1 .\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $
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6 .\" This code is derived from software contributed to The NetBSD Foundation
7 .\" by Heiko W.Rupp <hwr@pilhuhn.de>
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37 .Nd encapsulating network device
40 driver into the kernel, place the following line in the kernel
42 .Bd -ragged -offset indent
46 Alternatively, to load the
47 driver as a module at boot time, place the following line in
49 .Bd -literal -offset indent
55 network interface pseudo device encapsulates datagrams
57 These encapsulated datagrams are routed to a destination host,
58 where they are decapsulated and further routed to their final destination.
61 appears to the inner datagrams as one hop.
64 interfaces are dynamically created and destroyed with the
71 This driver currently supports the following modes of operation:
72 .Bl -tag -width indent
73 .It "GRE encapsulation (IP protocol number 47)"
74 Encapsulated datagrams are
75 prepended an outer datagram and a GRE header.
76 The GRE header specifies
77 the type of the encapsulated datagram and thus allows for tunneling other
79 GRE mode is also the default tunnel mode on Cisco routers.
80 This is also the default mode of operation of the
83 As part of the GRE mode,
85 also supports Cisco WCCP protocol, both version 1 and version 2.
86 Since there is no reliable way to distinguish between WCCP versions, it
87 should be configured manually using the
92 flag is not set (default), then WCCP version 1 is selected.
93 .It "MOBILE encapsulation (IP protocol number 55)"
95 encapsulated into IP, but with a shorter encapsulation.
97 IP header is modified and the modifications are inserted between the
98 so modified header and the original payload.
101 only for IP-in-IP encapsulation.
106 interfaces support a number of
109 .Bl -tag -width ".Dv GRESADDRS"
111 Set the IP address of the local tunnel end.
112 This is the source address
113 set by or displayed by
119 Set the IP address of the remote tunnel end.
120 This is the destination address
121 set by or displayed by
127 Query the IP address that is set for the local tunnel end.
129 address the encapsulation header carries as local address (i.e., the real
130 address of the tunnel start point).
132 Query the IP address that is set for the remote tunnel end.
134 address the encapsulated packets are sent to (i.e., the real address of
135 the remote tunnel endpoint).
137 Set the operation mode to the specified IP protocol value.
139 protocol is passed to the interface in
140 .Po Vt "struct ifreq" Pc Ns Li -> Ns Va ifr_flags .
141 The operation mode can also be given as
143 .Bl -tag -width ".Cm -link0" -compact
155 flag is not used to choose encapsulation, but to modify the
156 internal route search for the remote tunnel endpoint, see the
160 Query operation mode.
162 Set the GRE key used for outgoing packets.
163 A value of 0 disables the key option.
165 Get the GRE key currently used for outgoing packets.
166 0 means no outgoing key.
169 Note that the IP addresses of the tunnel endpoints may be the same as the
172 for the interface (as if IP is encapsulated), but need not be.
174 Configuration example:
176 Host X-- Host A ----------------tunnel---------- Cisco D------Host E
179 +------Host B----------Host C----------+
184 .Bd -literal -offset indent
187 ifconfig greN A D netmask 0xffffffff linkX up
188 ifconfig greN tunnel A D
193 .Bd -literal -offset indent
195 ip unnumbered D ! e.g. address from Ethernet interface
196 tunnel source D ! e.g. address from Ethernet interface
198 ip route C <some interface and mask>
200 ip route X mask tunnelX
207 .Bd -literal -offset indent
211 ifconfig greN tunnel D A
214 If all goes well, you should see packets flowing ;-)
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:
219 .Dl "ifconfig <etherif> alias Y"
223 .Dl "ip route Y mask tunnelX"
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:
228 192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.*
231 +------ the Internet ------+
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:
239 .Bd -literal -offset indent
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
247 .Bd -literal -offset indent
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
254 Note that this is a safe situation where the
256 flag (as discussed in the
258 section below) may (and probably should) be set.
262 interfaces is set to 1476 by default, to match the value used by Cisco routers.
263 If grekey is set this is lowered to 1472.
264 This may not be an optimal value, depending on the link between the two tunnel
266 It can be adjusted via
269 For correct operation, the
271 device needs a route to the destination that is less specific than the
273 (Basically, there needs to be a route to the decapsulating host that
274 does not run over the tunnel, as this would be a loop.)
275 If the addresses are ambiguous, doing the
276 .Nm ifconfig Cm tunnel
281 IP addresses will help to find a route outside the tunnel.
285 to actually mark the interface as
289 must be given last on its command line.
291 The kernel must be set to forward datagrams by setting the
292 .Va net.inet.ip.forwarding
294 variable to non-zero.
305 A description of GRE encapsulation can be found in RFC 1701 and RFC 1702.
307 A description of MOBILE encapsulation can be found in RFC 2004.
309 .An Heiko W.Rupp Aq hwr@pilhuhn.de
315 toggles the last bit of the
316 IP-address to provoke the search for a less specific route than the
317 one directly over the tunnel to prevent loops.
318 This is possibly not the best solution.
320 To avoid the address munging described above, turn on the
325 This implies that the GRE packet destination and the ifconfig remote host
326 are not the same IP addresses, and that the GRE destination does not route
331 The current implementation uses the key only for outgoing packets.
332 Incoming packets with a different key or without a key will be treated as if they
333 would belong to this interface.
335 RFC1701 is not fully supported, however all unsupported features have been
336 deprecated in RFC2784.