Debugging network issues in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What command shows the list of Docker networks on your system?

Use docker network ls to see all Docker networks available on your system.

beginner

How can you inspect details of a specific Docker network?

Use docker network inspect <network_name> to see detailed information about a Docker network, including connected containers and IP ranges.

intermediate

What is the purpose of the docker exec command in network debugging?

docker exec lets you run commands inside a running container. You can use it to run network tools like ping or curl inside the container to test connectivity.

intermediate

How do you test if a container can reach another container by name?

Inside one container, use ping <other_container_name> or curl http://<other_container_name> to check if the network name resolution and connectivity work.

intermediate

What common issue can cause containers not to communicate on a user-defined Docker network?

Containers must be connected to the same user-defined network to communicate by name. If they are on different networks or the default bridge network, name resolution may fail.

Which command lists all Docker networks?

A docker container ls

B docker network ls

C docker network inspect

D docker ps

docker network ls shows all Docker networks. Other commands list containers or inspect networks.

How do you run a ping command inside a running container named webapp?

A docker run webapp ping google.com

B docker network ping webapp google.com

C docker start webapp ping google.com

D docker exec webapp ping google.com

docker exec webapp ping google.com runs ping inside the running container named webapp.

If two containers cannot reach each other by name, what is a likely cause?

A Docker daemon is stopped

B They have the same IP address

C They are on different Docker networks

D Containers are not running

Containers must be on the same user-defined network to resolve each other's names.

Which command shows detailed info about a Docker network named mynetwork?

A docker network inspect mynetwork

B docker network ls mynetwork

C docker inspect mynetwork

D docker network show mynetwork

docker network inspect mynetwork shows detailed info about the network.

What tool can you use inside a container to test HTTP connectivity?

A curl

B ping

C docker exec

D docker network ls

curl tests HTTP connectivity inside a container.

Explain the steps you would take to debug why two Docker containers cannot communicate over the network.

Think about checking container status, network membership, and testing connectivity inside containers.

Describe how Docker network name resolution works and why it might fail.

Focus on network types and how Docker handles container names.