Exposing ports to host in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of the port mapping command?

You run the command docker run -d -p 8080:80 nginx. What does this command do?

A Starts an nginx container with port 80 inside mapped to port 80 on the host.

B Starts an nginx container with port 8080 inside mapped to port 80 on the host.

C Starts an nginx container with port 80 inside mapped to port 8080 on the host.

D Starts an nginx container with port 8080 inside mapped to port 8080 on the host.

💡 Hint

The format is -p hostPort:containerPort.

The command maps port 80 inside the container to port 8080 on the host machine, so accessing localhost:8080 reaches nginx's port 80.

🧠 Conceptual

intermediate

Why expose ports in Docker containers?

Why do we expose ports from a Docker container to the host machine?

A To allow network traffic from the host to reach services inside the container.

B To increase the container's CPU usage.

C To make the container use more memory.

D To stop the container from running.

💡 Hint

Think about how you access a website running inside a container.

Exposing ports lets the host machine send requests to the container's services, like a web server.

❓ Configuration

advanced

Which Docker Compose port mapping is correct?

Given this Docker Compose snippet, which option correctly maps port 5000 inside the container to port 3000 on the host?

Docker

services:
  app:
    image: myapp
    ports:

A - "3000"

B - "5000:3000"

C - "5000"

D - "3000:5000"

💡 Hint

Remember the format is hostPort:containerPort.

The correct syntax maps host port 3000 to container port 5000, allowing access via localhost:3000.

❓ Troubleshoot

advanced

Why can't you access the container service on the host port?

You ran docker run -d -p 8080:80 nginx but cannot access the web server at http://localhost:8080. What could be the reason?

A The container's service is not listening on port 80 inside the container.

B The host port 8080 is blocked by firewall.

C Docker daemon is not running.

D The container image is missing.

💡 Hint

Check if the service inside the container is running on the expected port.

If the service inside the container is not listening on port 80, mapping that port won't work.

✅ Best Practice

expert

What is the best practice for exposing ports in production Docker containers?

Which option is the best practice for exposing ports when running Docker containers in production?

A Expose ports randomly to confuse attackers.

B Expose only necessary ports and use firewall rules to restrict access.

C Do not expose any ports and rely on container internal networking only.

D Expose all container ports to the host for easy access.

💡 Hint

Think about security and minimal exposure.

Exposing only needed ports and controlling access improves security and reduces attack surface.