Connecting containers to multiple networks in Docker - Mini Project: Build & Apply

Connecting Containers to Multiple Networks

📖 Scenario: You are managing a small application that needs to communicate over two separate Docker networks. One network is for frontend communication and the other is for backend services. You will create these networks and connect a container to both networks.

🎯 Goal: Build a Docker setup where a container is connected to two different networks. This will help you understand how containers can communicate on multiple isolated networks.

📋 What You'll Learn

Create two Docker networks named frontend_net and backend_net

Run a container named my_app using the nginx image

Connect the my_app container to both frontend_net and backend_net

Verify the container is connected to both networks

💡 Why This Matters

🌍 Real World

In real projects, containers often need to communicate securely on different networks, such as separating frontend and backend traffic.

💼 Career

Understanding Docker networking is essential for DevOps roles to manage containerized applications and ensure proper network isolation and communication.

1

Create two Docker networks

Create two Docker networks named frontend_net and backend_net using the docker network create command.

Docker

# Create the frontend_net network
# Create the backend_net network

Hint

Use docker network create followed by the network name.

2

Run a container named my_app using nginx image

Run a container named my_app using the nginx image and connect it to the frontend_net network using the docker run command with the --network option.

Docker

docker network create frontend_net
docker network create backend_net
# Run the my_app container connected to frontend_net

Hint

Use docker run -d --name my_app --network frontend_net nginx to start the container.

3

Connect the my_app container to backend_net network

Use the docker network connect command to connect the running my_app container to the backend_net network.

Docker

docker network create frontend_net
docker network create backend_net
docker run -d --name my_app --network frontend_net nginx
# Connect my_app container to backend_net network

Hint

Use docker network connect backend_net my_app to add the container to the second network.

4

Verify the container is connected to both networks

Use the docker inspect command on the my_app container and print the networks it is connected to by filtering the .NetworkSettings.Networks field.

Docker

docker network create frontend_net
docker network create backend_net
docker run -d --name my_app --network frontend_net nginx
docker network connect backend_net my_app
# Inspect my_app container networks and print the network names

Hint

Use docker inspect -f '{{json .NetworkSettings.Networks}}' my_app to see connected networks.

Practice

1. What is the main benefit of connecting a Docker container to multiple networks?

easy

A. It increases the container's CPU usage automatically.

B. It makes the container run faster by using multiple networks.

C. It allows the container to communicate with different isolated groups of containers.

D. It automatically backs up container data across networks.

Solution

1. Step 1: Understand Docker networks

   Docker networks isolate groups of containers so they can communicate securely within the group.

2. Step 2: Multiple networks allow multi-group communication

   Connecting a container to multiple networks lets it talk to containers in each network separately.

3. Final Answer:

   It allows the container to communicate with different isolated groups of containers. -> Option C

4. Quick Check:

   Multiple networks = multi-group communication [OK]

Hint: Multiple networks let containers talk to separate groups [OK]

Common Mistakes:

• Thinking multiple networks speed up container
• Assuming multiple networks backup data
• Believing CPU usage increases automatically

2. Which Docker command correctly connects an existing container named webapp to a network called frontend?

easy

A. docker network add webapp frontend

B. docker connect network webapp frontend

C. docker container network add webapp frontend

D. docker network connect frontend webapp

Solution

1. Step 1: Recall Docker network connect syntax

   The correct syntax is docker network connect [network] [container].

2. Step 2: Match command to syntax

   docker network connect frontend webapp matches the syntax exactly: docker network connect frontend webapp.

3. Final Answer:

   docker network connect frontend webapp -> Option D

4. Quick Check:

   docker network connect = correct syntax [OK]

Hint: Use 'docker network connect [network] [container]' syntax [OK]

Common Mistakes:

• Swapping network and container order
• Using 'docker connect network' which is invalid
• Trying to add network with 'docker network add'

3. Given the commands below, what networks will the container db be connected to?

docker network create backend
docker network create monitoring

docker run -d --name db --network backend postgres

docker network connect monitoring db

medium

A. Only backend

B. Both backend and monitoring

C. Only monitoring

D. No networks connected

Solution

1. Step 1: Analyze initial container network

   The container db is started with --network backend, so it connects to backend first.

2. Step 2: Analyze network connect command

   The command docker network connect monitoring db adds monitoring network to the same container.

3. Final Answer:

   Both backend and monitoring -> Option B

4. Quick Check:

   Initial network + connect command = multiple networks [OK]

Hint: Initial network + connect command = multiple networks [OK]

Common Mistakes:

• Assuming second connect replaces first network
• Thinking container can only have one network
• Ignoring the network connect command

4. You tried to connect a container app to a network frontend using docker network connect frontend app, but got an error: Error response from daemon: network frontend not found. What is the most likely cause?

medium

A. The network frontend does not exist.

B. The container app is not running.

C. You used the wrong command syntax.

D. Docker daemon is not running.

Solution

1. Step 1: Understand the error message

   The error says network frontend not found, meaning Docker cannot find a network named frontend.

2. Step 2: Check possible causes

   If the network does not exist, the connect command fails. Container running state or syntax errors produce different messages.

3. Final Answer:

   The network frontend does not exist. -> Option A

4. Quick Check:

   Error 'network not found' = missing network [OK]

Hint: Check network existence if 'network not found' error appears [OK]

Common Mistakes:

• Assuming container must be running for connect
• Blaming command syntax without checking network
• Ignoring Docker daemon status

5. You have two networks: frontend and backend. You want a container api to be initially connected to frontend and communicate with containers in both networks but keep traffic isolated. Which sequence of commands correctly sets this up?

hard

A. docker network create frontend

docker network create backend

docker run -d --name api --network frontend myapi

docker network connect backend api

B. docker network create frontend

docker network create backend

docker run -d --name api --network backend myapi

docker network connect frontend api

C. docker network create frontend

docker network create backend

docker run -d --name api myapi

docker network connect frontend api

docker network connect backend api

D. docker network create frontend

docker network create backend

docker run -d --name api --network frontend --network backend myapi

Solution

1. Step 1: Create both networks

   Commands docker network create frontend and docker network create backend create the two isolated networks.

2. Step 2: Start container on one network and connect to the other

   Start api on frontend network, then connect it to backend using docker network connect backend api. This keeps traffic isolated but allows communication on both.

3. Final Answer:

   docker network create frontend docker network create backend docker run -d --name api --network frontend myapi docker network connect backend api -> Option A

4. Quick Check:

   Start on one network + connect second = multiple isolated networks [OK]

Hint: Start container on one network, then connect to others [OK]

Common Mistakes:

• Trying to specify two --network flags in docker run
• Not connecting container to second network after start
• Starting container without any network