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SSH Tunnel

· 2 min read
ひかり
ひかり
Main bloger

Local Port Forwarding​

ssh -L [client_port]:[host_to_forward]:[host_to_forward_port]

Suppose there is a client A, an SSH server B, and a host C, and you want to forward C:8080 to port 80 on A.

ssh -L80:C:8080 B

With this forwarding, accessing http://localhost from A will display the same content as http://C:8080 on B.

In summary, even if C is not visible from A, you can access the home network via SSH if port 22 is open to the outside. If B is visible from A, you can forward C's port to A via B.

Using -g allows access to C:8080 from computers on A's network using A's hostname.

Remote Port Forwarding​

ssh -R [client_port]:[host_to_forward]:[host_to_forward_port]

Unlike local forwarding, it forwards ports visible from the client instead of ports visible from the destination.

Suppose there is a client B, an SSH server A, and a host C, and you want to forward C:8080 to port 80 on A.

ssh -R80:C:8080 A

With this forwarding, accessing http://localhost from A will display the same content as http://C:8080 on B.

Even if B is not accessible from the outside, if a connection between B and A is established, you can forward the content of C to A. It is often used when you cannot directly operate B. When a connection between B and A is broken, tools like auto-ssh are often used to automatically reconnect.