifconfig greN tunnel B A
254 route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1
257 Note that this is a safe situation where the
259 flag (as discussed in the
261 section below) may (and probably should) be set.
265 interfaces is set to 1476 by default, to match the value used by Cisco routers.
266 This may not be an optimal value, depending on the link between the two tunnel
268 It can be adjusted via
271 For correct operation, the
273 device needs a route to the destination that is less specific than the
275 (Basically, there needs to be a route to the decapsulating host that
276 does not run over the tunnel, as this would be a loop.)
277 If the addresses are ambiguous, doing the
278 .Nm ifconfig Cm tunnel
283 IP addresses will help to find a route outside the tunnel.
287 to actually mark the interface as
291 must be given last on its command line.
293 The kernel must be set to forward datagrams by setting the
294 .Va net.inet.ip.forwarding
296 variable to non-zero.
306 A description of GRE encapsulation can be found in RFC 1701 and RFC 1702.
308 A description of MOBILE encapsulation can be found in RFC 2004.
310 .An Heiko W. Rupp Aq Mt hwr@pilhuhn.de
313 .Fn gre_compute_route
316 toggles the last bit of the
317 IP-address to provoke the search for a less specific route than the
318 one directly over the tunnel to prevent loops.
319 This is possibly not the best solution.
321 To avoid the address munging described above, turn on the
326 This implies that the GRE packet destination and the ifconfig remote host
327 are not the same IP addresses, and that the GRE destination does not route
332 The GRE RFCs are not yet fully implemented (no GRE options).