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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44 .Nd encapsulating network device
48 device into the kernel, place the following line in the kernel
50 .Bd -ragged -offset indent
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
87 protocols than IP like e.g.\& AppleTalk.
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, as e.g.\& when
176 encapsulating AppleTalk.
178 Configuration example:
180 Host X-- Host A ----------------tunnel---------- Cisco D------Host E
183 +------Host B----------Host C----------+
188 .Bd -literal -offset indent
191 ifconfig greN A D netmask 0xffffffff linkX up
192 ifconfig greN tunnel A D
197 .Bd -literal -offset indent
199 ip unnumbered D ! e.g. address from Ethernet interface
200 tunnel source D ! e.g. address from Ethernet interface
202 ip route C <some interface and mask>
204 ip route X mask tunnelX
211 .Bd -literal -offset indent
215 ifconfig greN tunnel D A
218 If all goes well, you should see packets flowing ;-)
220 If you want to reach Host A over the tunnel (from Host D (Cisco)), then
221 you have to have an alias on Host A for e.g.\& the Ethernet interface like:
223 .Dl "ifconfig <etherif> alias Y"
227 .Dl "ip route Y mask tunnelX"
229 A similar setup can be used to create a link between two private networks
230 (for example in the 192.168 subnet) over the Internet:
232 192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.*
235 +------ the Internet ------+
238 Assuming router A has the (external) IP address A and the internal address
239 192.168.1.1, while router B has external address B and internal address
240 192.168.2.1, the following commands will configure the tunnel:
243 .Bd -literal -offset indent
245 ifconfig greN 192.168.1.1 192.168.2.1 link1
246 ifconfig greN tunnel A B
247 route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1
251 .Bd -literal -offset indent
253 ifconfig greN 192.168.2.1 192.168.1.1 link1
254 ifconfig greN tunnel B A
255 route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1
258 Note that this is a safe situation where the
260 flag (as discussed in the
262 section below) may (and probably should) be set.
266 interfaces is set to 1476 by default, to match the value used by Cisco routers.
267 This may not be an optimal value, depending on the link between the two tunnel
269 It can be adjusted via
272 For correct operation, the
274 device needs a route to the destination that is less specific than the
276 (Basically, there needs to be a route to the decapsulating host that
277 does not run over the tunnel, as this would be a loop.)
278 If the addresses are ambiguous, doing the
279 .Nm ifconfig Cm tunnel
284 IP addresses will help to find a route outside the tunnel.
288 to actually mark the interface as
292 must be given last on its command line.
294 The kernel must be set to forward datagrams by setting the
295 .Va net.inet.ip.forwarding
297 variable to non-zero.
309 A description of GRE encapsulation can be found in RFC 1701 and RFC 1702.
311 A description of MOBILE encapsulation can be found in RFC 2004.
313 .An Heiko W.Rupp Aq hwr@pilhuhn.de
319 toggles the last bit of the
320 IP-address to provoke the search for a less specific route than the
321 one directly over the tunnel to prevent loops.
322 This is possibly not the best solution.
324 To avoid the address munging described above, turn on the
329 This implies that the GRE packet destination and the ifconfig remote host
330 are not the same IP addresses, and that the GRE destination does not route
335 The GRE RFCs are not yet fully implemented (no GRE options).