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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 .\"
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10 .\" modification, are permitted provided that the following conditions
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19 .\" This product includes software developed by the NetBSD
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 $
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
46 To compile this driver into the kernel, place the following line
47 in the kernel configuration file:
48 .Bd -ragged -offset indent
49 .Cd "pseudo-device gre"
50 .Ed
51 .Pp
52 Alternatively, to load this driver as a module at boot time,
53 place the following line in
54 .Xr loader.conf 5 :
55 .Bd -literal -offset indent
56 if_gre_load="YES"
57 .Ed
58 .Sh DESCRIPTION
59 The
60 .Nm
61 network interface pseudo device encapsulates datagrams
62 into IP.
63 These encapsulated datagrams are routed to a destination host,
64 where they are decapsulated and further routed to their final destination.
65 The
66 .Dq tunnel
67 appears to the inner datagrams as one hop.
68 .Pp
69 .Nm
70 interfaces are dynamically created and destroyed with the
71 .Xr ifconfig 8
72 .Cm create
73 and
74 .Cm destroy
75 subcommands.
76 .Pp
77 This driver currently supports the following modes of operation:
78 .Bl -tag -width indent
79 .It "GRE encapsulation (IP protocol number 47)"
80 Encapsulated datagrams are
81 prepended an outer datagram and a GRE header.
82 The GRE header specifies
83 the type of the encapsulated datagram and thus allows for tunneling other
84 protocols than IP.
85 GRE mode is also the default tunnel mode on Cisco routers.
86 This is also the default mode of operation of the
87 .Nm
88 interfaces.
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)"
100 Datagrams are
101 encapsulated into IP, but with a shorter encapsulation.
102 The original
103 IP header is modified and the modifications are inserted between the
104 so modified header and the original payload.
105 Like
106 .Xr gif 4 ,
107 only for IP-in-IP encapsulation.
108 .El
109 .Pp
110 The
111 .Nm
112 interfaces support a number of
113 .Xr ioctl 2 Ns s ,
114 such as:
115 .Bl -tag -width ".Dv GRESADDRS"
116 .It Dv GRESADDRS
117 Set the IP address of the local tunnel end.
118 This is the source address
119 set by or displayed by
120 .Xr ifconfig 8
121 for the
122 .Nm
123 interface.
124 .It Dv GRESADDRD
125 Set the IP address of the remote tunnel end.
126 This is the destination address
127 set by or displayed by
128 .Xr ifconfig 8
129 for the
130 .Nm
131 interface.
132 .It Dv GREGADDRS
133 Query the IP address that is set for the local tunnel end.
134 This is the
135 address the encapsulation header carries as local address (i.e., the real
136 address of the tunnel start point).
137 .It Dv GREGADDRD
138 Query the IP address that is set for the remote tunnel end.
139 This is the
140 address the encapsulated packets are sent to (i.e., the real address of
141 the remote tunnel endpoint).
142 .It Dv GRESPROTO
143 Set the operation mode to the specified IP protocol value.
144 The
145 protocol is passed to the interface in
146 .Po Vt "struct ifreq" Pc Ns Li -> Ns Va ifr_flags .
147 The 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
156 to
157 .Xr ifconfig 8 .
158 .Pp
159 The
160 .Cm link1
161 flag is not used to choose encapsulation, but to modify the
162 internal route search for the remote tunnel endpoint, see the
163 .Sx BUGS
164 section below.
165 .It Dv GREGPROTO
166 Query operation mode.
167 .El
168 .Pp
169 Note that the IP addresses of the tunnel endpoints may be the same as the
170 ones defined with
171 .Xr ifconfig 8
172 for the interface (as if IP is encapsulated), but need not be.
173 .Sh EXAMPLES
174 Configuration example:
175 .Bd -literal
176 Host X-- Host A ----------------tunnel---------- Cisco D------Host E
177 \\ |
178 \\ /
179 +------Host B----------Host C----------+
180 .Ed
181 .Pp
182 On host A
183 .Pq Dx :
184 .Bd -literal -offset indent
185 route add default B
186 ifconfig greN create
187 ifconfig greN A D netmask 0xffffffff linkX up
188 ifconfig greN tunnel A D
189 route add E D
190 .Ed
191 .Pp
192 On Host D (Cisco):
193 .Bd -literal -offset indent
194 Interface 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
198 ip route C <some interface and mask>
199 ip route A mask C
200 ip route X mask tunnelX
201 .Ed
202 .Pp
203 OR
204 .Pp
205 On Host D
206 .Pq Dx :
207 .Bd -literal -offset indent
208 route add default C
209 ifconfig greN create
210 ifconfig greN D A
211 ifconfig greN tunnel D A
212 .Ed
213 .Pp
214 If all goes well, you should see packets flowing ;-)
215 .Pp
216 If you want to reach Host A over the tunnel (from Host D (Cisco)), then
217 you 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
221 and on the Cisco:
222 .Pp
223 .Dl "ip route Y mask tunnelX"
224 .Pp
225 A 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
228 192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.*
229 \\ /
230 \\ /
231 +------ the Internet ------+
232 .Ed
233 .Pp
234 Assuming router A has the (external) IP address A and the internal address
235 192.168.1.1, while router B has external address B and internal address
236 192.168.2.1, the following commands will configure the tunnel:
237 .Pp
238 On router A:
239 .Bd -literal -offset indent
240 ifconfig greN create
241 ifconfig greN 192.168.1.1 192.168.2.1 link1
242 ifconfig greN tunnel A B
243 route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1
244 .Ed
245 .Pp
246 On router B:
247 .Bd -literal -offset indent
248 ifconfig greN create
249 ifconfig greN 192.168.2.1 192.168.1.1 link1
250 ifconfig greN tunnel B A
251 route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1
252 .Ed
253 .Pp
254 Note that this is a safe situation where the
255 .Cm link1
256 flag (as discussed in the
257 .Sx BUGS
258 section below) may (and probably should) be set.
259 .Sh NOTES
260 The MTU of
261 .Nm
262 interfaces is set to 1476 by default, to match the value used by Cisco routers.
263 This may not be an optimal value, depending on the link between the two tunnel
264 endpoints.
265 It can be adjusted via
266 .Xr ifconfig 8 .
267 .Pp
268 For correct operation, the
269 .Nm
270 device needs a route to the destination that is less specific than the
271 one over the tunnel.
272 (Basically, there needs to be a route to the decapsulating host that
273 does not run over the tunnel, as this would be a loop.)
274 If the addresses are ambiguous, doing the
275 .Nm ifconfig Cm tunnel
276 step before the
277 .Xr ifconfig 8
278 call to set the
279 .Nm
280 IP addresses will help to find a route outside the tunnel.
281 .Pp
282 In order to tell
283 .Xr ifconfig 8
284 to actually mark the interface as
285 .Dq up ,
286 the keyword
287 .Cm up
288 must be given last on its command line.
289 .Pp
290 The kernel must be set to forward datagrams by setting the
291 .Va net.inet.ip.forwarding
292 .Xr sysctl 8
293 variable 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
303 A description of GRE encapsulation can be found in RFC 1701 and RFC 1702.
304 .Pp
305 A 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
309 The
310 .Fn gre_compute_route
311 code in
312 .Pa if_gre.c
313 toggles the last bit of the
314 IP-address to provoke the search for a less specific route than the
315 one directly over the tunnel to prevent loops.
316 This is possibly not the best solution.
317 .Pp
318 To avoid the address munging described above, turn on the
319 .Cm link1
320 flag on the
321 .Xr ifconfig 8
322 command line.
323 This implies that the GRE packet destination and the ifconfig remote host
324 are not the same IP addresses, and that the GRE destination does not route
325 over the
326 .Nm
327 interface itself.
328 .Pp
329 The GRE RFCs are not yet fully implemented (no GRE options).