Container to container communication in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is container to container communication in Docker?

It is the process where two or more Docker containers exchange data or messages directly with each other.

beginner

How do Docker containers communicate by default when on the same user-defined network?

They can communicate using container names as hostnames because Docker's user-defined networks provide automatic DNS resolution.

beginner

What Docker command creates a user-defined network for container communication?

docker network create my-network

intermediate

Why is it recommended to use user-defined bridge networks for container communication instead of the default bridge network?

User-defined bridge networks provide better isolation, automatic DNS resolution, and easier container name communication compared to the default bridge network.

intermediate

How can you allow two containers to communicate if they are on different Docker networks?

You can connect a container to multiple networks using 'docker network connect' or create a shared network both containers join.

Which Docker network type allows containers to communicate using container names as hostnames?

A None network

B Host network

C Default bridge network

D User-defined bridge network

User-defined bridge networks provide automatic DNS resolution, allowing containers to use container names to communicate.

What command creates a new Docker network named 'app-net'?

A docker create network app-net

B docker network create app-net

C docker network new app-net

D docker create app-net network

The correct syntax to create a network is 'docker network create '.

If two containers are on the default bridge network, how do they communicate?

A Using IP addresses only

B Using container names as hostnames

C They cannot communicate

D Using host machine's IP

On the default bridge network, containers must use IP addresses to communicate because DNS resolution is not provided.

How can you connect an existing container to a new network?

A docker network connect <network> <container>

B docker connect network <container>

C docker network add <container> <network>

D docker container network connect <container> <network>

The command 'docker network connect ' connects a container to a network.

What is a benefit of using user-defined networks for container communication?

A No network isolation

B Containers cannot communicate

C Automatic DNS resolution

D Slower communication

User-defined networks provide automatic DNS resolution, making container communication easier.

Explain how Docker containers communicate when placed on the same user-defined network.

Think about how computers find each other on a network using names.

Describe the steps to enable communication between two containers on different Docker networks.

Consider how you would connect two groups of friends who are in different rooms.

Practice

1. What is the simplest way for two Docker containers to communicate on the same host?

easy

A. Use a shared volume for communication

B. Use the host's IP address directly inside containers

C. Run containers with --network=none and use port mapping

D. Connect both containers to the same user-defined Docker network

Solution

1. Step 1: Understand Docker networking basics

   Docker containers on the same user-defined network can resolve each other by container name and communicate directly.

2. Step 2: Compare other options

   Using host IP or port mapping is less direct and --network=none disables networking. Shared volumes are for file sharing, not network communication.

3. Final Answer:

   Connect both containers to the same user-defined Docker network -> Option D

4. Quick Check:

   Same network enables container communication [OK]

Hint: Use a user-defined network for easy container communication [OK]

Common Mistakes:

• Using default bridge network without user-defined network
• Trying to communicate via host IP without port mapping
• Assuming shared volumes enable network communication

2. Which Docker command correctly creates a user-defined network named mynetwork?

easy

A. docker network create mynetwork

B. docker create network mynetwork

C. docker network new mynetwork

D. docker network add mynetwork

Solution

1. Step 1: Recall Docker network creation syntax

   The correct command to create a network is docker network create <name>.

2. Step 2: Check other options for syntax errors

   Options B, C, and D use incorrect verbs or commands not recognized by Docker.

3. Final Answer:

   docker network create mynetwork -> Option A

4. Quick Check:

   Correct command syntax = docker network create [OK]

Hint: Remember: 'docker network create' is the correct syntax [OK]

Common Mistakes:

• Using 'docker create network' instead of 'docker network create'
• Using 'docker network new' which is invalid
• Trying 'docker network add' which does not exist

3. Given two containers web and db connected to the same user-defined network, what will be the output of docker exec web ping -c 1 db if db is running and reachable?

medium

A. ping: unknown host db

B. PING db (172.18.0.2): 56 data bytes\n64 bytes from 172.18.0.2: icmp_seq=0 ttl=64 time=0.123 ms\n\n--- db ping statistics ---\n1 packets transmitted, 1 packets received, 0% packet loss

C. connect: Network is unreachable

D. Request timed out

Solution

1. Step 1: Understand container DNS resolution on user-defined networks

   Containers on the same user-defined network can resolve each other by container name, so 'db' resolves to its IP.

2. Step 2: Predict ping command output

   Since 'db' is running and reachable, ping will succeed with a response showing bytes received and 0% packet loss.

3. Final Answer:

   Successful ping output with 1 packet transmitted and received -> Option B

4. Quick Check:

   Same network + running container = ping success [OK]

Hint: Ping container name on same network to test connectivity [OK]

Common Mistakes:

• Assuming default bridge network supports container name resolution
• Expecting ping to fail without port mapping
• Confusing network unreachable with DNS resolution failure

4. You created two containers on a user-defined network but web cannot reach db. Which command helps you debug the network connectivity issue?

medium

A. docker ps -a

B. docker container logs web

C. docker network inspect mynetwork

D. docker volume ls

Solution

1. Step 1: Identify the tool to check network details

   docker network inspect shows connected containers and network settings, useful for debugging connectivity.

2. Step 2: Evaluate other commands

   docker container logsdocker ps -a lists containers; docker volume ls lists volumes, none show network details.

3. Final Answer:

   docker network inspect mynetwork -> Option C

4. Quick Check:

   Inspect network to debug connectivity [OK]

Hint: Use 'docker network inspect' to check container connections [OK]

Common Mistakes:

• Checking container logs instead of network info
• Listing volumes which are unrelated
• Assuming 'docker ps' shows network details

5. You want two containers, app and cache, to communicate securely without exposing ports to the host. Which approach achieves this?

hard

A. Connect both containers to the same user-defined network and communicate via container names

B. Expose ports on both containers and use localhost to connect

C. Use host networking mode for both containers

D. Share a volume and write/read data files for communication

Solution

1. Step 1: Understand secure container communication

   Using a user-defined network allows containers to communicate internally without exposing ports to the host, keeping communication private.

2. Step 2: Evaluate other options for security

   Exposing ports or using host networking exposes services outside the host or risks port conflicts. Sharing volumes is not network communication.

3. Final Answer:

   Connect both containers to the same user-defined network and communicate via container names -> Option A

4. Quick Check:

   User-defined network keeps communication internal and secure [OK]

Hint: Use user-defined network to avoid exposing ports externally [OK]

Common Mistakes:

• Exposing ports unnecessarily to the host
• Using host network mode which is less secure
• Confusing volume sharing with network communication