1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
|
.\" $OpenBSD: gre.4,v 1.6 2000/11/10 20:02:16 todd Exp $
.\" $NetBSD: gre.4,v 1.10 1999/12/22 14:55:49 kleink Exp $
.\"
.\" Copyright 1998 (c) The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Heiko W.Rupp <hwr@pilhuhn.de>
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the NetBSD
.\" Foundation, Inc. and its contributors.
.\" 4. Neither the name of the The NetBSD Foundation nor the names of its
.\" contributors may be used to endorse or promote products derived
.\" from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd 13 September 1998
.Dt GRE 4
.Os
.Sh NAME
.Nm gre
.Nd encapsulating network device
.Sh SYNOPSIS
.Cd pseudo-device gre Op Ar count
.Sh NOTE
.Tn GRE
and
.Tn MobileIP
are enabled with the following
.Xr sysctl 3
variables respectively in
.Pa /etc/sysctl.conf :
.Bl -tag -width xxxxxxxxxxxxxxxxxxxxx
.It net.inet.gre.allow
Allow GRE packets in and out of the system.
.It net.inet.mobileip.allow
Allow MobileIP packets in and out of the system.
.El
.Pp
.Sh DESCRIPTION
The
.Nm gre
network interface allows tunnel construction using the Cisco GRE or
the Mobile-IP (RFC 2004) encapsulation protocols.
.Pp
This driver currently supports the following modes of operation:
.Bl -tag -width abc
.It GRE encapsulation (IP protocol number 47).
Encapsulated datagrams are prepended by a outer datagram and a GRE header.
The GRE header specifies the type of the encapsulated datagram
and thus allows for tunneling other protocols than IP like
e.g. AppleTalk (not yet supported).
GRE mode is the default tunnel mode on Cisco routers.
This is also the default mode of operation of the
.Sy gre Ns Ar X
interfaces.
.It MOBILE encapsulation (IP protocol number 55).
Datagrams are encapsulated into IP, but with a much smaller
encapsulation header.
This protocol only supports IP in IP encapsulation, and is intended
for use with mobile IP.
.El
.Pp
The network interfaces are named
.Sy gre Ns Ar 0 ,
.Sy gre Ns Ar 1 ,
etc.
The number of interfaces is given by the corresponding
.Sy pseudo-device
line in the system config file.
.Xr gre 4
interfaces support the following
.Xr ioctl 2 Ns s :
.Bl -tag -width aaa
.It GRESADDRS:
Set the IP address of the local tunnel end.
.It GRESADDRD:
Set the IP address of the remote tunnel end.
.It GREGADDRS:
Query the IP address that is set for the local tunnel end.
.It GREGADDRD:
Query the IP address that is set for the remote tunnel end.
.It GRESPROTO:
Set the operation mode to the specified IP protocol value.
The protocol is passed to the interface in (struct ifreq)->ifr_flags.
The operation mode can also be given as
.Bl -tag -width bbb
.It link0
IPPROTO_GRE
.It link2
IPPROTO_MOBILE
.El
.Pp
to
.Xr ifconfig 8 .
As the linkN flags are not mutually exclusive, modes must be set by applying
positive and negative flags, e.g.,
.Xr ifconfig 8
link0 -link1 -link2
.It GREGPROTO:
Query operation mode.
.El
.Pp
Note that the IP addresses of the tunnel endpoints may be the same as the
ones defined with
.Xr ifconfig 8
for the interface (as if IP is encapsulated), but need not be, as e.g. when
encapsulating AppleTalk.
.Pp
.Sh EXAMPLE
Configuration example:
.Bd -literal
Host X-- Host A ----------------tunnel---------- cisco D------Host E
\\ |
\\ /
+------Host B----------Host C----------+
.Ed
On host A (OpenBSD):
# route add default B
# ifconfig greN A D netmask 0xffffffff linkX up
# route add E D
On Host D (Cisco):
Interface TunnelX
ip unnumbered D ! e.g. address from Ethernet interface
tunnel source D ! e.g. address from Ethernet interface
tunnel destination A
ip route C <some interface and mask>
ip route A mask C
ip route X mask tunnelX
OR
On Host D (OpenBSD):
# route add default C
# ifconfig greN D A
.Pp
To reach Host A over the tunnel (from host D), there has to be an
alias on Host A for the Ethernet interface:
ifconfig <etherif> alias Y
and on the cisco
ip route Y mask tunnelX
.Sh NOTE
For correct operation, the
.Nm
device needs a route to the destination, that is less specific than the
one over the tunnel.
(There needs to be a route to the decapsulating host that
does not run over the tunnel, as this would create a loop.)
.Pp
In order for
.Xr ifconfig 8
to actually mark the interface as up, the keyword ``up'' must be given
last on its command line.
.Pp
The kernel must be set to forward datagrams by including option
``GATEWAY'' in the kernel config file and issuing the appropriate
option to
.Xr sysctl 8 .
.Sh SEE ALSO
.Xr netintro 4 ,
.Xr ip 4 ,
.Xr atalk 4 ,
.Xr inet 4 ,
.Xr ifconfig 8 ,
.Xr options 4 ,
.Xr protocols 5 ,
.Xr sysctl 8
.Pp
A description of GRE encapsulation can be found in RFC 1701, RFC 1702.
.Pp
A description of MOBILE encapsulation can be found in RFC 2004.
.Sh BUGS
The compute_route() code in net/if_gre.c toggles the last bit of the
IP-address to provoke the search for a less specific route than the
one directly over the tunnel to prevent loops.
This is possibly not the best solution.
.Pp
To avoid the address munging described above, turn on the link1 flag
on the ifconfig command line.
This implies that the GRE packet destination and the remote host are not
the same IP addresses, and that the GRE destination does not route over
the greX interface itself.
.Pp
GRE RFC not yet fully implemented (no GRE options), no other protocols
yet than IP over IP.
.Pp
.Sh AUTHORS
Heiko W.Rupp <hwr@pilhuhn.de>
|
