Netcat with SSH Port Forwarding

Netcat with SSH Port Forwarding

I have encountered a problem with getting access to the private local service which available from the local machine. And I need to write application using this service in another network. I have only one port open to access the local machine ssh daemon. After a few minutes of problem discussion, we decided to use NetCat.

$ mkfifo pipe
$ while [ 1 ]; do nc -l -p 8080 < pipe | ssh gw_to_private_net \ -p 22977  "nc 192.168.12.230 80" | tee pipe; done

So we need run this only on local machine. Now we have the open local port 8080 forwarded to private machine 192.168.12.230:80.

Let me describe these commands

First open a local port to hear from our network connections. So this port will be proxy the data to private net:

$ nc -l -p 8080

Next step is to connect is to login to private getaway host via ssh and open connection to local private host with ip 192.168.12.230 on port 80:

$ ssh gw_to_private_net -p 22977  "nc 192.168.12.230 80"

So what we have. We can read data from local port to output and we can write data to remote host. Let’s combine these commands:

$ nc -l -p 8080 | ssh gw_to_private_net -p 22977 \
 "nc 192.168.12.230 80"

After running this command we can send request to the local port 8080 and see a response from private host in the output. It is not that we wanted, lets redirect the response to local connection. We use for this pipe or Unix sockets.

$ mkfifo pipe; nc -l -p 8080 < pipe | ssh gw_to_private_net \ 
-p 22977  "nc 192.168.12.230 80" | tee pipe

Wow, that works, but after each request the connection gets closed. That’s why we need to reconnect each time via simple loop.

$ mkfifo pipe
$ while [ 1 ]
> do
>   nc -l -p 8080 < pipe | ssh gw_to_private_net \
-p 22977  "nc 192.168.12.230 80" | tee pipe
> done

Michael Nikitochkin is a Lead Software Engineer. Follow him on LinkedIn or GitHub.

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