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.