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Blog di Bernardino (Dino) Ciuffetti
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17 Set 12 SSH local and remote tunnels

Sometimes you may need to forward remote traffic to a local host through a SSH connection. In other words you can bind a given TCP port to a server running SSH and make remote clients connecting to it, letting the traffic to be redirected to a local server.

    You may need to add the parameter GatewayPorts clientspecified to /etc/ssh/sshd_config on the SSH server and restart sshd. This is to enable ssh clients to bind remote connections on a given ip, otherwise you can only connect to the remote port just from 127.0.0.1.

    At this point, let me do an example:

    • A TCP client wants to connect to 192.168.1.2 on TCP port 18443
    • You want to forward TCP traffic from 192.168.1.2:18443 to 11.22.33.44:18443
    • You have a client host with IP address 11.22.33.41 that can reach 11.22.33.44
    • You can establish a SSH connection from your client (11.22.33.41) to the remote server (192.168.1.2)

    If you have the given situation, you can execute the following command to bind the TCP port 18443 on the remote server:

    ssh -l root 192.168.1.2 -R:18443:11.22.33.44:18443

    Now, you can apply your changes:

    • Substitute “root” with your SSH user on the remote SSH server
    • Substitute “192.168.1.2” with your remote SSH server IP/host
    • Substitute the first “18443” with the port your remote TCP clients need to connect
    • Substitute “11.22.33.44” with your internal TCP server to forward traffic coming from the outside
    • Substitute the second “18443” with the TCP port listening on the internal host

    You can even do the reverse, letting local traffic flowing to an external host, passing through a SSH connection.

    Let me do another example:

    • You have a client host with IP address 11.22.33.41
    • A TCP client wants to connect to 11.22.33.41 (your IP) on TCP port 18443
    • You want to forward local TCP traffic from 11.22.33.41:18443 to 192.168.1.3:18443
    • You can establish a SSH connection from your client (11.22.33.41) to a remote server (192.168.1.2) that can reach 192.168.1.3

    If you have the given situation, you can execute the following command to bind the TCP port 18443 of your computer to the remote server:

    ssh -g -l root 192.168.1.2 -L18443:192.168.1.3:18443

    Now, apply your changes:

    • Substitute “root” with your SSH user on the remote SSH server
    • Substitute “192.168.1.2 with your remote SSH server IP/host
    • Substitute the first “18443” with the port your local TCP clients need to connect
    • Substitute “192.168.1.3” with the remote TCP server you want to reach from your local TCP clients
    • Substitute the second “18443” with the TCP port of the remote server