SSH -N -f Background Tunnel Explained | Generated by AI

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Question: Why does the SSH command with -N -f exit immediately, while similar SSH commands without those flags stay running until Ctrl+C?

Answer:

The difference comes down to the meaning of the SSH flags you used:

1. -L 5001:127.0.0.1:5001

   • This sets up a local port forwarding: your local port 5001 forwards to 127.0.0.1:5001 on the remote host.

2. -N

   • Tells SSH not to execute any remote commands. It only sets up the port forwarding.

3. -f

   • Requests SSH to go into the background just before executing the command. This is typically used with -N so the SSH tunnel runs in the background without keeping a terminal open.

So your command:

ssh -L 5001:127.0.0.1:5001 amd-cloud -N -f

• Sets up the tunnel, then immediately backgrounds itself, which is why your terminal prompt returns right away.
• The tunnel is still running in the background; you can check it with ps aux | grep ssh or try connecting to localhost:5001.

In contrast, when you SSH without -N -f, like:

ssh -L 5001:127.0.0.1:5001 openclaw

• SSH logs you into the remote host and keeps the session interactive.
• The process stays in the foreground until you press Ctrl+C or exit.
• The port forwarding works as long as the session is alive.

Key takeaway:

• -N -f → background tunnel only, exits terminal immediately.
• No -N -f → foreground interactive session, stays until manually terminated.

You can also combine them depending on whether you want to keep the terminal free or stay logged in.

References:

• SSH Manual: -N and -f
• SSH Port Forwarding

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