xref: /openssh-portable/README.tun (revision ddd63ab1)
1How to use OpenSSH-based virtual private networks
2-------------------------------------------------
3
4OpenSSH contains support for VPN tunneling using the tun(4) network
5tunnel pseudo-device which is available on most platforms, either for
6layer 2 or 3 traffic.
7
8The following brief instructions on how to use this feature use
9a network configuration specific to the OpenBSD operating system.
10
11(1) Server: Enable support for SSH tunneling
12
13To enable the ssh server to accept tunnel requests from the client, you
14have to add the following option to the ssh server configuration file
15(/etc/ssh/sshd_config):
16
17	PermitTunnel yes
18
19Restart the server or send the hangup signal (SIGHUP) to let the server
20reread it's configuration.
21
22(2) Server: Restrict client access and assign the tunnel
23
24The OpenSSH server simply uses the file /root/.ssh/authorized_keys to
25restrict the client to connect to a specified tunnel and to
26automatically start the related interface configuration command. These
27settings are optional but recommended:
28
29	tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... reyk@openbsd.org
30
31(3) Client: Configure the local network tunnel interface
32
33Use the hostname.if(5) interface-specific configuration file to set up
34the network tunnel configuration with OpenBSD. For example, use the
35following configuration in /etc/hostname.tun0 to set up the layer 3
36tunnel on the client:
37
38	inet 192.168.5.1 255.255.255.252 192.168.5.2
39
40OpenBSD also supports layer 2 tunneling over the tun device by adding
41the link0 flag:
42
43	inet 192.168.1.78 255.255.255.0 192.168.1.255 link0
44
45Layer 2 tunnels can be used in combination with an Ethernet bridge(4)
46interface, like the following example for /etc/bridgename.bridge0:
47
48	add tun0
49	add sis0
50	up
51
52(4) Client: Configure the OpenSSH client
53
54To establish tunnel forwarding for connections to a specified
55remote host by default, use the following ssh client configuration for
56the privileged user (in /root/.ssh/config):
57
58	Host sshgateway
59		Tunnel yes
60		TunnelDevice 0:any
61		PermitLocalCommand yes
62	        LocalCommand sh /etc/netstart tun0
63
64A more complicated configuration is possible to establish a tunnel to
65a remote host which is not directly accessible by the client.
66The following example describes a client configuration to connect to
67the remote host over two ssh hops in between. It uses the OpenSSH
68ProxyCommand in combination with the nc(1) program to forward the final
69ssh tunnel destination over multiple ssh sessions.
70
71	Host access.somewhere.net
72	        User puffy
73	Host dmzgw
74	        User puffy
75	        ProxyCommand ssh access.somewhere.net nc dmzgw 22
76	Host sshgateway
77	        Tunnel Ethernet
78	        TunnelDevice 0:any
79	        PermitLocalCommand yes
80	        LocalCommand sh /etc/netstart tun0
81	        ProxyCommand ssh dmzgw nc sshgateway 22
82
83The following network plan illustrates the previous configuration in
84combination with layer 2 tunneling and Ethernet bridging.
85
86+--------+       (          )      +----------------------+
87| Client |------(  Internet  )-----| access.somewhere.net |
88+--------+       (          )      +----------------------+
89    : 192.168.1.78                             |
90    :.............................         +-------+
91     Forwarded ssh connection    :         | dmzgw |
92     Layer 2 tunnel              :         +-------+
93                                 :             |
94                                 :             |
95                                 :      +------------+
96                                 :......| sshgateway |
97                                      | +------------+
98--- real connection                 Bridge ->  |          +----------+
99... "virtual connection"                     [ X ]--------| somehost |
100[X] switch                                                +----------+
101                                                          192.168.1.25
102
103(5) Client: Connect to the server and establish the tunnel
104
105Finally connect to the OpenSSH server to establish the tunnel by using
106the following command:
107
108	ssh sshgateway
109
110It is also possible to tell the client to fork into the background after
111the connection has been successfully established:
112
113	ssh -f sshgateway true
114
115Without the ssh configuration done in step (4), it is also possible
116to use the following command lines:
117
118	ssh -fw 0:1 sshgateway true
119	ifconfig tun0 192.168.5.1 192.168.5.2 netmask 255.255.255.252
120
121Using OpenSSH tunnel forwarding is a simple way to establish secure
122and ad hoc virtual private networks. Possible fields of application
123could be wireless networks or administrative VPN tunnels.
124
125Nevertheless, ssh tunneling requires some packet header overhead and
126runs on top of TCP. It is still suggested to use the IP Security
127Protocol (IPSec) for robust and permanent VPN connections and to
128interconnect corporate networks.
129
130	Reyk Floeter
131
132$OpenBSD: README.tun,v 1.4 2006/03/28 00:12:31 deraadt Exp $
133