Exposing ports to host in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to expose a port in Docker?

Exposing a port means making a port inside the Docker container accessible from the host machine or other containers. It allows communication between the container and the outside world.

beginner

Which Docker command option is used to expose a container port to the host?

The -p or --publish option is used with docker run to map a container port to a host port, like -p 8080:80.

beginner

Explain the syntax -p 8080:80 in Docker port mapping.

It means map port 80 inside the container to port 8080 on the host. So, accessing localhost:8080 on the host reaches port 80 in the container.

intermediate

What is the difference between EXPOSE in Dockerfile and -p in docker run?

EXPOSE in Dockerfile is a hint that the container listens on a port but does not publish it. -p in docker run actually maps the port to the host.

beginner

Can you expose multiple ports from a container to the host? How?

Yes, by using multiple -p options in docker run, for example: -p 8080:80 -p 8443:443 to expose HTTP and HTTPS ports.

Which Docker command option exposes a container port to the host?

A -p

B -e

C -v

D -d

The -p option maps container ports to host ports, exposing them.

What does -p 3000:3000 do in docker run?

A Maps container port 3000 to host port 80

B Runs container in detached mode

C Exposes port 80 inside the container

D Maps host port 3000 to container port 3000

It maps port 3000 on the host to port 3000 inside the container.

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

A To publish ports to the host

B To document which ports the container listens on

C To start the container

D To set environment variables

EXPOSE is a documentation hint and does not publish ports.

How do you expose multiple ports from a container?

A Use -e option multiple times

B Use multiple EXPOSE lines in Dockerfile only

C Use multiple -p options in docker run

D Use -d option multiple times

Multiple -p options map multiple ports to the host.

If you run docker run -p 8080:80, how do you access the container's service?

A By visiting localhost:8080 on the host

B By visiting localhost:80 on the host

C By visiting container IP:8080

D By visiting container IP:80

Port 80 in the container is mapped to port 8080 on the host.

Describe how to expose a container port to the host using Docker commands.

Think about how to connect container ports to your computer ports.

Explain the difference between EXPOSE in Dockerfile and the -p option in docker run.

One is for documentation, the other for actual port access.

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