EXPOSE instruction for ports in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the EXPOSE instruction do in a Dockerfile?

It tells Docker which ports the container will listen on at runtime. It acts as documentation and helps Docker map ports when running containers.

beginner

Does the EXPOSE instruction publish ports to the host machine automatically?

No. EXPOSE only documents the ports. To make ports accessible outside the container, you must use the -p or --publish option with docker run.

intermediate

How do you expose multiple ports in a Dockerfile?

You can use multiple EXPOSE instructions, one per port, or list multiple ports separated by spaces in a single EXPOSE instruction.

beginner

Example: What does this line do? EXPOSE 80 443

It declares that the container listens on ports 80 and 443. This helps users know which ports to map when running the container.

intermediate

Can EXPOSE specify the protocol (TCP/UDP) for a port?

Yes. You can specify the protocol by adding /tcp or /udp after the port number, like EXPOSE 53/udp. If omitted, TCP is assumed.

What is the main purpose of the EXPOSE instruction in a Dockerfile?

A To set environment variables

B To automatically publish ports to the host

C To build the container image

D To document which ports the container listens on

EXPOSE documents ports but does not publish them automatically.

How do you make a container port accessible from the host machine?

A Use the EXPOSE instruction in the Dockerfile

B Use the -p or --publish option with docker run

C Add the port to the CMD instruction

D Set the port in the ENV instruction

The -p or --publish option maps container ports to host ports.

Which of these is a valid way to expose ports 80 and 443 in a Dockerfile?

A EXPOSE 80 443

B EXPOSE 80,443

C EXPOSE 80; EXPOSE 443

D EXPOSE 80:443

Ports can be listed separated by spaces in one EXPOSE instruction.

What protocol does EXPOSE assume if none is specified?

A UDP

B HTTP

C TCP

D FTP

TCP is the default protocol if none is specified.

Which command publishes container port 8080 to host port 80?

A docker run -p 80:8080 image

B docker run -p 8080:80 image

C docker run --expose 80:8080 image

D docker run --publish 8080 image

The syntax -p hostPort:containerPort maps ports correctly.

Explain the role of the EXPOSE instruction in a Dockerfile and how it relates to port publishing.

Think about what EXPOSE tells Docker and what it does not do.

Describe how to expose multiple ports and specify protocols using the EXPOSE instruction.

Consider how to list ports and add protocol details.