SSH Port Forwarding and Tunnelling – 101

Introduction

SSH is a very common and widely utilized protocol to securely interact with machines. Many developers and those with technical duties in the wide landscape of software engineering and information technology swear by it every day. However, most SSH users just utilize this protocol to interact with the server via the command line. SSH however is capable of a lot more. This blog will be a humble attempt to explain one such technique called SSH Port Forwarding or SSH Tunnelling.

So, what is it ? Port-Forwarding or Tunnelling ?

First let’s understand the difference between port forwarding and tunnelling.

What is port-forwarding?

Port Forwarding, also known as “port-based forwarding” or “port mapping”, maps an IP address and TCP port to another IP address and another TCP port.

What is tunnelling?

Tunnelling is encapsulating and wrapping packets in layers of headers to form a tunnel. A packet like this will have a header that will indicate the packet’s destination and the protocol it uses. The other part of the packet will contain the actual payload/content. It can be understood as a packet within a packet.

Now, that we know both of them, which one is it ?

Types of SSH port-forwarding?

  • Local Port Forwarding
  • Remote Port Forwarding
  • Dynamic Port Forwarding

Local Port Forwarding

Local port forwarding is useful when you want to use a service that’s local to another system. Though it’s not necessary for the service to be local, in most cases it’s used for that i.e when a direct connection is not possible. This allows us to run services locally and not expose them to the internet while still being able to access it like a regular service.

You can do local port-forwarding by following the syntax below: –

1ssh -L 8080:localhost:8384 [email protected]

Here we have opened our port 8080 and any traffic for port 8080 will be redirected to myinstance’s localhost and port 8384.

You can also open multiple ports in a single command like the following

1ssh -L 8080:localhost:8384 9090:localhost:9494 [email protected]

You can also bind to a particular network interface if its required. Here is how you can achieve it

1ssh -L 192.168.0.200:8080:localhost:8384 [email protected]

This would then bind port 8080 on IP address 192.168.0.200 and will make service of myinstance available on that IP and port.

Remote Port Forwarding

Remote Port Forwarding is useful when you want to expose a service running on your local system to the internet. This could be useful when we are behind a NAT service for instance and want to expose a service that otherwise would be very difficult to manage. To achieve this, we can use the following command –

1ssh -R 8080:localhost:8000 [email protected]

If we have a local service running on port 8000 then we can use the above command and it will open port 8080 on myinstance. Once a user visits port 8080 on myinstance it will forward the traffic back to our local machine on port 8000.

A very important configuration parameter to look for when working with Remote Port Forwarding is to check GatewayPorts.

Here are the possible alternatives to look for in the sshd_config which is usually found in the /etc/ssh folder.

  • If GatewayPorts is set to no, it would prevent connecting to forwarded ports from any computer outside of the server from connecting to it.
  • If GatewayPorts is set to yes, it would allow connecting to forwarded ports from any computer outside of the server.
  • If GatewayPorts is set to clientspecified, only connections from the specified address will be allowed, which would look like below
1ssh -R 1.1.1.1:8080:localhost:8000 [email protected]

Dynamic Port Forwarding

Dynamic Port Forwarding turns an SSH client into a SOCKS proxy server. With the help of system tools like proxychains and modifying our proxy settings in the browser settings we can interact with those services.

It can be done by the following command:

1ssh -D 8080 [email protected]

The above command will open the SOCKS port on 8080 and for applications configured with proxy settings all traffic will pass through myinstance.

Suggested Read : Intercepting Request Which Requires VPN + SOCKS Proxy

Scenarios

Let’s solidify above concepts using real life examples.

Scenario 1

Problem: You have a service running in a docker container (172.20.0.3) on port 8888 on the server (192.168.0.111). The docker container is not configured to use host networking. You want to interact with that service on your local machine. You also have SSH access to the machine.

We can use SSH Local Port Forwarding in this case which would mean we will tunnel the service running on the server to a port on our local machine. It could be achieved with the following command:

1ssh -L 8080:172.20.0.3:8888 [email protected]

In case you want to bind the service only to a particular network device on your device below command can be utilized.

1ssh -L 192.168.0.222:8080:172.20.0.3:8888 [email protected]

Here 192.168.0.222 is our Local IP address and 172.20.0.3 is IP address of the docker container on the server host.

fig1

Scenario 2

Problem: We have a service running locally on our system on port 8888. We need to host it on the server for some time to discuss design changes with our colleagues.

With the help of SSH Remote Port Forwarding we can use the below command:

1ssh -R 8080:localhost:8888 [email protected]

Now, the local service running on our local machine’s port 8888 will be able on port 8080 on the server.

Note: For this to work it’s imperative that GatewayPorts is yes in /etc/ssh/sshd_config file. If not, you will have to make necessary changes by using superuser permissions.

fig2

Scenario 3

Problem Let’s say you are facing some problems with your Windows PC. A very tech-savvy friend of yours could have the answer to your problem. You don’t like installing third-party solutions on your host. So, how will you give your friend the ability to RDP login to the machine?

With the help of SSH Remote Port Forwarding, we can tunnel our RDP service on the server so our friend will be able to access our PC from the internet. We assume that you have allowed RDP connections to your host, if not you can follow this guide.

Issuing the following command on your Windows host will make RDP service accessible to users who can also access server 192.168.0.170.

1ssh -R 3389:localhost:3389 [email protected]

fig3

Now our friend can use a service like Remmina to connect to the machine.

fig4

fig5

fig6

Note: Here it’s assumed that the user and user’s friend can access the server (192.168.0.170) but cannot access each other’s network.

Scenario 4

Problem We have compromised a jump server (10.10.10.123) and during our enumeration we discovered an entirely different subnet range of 172.16.1.1/24. Our objective is now to discover which services are running on the internal network and try to access those services. We also need to configure C2 to accept connection on the victim jump server.

With the help of dynamic port forwarding, we can access internal services by using the jump server as a proxy. To set this thing we can issue the following command:

1ssh -D 1080 [email protected] -i /path/to/my/ssh_key

This will open a SOCKS5 proxy on our localhost and we can then use utilities like proxychains to interact with remote services.

Note: A better alternative to this according to me is, sshuttle you can open an entire subnet range with the help of this utility, and it will work as if the internal network was reachable without a proxy. You can issue the following command to work with it.

1sshuttle -r [email protected] 172.16.1.1/24 --ssh-cmd 'ssh -i /path/to/my/sshkey'

To configure our C2 we can do a Remote Port Forward

1ssh -N -R 127.0.0.1:41000:172.16.1.23:41000 [email protected] -i /path/to/ssh/key

This will now listen for connections on port 41000 on the server 172.16.1.23 and accept connections from our implants and relaying everything back to us.

Note:

  1. This usually requires changes in C2 profile/settings to accommodate the changes.
  2. -N will just hang the terminal when trying to SSH into machine and not execute enter a shell.

fig7

In conclusion, SSH is a far more capable utility then what its usually utilized for. These abilities are very useful in System Administration, Development and in Penetration Testing environments. A solid understanding of these techniques go hand-in-hand with most things in Information Technology.

References:

About Payatu

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