EXPOSE instruction for ports in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to expose port 80.

Docker

EXPOSE [1]

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

A22

B443

C80

D8080

Attempts:

3 left

💡 Hint

Common Mistakes

Using port 22 which is for SSH, not HTTP.

Confusing port 443 which is for HTTPS.

The EXPOSE instruction tells Docker which port the container listens on at runtime. Port 80 is the default HTTP port.

2fill in blank

medium

Complete the Dockerfile line to expose port 5432 for a PostgreSQL database.

Docker

EXPOSE [1]

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

A5432

B3306

C6379

D27017

Attempts:

3 left

💡 Hint

Common Mistakes

Using port 3306 which is for MySQL.

Using port 6379 which is for Redis.

Port 5432 is the default port for PostgreSQL database connections.

3fill in blank

hard

Fix the error in the Dockerfile line to expose port 5000 (remove the wrong character).

Docker

EXPOSE [1]

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

A5000/

B5000

C5000/tcp

Dtcp/5000

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a slash after the port number.

Including protocol in the wrong format.

The EXPOSE instruction should have only the port number without extra characters. '5000/' is invalid syntax.

4fill in blank

hard

Fill both blanks to expose HTTP and HTTPS ports in the Dockerfile.

Docker

EXPOSE [1] [2]

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

A80

B22

C443

D8080

Attempts:

3 left

💡 Hint

Common Mistakes

Using port 22 which is for SSH.

Confusing port 8080 with standard HTTP port.

Port 80 is for HTTP and port 443 is for HTTPS. Both are commonly exposed for web servers.

5fill in blank

hard

Fill all three blanks to expose ports for HTTP, PostgreSQL, and Redis.

Docker

EXPOSE [1] [2] [3]

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

A80

B5432

C6379

D22

Attempts:

3 left

💡 Hint

Common Mistakes

Including port 22 which is for SSH and not relevant here.

Mixing up database ports.

Port 80 is for HTTP, 5432 for PostgreSQL, and 6379 for Redis. These are standard ports to expose for these services.

Practice

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

easy

A. To start the container with a specific port

B. To automatically publish ports to the host machine

C. To document which ports the container listens on

D. To stop the container from using any ports

Solution

1. Step 1: Understand the role of EXPOSE

   The EXPOSE instruction tells Docker which ports the container will listen on internally.

2. Step 2: Clarify what EXPOSE does not do

   EXPOSE does not publish or map ports to the host automatically; it only documents them.

3. Final Answer:

   To document which ports the container listens on -> Option C

4. Quick Check:

   EXPOSE documents ports, does not publish them [OK]

Hint: EXPOSE documents ports, doesn't publish them automatically [OK]

Common Mistakes:

• Thinking EXPOSE publishes ports to host
• Confusing EXPOSE with port mapping
• Assuming EXPOSE starts container on a port

2. Which of the following is the correct syntax to expose port 8080 in a Dockerfile?

easy

A. EXPOSE 8080

B. EXPOSE:8080

C. EXPOSE(port=8080)

D. EXPOSE[8080]

Solution

1. Step 1: Recall Dockerfile EXPOSE syntax

   The correct syntax is simply EXPOSE followed by the port number without any punctuation.

2. Step 2: Check each option

   Options A, B, and C use invalid syntax with brackets, colons, or parentheses which are not allowed.

3. Final Answer:

   EXPOSE 8080 -> Option A

4. Quick Check:

   Correct syntax is EXPOSE followed by port number [OK]

Hint: EXPOSE followed by space and port number is correct [OK]

Common Mistakes:

• Using colons or parentheses in EXPOSE
• Adding brackets around port number
• Writing EXPOSE in lowercase

3. Given this Dockerfile snippet:

FROM nginx
EXPOSE 80
EXPOSE 443

What ports will the container listen on internally?

medium

A. Ports 80 and 443

B. Only port 443

C. Only port 80

D. No ports are exposed

Solution

1. Step 1: Identify all EXPOSE instructions

   The Dockerfile exposes port 80 and port 443 explicitly.

2. Step 2: Understand multiple EXPOSE usage

   Multiple EXPOSE lines add ports cumulatively, so both ports are exposed internally.

3. Final Answer:

   Ports 80 and 443 -> Option A

4. Quick Check:

   Multiple EXPOSE lines add ports together [OK]

Hint: Multiple EXPOSE lines add ports cumulatively [OK]

Common Mistakes:

• Assuming only the last EXPOSE counts
• Thinking EXPOSE publishes ports to host
• Ignoring multiple EXPOSE instructions

4. You wrote this Dockerfile snippet:

EXPOSE 80 443

But Docker build gives an error. What is wrong?

medium

A. Port numbers must be in quotes

B. Ports must be separated by commas

C. EXPOSE must be lowercase

D. EXPOSE cannot list multiple ports on one line

Solution

1. Step 1: Check EXPOSE syntax rules

   EXPOSE accepts multiple ports on one line separated by spaces; it does not require multiple EXPOSE lines.

2. Step 2: Validate other options

   Ports do not need quotes, EXPOSE is uppercase by convention, and commas are not used to separate ports.

3. Final Answer:

   EXPOSE cannot list multiple ports on one line -> Option D

4. Quick Check:

   One port per EXPOSE line only [OK]

Hint: Use one EXPOSE per port, no spaces or commas [OK]

Common Mistakes:

• Listing multiple ports on one EXPOSE line
• Adding quotes around port numbers
• Writing EXPOSE in lowercase

5. You want to run a web app container that listens on port 3000 inside the container. You want to access it on port 8080 on your host machine. Which Docker commands and Dockerfile instructions correctly achieve this?

hard

A. Dockerfile: EXPOSE 8080Run: docker run -p 3000:8080 image

B. Dockerfile: EXPOSE 3000Run: docker run -p 8080:3000 image

C. Dockerfile: EXPOSE 3000 8080Run: docker run -p 3000:8080 image

D. Dockerfile: EXPOSE 8080Run: docker run -p 8080:8080 image

Solution

1. Step 1: Understand EXPOSE and port mapping

   EXPOSE should list the internal container port (3000). The run command maps host port 8080 to container port 3000.

2. Step 2: Check each option's correctness

   Dockerfile: EXPOSE 3000
   Run: docker run -p 8080:3000 image correctly exposes 3000 and maps 8080 on host to 3000 in container. Others either reverse ports or use invalid EXPOSE syntax.

3. Final Answer:

   Dockerfile: EXPOSE 3000 and run with -p 8080:3000 -> Option B

4. Quick Check:

   EXPOSE internal port, map host:container ports correctly [OK]

Hint: EXPOSE internal port; map host port to container port with -p [OK]

Common Mistakes:

• Exposing wrong port inside Dockerfile
• Reversing host and container ports in -p
• Listing multiple ports in one EXPOSE line