Exposing ports to host in Docker - Mini Project: Build & Apply

Exposing Ports to Host with Docker

📖 Scenario: You are working on a web application inside a Docker container. To test it on your computer's browser, you need to make the container's web server accessible through your computer's network.

🎯 Goal: Learn how to expose a port from a Docker container to your host machine so you can access the container's service from your browser.

📋 What You'll Learn

Create a Dockerfile with a simple web server

Define the port the container listens on

Expose the container port to the host machine

Run the container with port mapping

Verify the service is accessible from the host

💡 Why This Matters

🌍 Real World

Developers often run web servers or APIs inside containers and need to access them from their computers or other devices on the network.

💼 Career

Understanding how to expose container ports to the host is essential for testing, debugging, and deploying containerized applications.

1

Create a Dockerfile with a simple web server

Create a file named Dockerfile with these exact lines to set up a basic Python web server listening on port 8000: FROM python:3.12-slim CMD ["python", "-m", "http.server", "8000"]

Docker

# Create a Dockerfile with the specified content
# Your code here

Hint

This Dockerfile uses Python's built-in HTTP server on port 8000.

2

Add EXPOSE instruction for port 8000

Add a line EXPOSE 8000 to the Dockerfile to declare that the container listens on port 8000.

Docker

FROM python:3.12-slim
CMD ["python", "-m", "http.server", "8000"]
# Add EXPOSE 8000 below
# Your code here

Hint

The EXPOSE instruction tells Docker which port the container will use.

3

Run the container with port mapping

Run the Docker container using the command docker run -d -p 8080:8000 webserver to map container port 8000 to host port 8080. Assume the image is named webserver.

Docker

FROM python:3.12-slim
CMD ["python", "-m", "http.server", "8000"]
EXPOSE 8000
# Run the container with port mapping below
# Your code here

Hint

The -p flag maps host port 8080 to container port 8000.

4

Verify the web server is accessible on host port 8080

Write the command curl http://localhost:8080 to check if the web server running inside the container is reachable from your host machine.

Docker

FROM python:3.12-slim
CMD ["python", "-m", "http.server", "8000"]
EXPOSE 8000
# Run the container with port mapping
# docker run -d -p 8080:8000 webserver

# Verify access with curl
# Your code here

Hint

The curl command fetches the web page served by the container.

Practice

1.

What does the Docker option -p 8080:80 do when running a container?

easy

A. It maps port 8080 inside the container to port 80 on the host machine.

B. It maps port 80 inside the container to port 8080 on the host machine.

C. It exposes port 8080 inside the container without mapping to the host.

D. It blocks port 80 on the host machine.

Solution

1. Step 1: Understand Docker port mapping syntax

   The syntax -p host_port:container_port means the host port is on the left and container port is on the right.

2. Step 2: Apply to given ports

   Given -p 8080:80, port 80 inside the container is accessible via port 8080 on the host.

3. Final Answer:

   It maps port 80 inside the container to port 8080 on the host machine. -> Option B

4. Quick Check:

   host_port:container_port = 8080:80 [OK]

Hint: Remember host port is before colon, container port after [OK]

Common Mistakes:

• Confusing host and container ports order
• Thinking ports are blocked instead of mapped
• Assuming ports are exposed but not mapped

2.

Which of the following is the correct Docker command to expose container port 5000 to host port 3000?

docker run ... myapp

easy

A. docker run -p 3000:5000 myapp

B. docker run -p 5000 myapp

C. docker run --expose 3000:5000 myapp

D. docker run -p 5000:3000 myapp

Solution

1. Step 1: Recall port mapping syntax

   The correct syntax is -p host_port:container_port.

2. Step 2: Match ports to command

   To expose container port 5000 on host port 3000, use -p 3000:5000.

3. Final Answer:

   docker run -p 3000:5000 myapp -> Option A

4. Quick Check:

   host_port:container_port = 3000:5000 [OK]

Hint: Host port is always before colon in -p option [OK]

Common Mistakes:

• Swapping host and container ports
• Using --expose with port mapping syntax
• Omitting container port in -p option

3.

What will be the output when running this command and then accessing http://localhost:8080?

docker run -d -p 8080:80 nginx

medium

A. Docker will throw a syntax error and not run.

B. Connection refused error because port 80 is not exposed.

C. The container will run but port 8080 will be closed.

D. You will see the default Nginx welcome page in your browser.

Solution

1. Step 1: Understand the command

   The command runs Nginx container detached with port 80 inside mapped to host port 8080.

2. Step 2: Accessing localhost:8080

   Since port 8080 on host maps to container's port 80 where Nginx serves, accessing localhost:8080 shows Nginx default page.

3. Final Answer:

   You will see the default Nginx welcome page in your browser. -> Option D

4. Quick Check:

   Port mapping works, Nginx serves on port 80 [OK]

Hint: Map container port 80 to host to access web server [OK]

Common Mistakes:

• Assuming port 80 is not exposed automatically
• Expecting syntax error from correct command
• Thinking container runs but ports are closed

4.

Identify the error in this Docker command that tries to expose port 3306:

docker run -p 3306: mysql

medium

A. Missing container port in the -p option.

B. Port 3306 is not allowed to be exposed.

C. The image name 'mysql' is incorrect.

D. The command should use --expose instead of -p.

Solution

1. Step 1: Check -p option syntax

   The -p option requires both host and container ports in host_port:container_port format.

2. Step 2: Analyze given command

   The command uses -p 3306: without specifying container port, which is invalid syntax.

3. Final Answer:

   Missing container port in the -p option. -> Option A

4. Quick Check:

   -p requires host_port:container_port [OK]

Hint: Always specify both host and container ports with -p [OK]

Common Mistakes:

• Using only one port number with -p
• Confusing --expose and -p options
• Assuming image name causes port errors

5.

You want to run two containers both exposing port 80 inside, but accessible on host ports 8081 and 8082 respectively. Which Docker commands correctly achieve this?

hard

A. docker run -d -p 8081:8081 app1 && docker run -d -p 8082:8082 app2

B. docker run -d -p 80:8081 app1 && docker run -d -p 80:8082 app2

C. docker run -d -p 8081:80 app1 && docker run -d -p 8082:80 app2

D. docker run -d --expose 8081:80 app1 && docker run -d --expose 8082:80 app2

Solution

1. Step 1: Understand port mapping for multiple containers

   Each container exposes port 80 internally. To avoid conflict, map to different host ports like 8081 and 8082.

2. Step 2: Check each option's correctness

   docker run -d -p 8081:80 app1 && docker run -d -p 8082:80 app2 correctly maps host ports 8081 and 8082 to container port 80 for app1 and app2 respectively. Others misuse port order or --expose.

3. Final Answer:

   docker run -d -p 8081:80 app1 && docker run -d -p 8082:80 app2 -> Option C

4. Quick Check:

   Host ports differ, container ports 80 mapped correctly [OK]

Hint: Map different host ports to same container port for multiple containers [OK]

Common Mistakes:

• Swapping host and container ports
• Using same host port for multiple containers
• Confusing --expose with port mapping