SSH tunneling (port forwarding) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the effect of this SSH command?

You run this command on your local machine:

ssh -L 8080:internal.server.com:80 user@remotehost

What does this command do?

A It forwards local port 8080 to port 80 on internal.server.com through remotehost.

B It forwards remotehost's port 8080 to your local port 80.

C It creates a reverse tunnel from internal.server.com port 80 to your local port 8080.

D It forwards remotehost's port 80 to internal.server.com port 8080.

💡 Hint

Remember that -L means local port forwarding from your machine.

The -L option forwards a local port (8080) to a destination (internal.server.com:80) via the SSH server (remotehost).

💻 Command Output

intermediate

What output does this SSH reverse tunnel command produce?

You run this command:

ssh -R 9090:localhost:3000 user@remotehost

What does this command do?

A It forwards your local port 3000 to remotehost's port 9090.

B It forwards your local port 9090 to remotehost's port 3000.

C It forwards remotehost's port 3000 to your local port 9090.

D It forwards remotehost's port 9090 to your local machine's port 3000.

💡 Hint

The -R option creates a reverse tunnel from remotehost to your local machine.

The -R option forwards a port on the remote server (9090) back to a port on your local machine (3000).

🔧 Debug

advanced

Why does this SSH tunnel command fail with 'bind: Address already in use'?

You run:

ssh -L 8080:localhost:80 user@remotehost

But get the error:

bind: Address already in use

What is the most likely cause?

A Your SSH user does not have permission to connect to remotehost.

B Remotehost's port 80 is blocked by firewall.

C Local port 8080 is already used by another process on your machine.

D You typed the command with wrong syntax.

💡 Hint

The error mentions 'bind' which relates to local port usage.

The error means your local machine cannot bind to port 8080 because it is already in use.

🧠 Conceptual

advanced

Which SSH option allows dynamic port forwarding?

You want to create a SOCKS proxy using SSH. Which option should you use?

A -D

B -L

C -R

D -X

💡 Hint

Dynamic forwarding creates a proxy that can handle many ports dynamically.

The -D option sets up a dynamic SOCKS proxy on the local machine.

🚀 Application

expert

How to securely access a web server behind a firewall using SSH tunneling?

You have a web server running on port 80 inside a private network. You can SSH into a gateway server in that network but cannot access the web server directly. Which SSH command lets you access the web server from your local browser at http://localhost:8080?

A ssh -D 8080 user@gateway

B ssh -L 8080:webserver.internal:80 user@gateway

C ssh -R 8080:webserver.internal:80 user@gateway

D ssh -L 80:localhost:8080 user@gateway

💡 Hint

You want to forward a local port to the internal web server through the gateway.

Local forwarding (-L) forwards your local port 8080 to the internal web server's port 80 via the gateway.

Practice

1. What does the SSH command ssh -L 8080:localhost:80 user@remote do?

easy

A. Creates a VPN connection between your machine and the remote server

B. Forwards remote port 8080 to your local port 80

C. Forwards your local port 8080 to port 80 on the remote server's localhost

D. Copies files from port 8080 on remote to port 80 locally

Solution

1. Step 1: Understand the -L option in SSH

   The -L option sets up local port forwarding, mapping a local port to a remote address and port.

2. Step 2: Analyze the command ports and hosts

   Local port 8080 is forwarded to port 80 on the remote server's localhost, allowing local access to remote web service.

3. Final Answer:

   Forwards your local port 8080 to port 80 on the remote server's localhost -> Option C

4. Quick Check:

   ssh -L local_port:remote_host:remote_port = local to remote [OK]

Hint: Remember -L means local port to remote host:port [OK]

Common Mistakes:

• Confusing -L with -R (remote forwarding)
• Mixing up local and remote ports
• Thinking it copies files instead of forwarding ports

2. Which of the following is the correct syntax to forward local port 3306 to remote port 3306 on host db.example.com using SSH?

easy

A. ssh -L 3306:db.example.com:3306 user@remote

B. ssh -R 3306:db.example.com:3306 user@remote

C. ssh -F 3306:db.example.com:3306 user@remote

D. ssh -P 3306:db.example.com:3306 user@remote

Solution

1. Step 1: Identify the correct option for local port forwarding

   The -L flag is used for local port forwarding, which forwards a local port to a remote host and port.

2. Step 2: Check the syntax correctness

   ssh -L 3306:db.example.com:3306 user@remote correctly forwards local port 3306 to remote db.example.com port 3306.

3. Final Answer:

   ssh -L 3306:db.example.com:3306 user@remote -> Option A

4. Quick Check:

   -L for local forwarding syntax [OK]

Hint: Use -L for local forwarding, not -R or others [OK]

Common Mistakes:

• Using -R instead of -L for local forwarding
• Using invalid flags like -F or -P
• Swapping local and remote ports

3. What will be the output of the following command if you run ssh -L 9000:localhost:5432 user@remote and then connect your local machine to port 9000?

Assuming PostgreSQL runs on port 5432 on the remote server, what happens?

medium

A. You connect to the remote PostgreSQL database through local port 9000

B. You connect to a local PostgreSQL database on your machine on port 5432

C. The SSH connection fails because port 9000 is blocked

D. You connect to port 9000 on the remote server directly

Solution

1. Step 1: Understand the port forwarding setup

   The command forwards local port 9000 to remote localhost port 5432, where PostgreSQL runs.

2. Step 2: Analyze the connection effect

   Connecting locally to port 9000 actually connects through SSH tunnel to remote PostgreSQL on port 5432.

3. Final Answer:

   You connect to the remote PostgreSQL database through local port 9000 -> Option A

4. Quick Check:

   Local port 9000 tunnels to remote 5432 [OK]

Hint: Local port connects to remote service via tunnel [OK]

Common Mistakes:

• Thinking it connects to local PostgreSQL instead
• Assuming SSH connection fails without checking firewall
• Confusing local and remote ports

4. You tried to run ssh -L 8080:localhost:80 user@remote but get an error: "bind: Address already in use". What is the most likely cause and fix?

medium

A. Remote port 80 is blocked; use a different remote port

B. Local port 8080 is already used; choose a different local port

C. User authentication failed; check username and password

D. SSH service is not running on remote server

Solution

1. Step 1: Understand the error message

   "bind: Address already in use" means the local port 8080 is occupied by another process.

2. Step 2: Fix by changing local port

   Choose a different unused local port (e.g., 8081) to avoid conflict.

3. Final Answer:

   Local port 8080 is already used; choose a different local port -> Option B

4. Quick Check:

   Port bind error means local port busy [OK]

Hint: Check if local port is free before forwarding [OK]

Common Mistakes:

• Assuming remote port or authentication causes bind error
• Ignoring local port conflicts
• Not checking running processes using the port

5. You want to securely access a web service running on port 5000 of a remote server at 192.168.1.10, but the server only allows SSH connections. Which command correctly sets up a tunnel so you can open http://localhost:7000 in your browser to access the remote service?

hard

A. ssh -L 5000:localhost:7000 user@192.168.1.10

B. ssh -R 7000:localhost:5000 user@192.168.1.10

C. ssh -L 7000:192.168.1.10:5000 user@localhost

D. ssh -L 7000:localhost:5000 user@192.168.1.10

Solution

1. Step 1: Identify the correct port forwarding direction

   You want to forward your local port 7000 to remote port 5000 on the server 192.168.1.10.

2. Step 2: Verify the command syntax

   ssh -L 7000:localhost:5000 user@192.168.1.10 forwards local port 7000 to remote localhost port 5000 correctly.

3. Final Answer:

   ssh -L 7000:localhost:5000 user@192.168.1.10 -> Option D

4. Quick Check:

   Local port 7000 tunnels to remote 5000 [OK]

Hint: Local port first, then remote host:port in -L [OK]

Common Mistakes:

• Using -R instead of -L for local forwarding
• Swapping local and remote ports
• Incorrect remote host or user syntax