SSH tunneling (port forwarding) in Linux CLI - Step-by-Step Execution

Concept Flow - SSH tunneling (port forwarding)

Start SSH command

↓

Specify local port

↓

Specify remote host and port

↓

Establish SSH connection

↓

Create encrypted tunnel

↓

Forward local port traffic through tunnel

↓

Access remote service via local port

↓

End

The flow shows how SSH sets up a secure tunnel by forwarding a local port to a remote service through an encrypted connection.

Execution Sample

Linux CLI

ssh -L 8080:remotehost:80 user@sshserver

This command forwards local port 8080 to port 80 on remotehost via sshserver.

Execution Table

Step	Action	Details	Result
1	Start SSH command	ssh -L 8080:remotehost:80 user@sshserver	SSH client prepares connection
2	Specify local port	Local port 8080 chosen	Local port 8080 reserved for forwarding
3	Specify remote host and port	remotehost:80	Target remote service identified
4	Establish SSH connection	Connect to sshserver as user	Encrypted SSH session established
5	Create encrypted tunnel	Tunnel from local 8080 to remotehost:80	Tunnel ready to forward traffic
6	Forward local port traffic	Traffic to localhost:8080	Forwarded through tunnel to remotehost:80
7	Access remote service	User accesses http://localhost:8080	Remote web service accessed securely
8	Exit	User closes SSH session	Tunnel closed, forwarding stops

💡 SSH session ends, tunnel closes, local port forwarding stops

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
local_port	none	8080 reserved	8080 reserved	8080 forwarding active	none (after exit)
remote_host	none	remotehost:80 set	remotehost:80 set	remotehost:80 set	none (after exit)
ssh_connection	none	none	established	established	closed

Key Moments - 3 Insights

Why do we specify a local port in SSH tunneling?

The local port is where your computer listens for traffic to forward through the tunnel. See execution_table step 2 where local port 8080 is reserved.

What happens if the SSH connection drops?

The tunnel closes and port forwarding stops, as shown in execution_table step 8 where the SSH session ends.

Can you access the remote service directly without SSH tunneling?

Only if the remote service is publicly accessible. SSH tunneling allows secure access even if the service is behind a firewall, shown in step 6 forwarding traffic.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the encrypted SSH session established?

A Step 2

B Step 4

C Step 6

D Step 8

💡 Hint

Check the 'Result' column for when the SSH session is established.

According to variable_tracker, what is the state of local_port after step 6?

A none

B 8080 reserved

C 8080 forwarding active

D closed

💡 Hint

Look at the 'local_port' row under 'After Step 6' column.

If the SSH session closes, what happens to the port forwarding?

A It stops

B It switches to another port

C It continues running

D It becomes unencrypted

💡 Hint

Refer to execution_table step 8 and exit_note about tunnel closing.

Concept Snapshot

SSH tunneling forwards a local port to a remote service securely.
Use: ssh -L local_port:remote_host:remote_port user@ssh_server
Local port listens on your machine.
Traffic is encrypted and sent through SSH.
Tunnel closes when SSH session ends.

Full Transcript

SSH tunneling, also called port forwarding, lets you securely access a remote service by forwarding a local port through an encrypted SSH connection. You start by running an SSH command with the -L option, specifying a local port, the remote host and port, and the SSH server to connect to. The SSH client reserves the local port and establishes a secure connection to the SSH server. Then it creates a tunnel that forwards any traffic sent to the local port through the SSH connection to the remote service. When you access the local port, you are actually reaching the remote service securely. When the SSH session ends, the tunnel closes and forwarding stops.