I create SSH tunnels every day and this article is a small introduction into the SSH tunneling (also known as “port forwarding”). A secure shell (SSH) tunnel consists of an encrypted tunnel created through a SSH protocol connection. Users may set up SSH tunnels to transfer unencrypted traffic over a network through an encrypted channel. SSH is typically used to log into a remote machine and execute commands, but it also supports tunneling, forwarding TCP ports and X11 connections; it can transfer files using the associated SSH file transfer (SFTP) or secure copy (SCP) protocols. Also tunnels provide an ability to bypass firewalls :)

So, let’s start. Ports can be forwarded in three ways:

1. Local port forwarding;
2. Remote port forwarding;
3. Dynamic port forwarding (proxy).

Wikipedia says that:

Port forwarding or port mapping is a name given to the combined technique of
* translating the address and/or port number of a packet to a new destination
* possibly accepting such packet(s) in a packet filter (firewall)
* forwarding the packet according to the routing table.

Tunneling with local port forwarding

Let’s imagine that there are some computers with specific abilities:

• work - can’t access target (for example, target is blocked by firewall or target can be unaccessible from the work’s network)
• bridge (gateway) - accessible from work and can access target (need to have IP accessible from work)

So, we want to access target throw the global machine.

Access from work computer to some external banned resource

To create the SSH tunnel you should execute the following command from work (local) machine:

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ssh -L [<local-address-to-listen>:]<local-port-to-listen>:<target-host>:<target-port> <bridge>

Now the SSH client at work will connect to the SSHd running at bridge and bind <target-host>:<target-port> to listen for local requests to [<local-address-to-listen>:]<local-port-to-listen>. At the bridge end the connection to <target-host>:<target-port> will be created. So work doesn’t need to know how to connect to <target-host>:<target-port>, only bridge needs to worry about that. The channel between work and bridge will be encrypted (SSH tunnel), but the connection between bridge and <target-host>:<target-port> will be unencrypted.

After executing this command, it is possible to access <target-host>:<target-port> by accessing [<local-address-to-listen>:]<local-port-to-listen> at work computer. The bridge computer will act as a gateway which would accept requests from work machine and fetch data/tunnelling it back.

Tunneling with remote port forwarding (reversed tunneling)

Let’s reverse the problem.

• home - can’t access target (for example, target is protected by firewall or target can be unacessible from the home’s network)
• bridge (gateway or work) - accessible from home and can access target (need to have IP accessible from home)

To create the SSH tunnel you should execute the following command from bridge (gateway/work) machine:

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ssh -R [<home-address-to-listen>:]<home-port-to-listen>:<target-host>:<target-port> <home>

After executing of this command, the SSH client at bridge will connect to SSHd running at home and create the tunnel. Then the server will bind [<home-address-to-listen>:]<home-port-to-listen> to listen for incoming requests which would be routed through the created SSH tunnel between home and gateway. Now it’s possible to access the “internal” resource <target-host>:<target-port> by accessing [<home-address-to-listen>:]<home-port-to-listen> from the home machine. The gateway will then create a connection to <target-host>:<target-port> and relay back the response to home via the created SSH tunnel.

With this approach we need to create the splitted SSH tunnel for each new resource. But SSH provides us an ability to proxy traffic to any resource using SSH tunnel and it named dynamic port forwarding.

Dynamic port forwarding (proxy)

Dynamic port forwarding provides us an ability to configure one local port for tunnelling data to all remote destinations using the SOCKS protocol. At the client side of the SSH tunnel a SOCKS proxy would be created and any application that can work throw SOCKS protocol can use it to access some resources that accessible only from the other host.

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ssh -D [<local-address-to-listen>:]<local-port-to-listen> <gateway>

Additional SSH flags

There are several intresting ssh command flags:

• -v - verbose output
• -q - quiet mode
• -1 - forces ssh to try protocol version 1 only
• -2 - forces ssh to try protocol version 2 only
• -4 - forces ssh to use IPv4 addresses only
• -6 - forces ssh to use IPv6 addresses only