Exposing ports to host in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to expose port 80 of the container to port 8080 on the host.

Docker

docker run -p [1]:80 nginx

Drag options to blanks, or click blank then click option'

A3000

B8080

C443

D80

Attempts:

3 left

💡 Hint

Common Mistakes

Using the container port as the host port.

Swapping the ports in the mapping.

The -p option maps a host port to a container port. Here, port 8080 on the host is mapped to port 80 inside the container.

2fill in blank

medium

Complete the command to expose port 5432 of the container to the same port on the host.

Docker

docker run -p [1]:5432 postgres

Drag options to blanks, or click blank then click option'

A3306

B8080

C5432

D5000

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different host port without reason.

Confusing container port with host port.

Mapping port 5432 on the host to port 5432 in the container allows direct access to the database service.

3fill in blank

hard

Fix the error in the command to expose port 5000 of the container to port 5000 on the host.

Docker

docker run -p 5000:[1] myapp

Drag options to blanks, or click blank then click option'

A5000

B443

C80

D8080

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping host and container ports.

Using a default port not matching the app.

The container port must match the port your app listens on, here 5000.

4fill in blank

hard

Fill both blanks to expose port 3000 of the container to port 4000 on the host and run the container in detached mode.

Docker

docker run [1] -p [2]:3000 myapp

Drag options to blanks, or click blank then click option'

A-d

B-it

C4000

D-rm

Attempts:

3 left

💡 Hint

Common Mistakes

Using interactive mode instead of detached.

Swapping host and container ports.

-d runs the container detached (in background). The port mapping 4000:3000 exposes container port 3000 on host port 4000.

5fill in blank

hard

Fill all three blanks to expose port 22 of the container to port 2222 on the host, name the container 'ssh-server', and run it in detached mode.

Docker

docker run [1] --name [2] -p [3]:22 sshd

Drag options to blanks, or click blank then click option'

A-d

Bssh-server

C2222

D-it

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to name the container.

Using wrong port mapping order.

Using interactive mode instead of detached.

-d runs the container detached, --name ssh-server names the container, and -p 2222:22 maps host port 2222 to container port 22.

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