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t007-ssh-tunnels.txt (5707B)
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1 ## Rationale
2
3 Corporate and academic networks are closed by design, with routers
4 and firewalls forwarding and filtering content going to and from
5 the wider internet. For security reasons this is an absolute
6 necessity, as the guardkeeping prevents unwanted incoming connections
7 to the networked devices.
8
9 However, it is often necessary to connect to internal devices or
10 services from the outside. This could be the case if an employee
11 needs to access a shared database on the company network, or a
12 subscription website only allows full access from a certain range
13 of IP addresses. Network administrators usually offer virtual
14 private network (VPN) access to achieve such goals. Unfortunately,
15 VPN access occasionally requires particular software that may not
16 work on all operating systems. In other cases, the network
17 administrators may enforce strict requirements to the remote systems
18 before allowing VPN access.
19
20 ###### Closed Network ######
21 # #
22 # +----------+ +----------+ +----------+
23 # | Office | | Router/ | ? | Outside |
24 # | Computer |<~~~~>| Firewall | ? ? | Computer |
25 # +----------+ +----------+ +----------+
26 # #
27 ############################
28
29 So what do you do if you need outside access to a network, have no
30 administrative rights over the router and firewall, and cannot (or
31 don't want to) access via VPN? Fortunately, OpenSSH, the widely
32 used secure shell (SSH) implementation, offers simple and secure
33 solutions to this problem. Almost all Linux/BSD/UNIX/MacOS systems
34 come with OpenSSH preinstalled, so you might already have it on
35 your system.
36
37 If you can access the closed network from the outside via SSH, this
38 makes things straightforward as described in Scenario 1 below. If
39 not, see Scenario 2.
40
41
42 ## Scenario 1: SSH access available from the outside
43
44 Some networks are configured to allow outsiders to connect to an
45 internal SSH server through port forwarding on the network router:
46
47 ###### Closed Network ######
48 # #
49 # +----------+ +----------+ +----------+
50 # | Office | SSH | Router/ | SSH | Outside |
51 # | Computer |<~~~~~| Firewall |<~~~~~| Computer |
52 # +----------+ +----------+ +----------+
53 # #
54 ############################
55
56 For the purposes described here, this is an ideal situation since
57 it is easy to create a tunnel that connects the outside computer
58 with the internal network via SSH. The following command creates
59 the tunnel when executed on the outside computer:
60
61 $ ssh -D 1337 -C -N company-domain.com
62
63 Note that the port number specified with the -D option should be
64 greater than 1000 when running as an unpriviledged (non-root) user.
65 The -C option turns on compression, which is useful for slow network
66 connections at the cost of little CPU overhead.
67
68 With the SSH tunnel in place, you can make most webbrowsers and
69 other network programs on the outside computer use the tunnel for
70 all their network traffic by pointing them to the SOCKSv5 proxy
71 "socks://localhost:1337". This allows access from programs on the
72 outside computer to any device within the closed network. Connections
73 to the wider internet utilizing the tunnel will originate from an
74 IP address associated with the closed network, achieving the
75 objectives stated above.
76
77
78 ## Scenario 2: SSH access unavailable from the outside
79
80 Unfortunately, outside SSH access to corporate networks is becoming
81 increasingly rare. However, the OpenSSH toolset again offers a
82 solution if you have a persistent SSH server outside of the network
83 at your disposal:
84
85 ###### Closed Network ######
86 # #
87 # +----------+ +----------+ +---------+ +---------+
88 # | Office | SSH | Router/ | SSH | Outside | SSH | Outside |
89 # | Computer |<~~~~>| Firewall |<~~~~>| Server |<~~~~~| Laptop |
90 # +----------+ +----------+ +---------+ +---------+
91 # #
92 ############################
93
94 As long as you can initiate *outgoing* SSH connections from inside
95 the closed network to your outside SSH server, you can create a
96 reverse ssh tunnel and utilize it in a similar manner as in the
97 previous scenario. On the office computer, create a reverse tunnel
98 to the outside server:
99
100 $ ssh -f -N -R 10022:localhost:22 outside-server.com
101
102 As long as the above command runs, you can initiate new SSH connections
103 from the outside server to the office computer with the command
104 `ssh -p 10022 localhost`. If you're working from an outside laptop,
105 you can utilize this reverse tunnel to connect to the office computer
106 and network. Add the following configuration to `~/.ssh/config`
107 on the outside laptop:
108
109 Host office_computer
110 ProxyCommand ssh -q outside-server.com nc localhost 10022
111
112 With the above configuration, it is very easy to establish a SSH
113 connection from the outside laptop to the office computer:
114
115 $ ssh office_computer
116
117 As in the previous example, you can use this setup to create a SSH
118 tunnel all the way from the outside laptop to the office computer:
119
120 $ ssh -D 1337 -C -N office_computer
121
122 Again, this creates a SOCKSv5 proxy that you can use for tunneling
123 network traffic from the outside laptop to the closed network. It
124 is useful to automatically monitor the tunnel status using pgrep(1),
125 and reinitialize it if the ssh command unexpectedly quits.
126
127
128 References:
129
130 - OpenSSH: https://www.openssh.com/
131 - ssh(1) manual page: https://man.openbsd.org/ssh
132 - gramscii(1), used for drawings in this post: git://bitreich.org/gramscii
133
134 Thanks to KatolaZ for feedback on this post.