Inspecting container network settings in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Inspect the IP address of a running container

You have a running Docker container named webapp. Which command will show you the container's IP address?

A docker network inspect webapp

B docker logs webapp | grep IP

C docker ps -q webapp

D docker inspect -f '{{ .NetworkSettings.IPAddress }}' webapp

💡 Hint

Use the docker inspect command with a format option to get specific network info.

The docker inspect -f '{{ .NetworkSettings.IPAddress }}' webapp command extracts the IP address of the container named webapp. Other options either inspect networks or show logs, which do not directly provide the IP address.

💻 Command Output

intermediate

Check which network a container is connected to

You want to find out which Docker networks the container dbserver is connected to. Which command gives you this information?

A docker inspect -f '{{ json .NetworkSettings.Networks }}' dbserver

B docker network inspect dbserver

C docker network ls dbserver

D docker ps --filter network=dbserver

💡 Hint

Look inside the container's network settings using docker inspect with a format.

The command docker inspect -f '{{ json .NetworkSettings.Networks }}' dbserver shows the networks the container is connected to in JSON format. Other commands either list networks or filter containers but do not show the container's network connections.

❓ Troubleshoot

advanced

Diagnose why a container cannot reach another container by name

Two containers, frontend and backend, are running on the default bridge network. The frontend container cannot ping backend by its container name. What is the most likely reason?

A The backend container is stopped, so it cannot respond to pings.

B Containers on the default bridge network cannot resolve each other's names; user-defined networks are needed for name resolution.

C The frontend container's firewall is blocking ICMP packets.

D Docker daemon is not running, so containers cannot communicate.

💡 Hint

Think about how Docker handles DNS resolution on different network types.

By default, Docker's default bridge network does not support automatic DNS resolution between containers by name. To enable this, containers must be connected to a user-defined bridge network. Other options are possible issues but less likely in this scenario.

❓ Configuration

advanced

Configure a container to use a specific static IP address

You want to run a container with a fixed IP address 172.18.0.50 on a user-defined network named mynetwork. Which command correctly assigns this IP?

A docker network create --subnet=172.18.0.0/16 mynetwork && docker run --net mynetwork --ip 172.18.0.50 alpine

B docker run --net mynetwork --ip 172.18.0.50 alpine

C docker network create mynetwork && docker run --net mynetwork --ip 172.18.0.50 alpine

D docker run --ip 172.18.0.50 alpine

💡 Hint

You must create the network with a subnet before assigning static IPs.

To assign a static IP, the network must be created with a subnet range. Option A creates the network with the subnet and then runs the container with the specified IP. Option A fails because the network does not exist with a subnet. Option A creates the network without subnet, so IP assignment fails. Option A does not specify the network.

✅ Best Practice

expert

Best practice for inspecting container network settings in a multi-host Docker Swarm

In a Docker Swarm cluster with multiple hosts, you want to inspect the network settings of a service's tasks (containers) across nodes. What is the best approach?

A Use <code>docker service inspect [service]</code> to get all network details of tasks directly from the manager node.

B Run <code>docker network inspect [network]</code> on the manager node only to see all container IPs.

C Use <code>docker service ps --no-trunc --filter desired-state=running [service]</code> to list tasks, then <code>docker inspect</code> on each task container ID on its host.

D SSH into each worker node and run <code>docker ps</code> to find containers, then <code>docker logs</code> for network info.

💡 Hint

Consider how Docker Swarm distributes containers and where you can run commands.

In Docker Swarm, tasks run on different nodes. To inspect network settings of each container, you must identify the task containers and inspect them on their respective hosts. Option C describes this process. Option C only shows network info but not per-container IPs. Option C is manual and incomplete. Option C does not provide detailed container network info.