Encrypted SSH

SSH port forwarding and some cool uses


I'll begin with some of the bare minimum and absolutely amazing things ssh port forwarding is capable of, and see if I can convert this to a proper blog post later.


1. Local port forwarding

Syntax:

ssh -L8081:example.com:80 pallav@myserver.com

Meaning: Opens a SSH connection to myserver.com, and forward all requests to my port 8081 to port 80 (default http port) of example.com via myserver. Thus, if example.com is blocked on my PC, but it can be accessed by some other PC which I have ssh access to, then I can use this command to gain access to the blocked website (The website is now accessible on localhost:8081 in my browser). Also, ssh connections are encrypted, so the only thing ISP sees is an open connection to myserver, and not the website I'm trying to access. Unfortunately, many websites you see day to day check the hostname of request (which would be localhost) and won't work this way. However, this trick is great to forward SSH connections over multiple hops. (For bypassing website restrictions, read on)

Apart from this, you can forward any arbitrary port data to any arbitrary port, from this point, only thing stopping you is your imagination.


2. Remote port forwarding

Syntax:

ssh -R8081:example.com:22 pallav@myserver.com

This is extremely similar to local port forwarding, which is why it causes a lot of people confusion. In previous case, the port being mapped was on local machine and the request was being sent to example.com via remote server. In this case, the port being mapped is on the remote server, while the request being sent is from local computer. Get the difference? Good.

This is typically used if the local computer doesn't have a public IP address, and you need access to it from outside. In that case, something like

ssh -R8081:localhost:22 pallav@myserver.com

Would give ssh access to (previously publically unaccessible) local PC via port 8081 of remote server.

3. Dynamic port forwarding

Syntax:

ssh -D8081 pallav@myserver.com

In local port forwarding, we were restricted to forwarding request to a particular port of example.com. But what if we want the requests to reach different destinations, on different ports? By the above command, the port 8081 on localhost is dynamically mapped such that, localhost:8081 can be used as a socks proxy for any request. Thus, this means you can use remote server as a free VPN, by using the above command, and then setting localhost:8081 as the socks proxy in the browser (leave other fields empty).

Quiz time

Let's say you are in a hotel that has free wifi. But like every hotel, once you connect to the wifi, it redirects you to the login page, which only allows one login per room ☹. But you are a hacker, and decide to share the connection with you brother, who also has a laptop. What do you do?

Turn on sshd service on one laptop (whichever one logins to internet) Use the other laptop to ssh into this one using local ip address (shown in ifconfig) . While SSHing, set up dynamic port forwarding. Now internet is accessible via socks proxy through the tunnel.




ABOUT THE AUTHOR
Pallav Agarwal Image
My name is Pallav Agarwal. I am a recent graduate of the department of Computer Science and Engineering, Indian Institute of Technology Kanpur, India. I love experimenting with tech, and this blog is a way for me to give a little helping hand to other's who are like me (but don't know it yet).

I am ambitious, intelligent, competitve (sometimes too much), loyal and brutally honest. People I respect the most are teachers, which is partially why I myself like to teach too. Apart from programming, I also like travelling, adventure sports and trying new food items. If you like a post, have a query, or just want to chit-chat, let me know here