EXPOSE instruction for ports in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What does the EXPOSE instruction do in a Dockerfile?

Choose the correct explanation of the EXPOSE instruction in a Dockerfile.

A It sets environment variables inside the container related to network ports.

B It documents the ports the container listens on at runtime but does not publish them automatically.

C It blocks all network traffic to the container except on the exposed ports.

D It automatically publishes the specified ports to the host machine when the container runs.

💡 Hint

Think about whether EXPOSE opens ports by itself or just informs about them.

The EXPOSE instruction in a Dockerfile is used to document which ports the container will listen on at runtime. It does not publish or open these ports automatically; that requires additional options when running the container.

💻 Command Output

intermediate

What is the output of 'docker port' after EXPOSE?

Given a Dockerfile with EXPOSE 8080 and a container started without port mapping, what does docker port <container_id> show?

Docker

Dockerfile:
FROM nginx
EXPOSE 8080

Commands:
docker build -t test-expose .
docker run -d --name test-container test-expose
docker port test-container

A No output, because no ports are published to the host.

B Shows 8080/tcp -> 0.0.0.0:8080, mapping container port to host port.

C Shows an error because EXPOSE is not a valid command.

D Shows 8080/tcp -> <container_ip>:8080, internal container mapping.

💡 Hint

Consider if docker run without -p or --publish publishes ports.

Without explicit port publishing using -p or --publish, the docker port command shows no output because the container ports are not mapped to the host.

❓ Configuration

advanced

Which Dockerfile snippet correctly exposes multiple ports?

Select the Dockerfile snippet that correctly exposes ports 80 and 443.

A EXPOSE 80 443

B

EXPOSE 80
EXPOSE 443

C EXPOSE [80, 443]

D EXPOSE "80,443"

💡 Hint

Think about how Dockerfile instructions handle multiple lines versus single line with multiple ports.

Dockerfile supports multiple EXPOSE instructions, each exposing one or more ports. Using separate EXPOSE lines for each port is correct. The syntax EXPOSE 80 443 is valid because EXPOSE expects one port per instruction or multiple ports separated by spaces but not as a list or string.

❓ Troubleshoot

advanced

Why can't I access my container's service on the exposed port?

You have a Dockerfile with EXPOSE 5000 and run the container with docker run myapp. You try to access the service on port 5000 from your host but fail. What is the most likely reason?

A Docker automatically blocks all ports unless you disable the firewall on your host.

B The container's firewall is blocking port 5000 internally.

C The EXPOSE instruction does not publish ports to the host; you must use <code>-p 5000:5000</code> when running the container.

D The EXPOSE instruction is missing the protocol, so it defaults to UDP which is not accessible.

💡 Hint

Remember what EXPOSE does and what is needed to make ports accessible from outside the container.

EXPOSE only documents ports but does not publish them. To access a container's port from the host, you must publish it using -p host_port:container_port when running the container.

✅ Best Practice

expert

What is the best practice for using EXPOSE in multi-stage Dockerfiles?

In a multi-stage Dockerfile, where should you place the EXPOSE instruction to correctly document the ports for the final image?

A Do not use EXPOSE in multi-stage Dockerfiles because it causes conflicts.

B Place EXPOSE in every stage to document all ports used during build and runtime.

C Place EXPOSE only in the first stage to optimize build caching.

D Place EXPOSE only in the final stage to document ports for the final image.

💡 Hint

Think about which stage produces the final image used to run containers.

EXPOSE should be used in the final stage of a multi-stage Dockerfile to document the ports that the final image will listen on. Ports used only during build stages do not need to be exposed.