Container networking in Nginx - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Check container network interfaces

You run docker exec mynginx ip addr show inside a running nginx container. What output will you see related to network interfaces?

A Shows only <code>lo</code> interface with no IP addresses

B Lists interfaces including <code>eth0</code> with an IP address assigned by Docker bridge network

C Shows <code>wlan0</code> interface with the host Wi-Fi IP address

D Returns an error because <code>ip</code> command is not available in nginx container

💡 Hint

Docker containers usually have a virtual Ethernet interface named eth0 connected to a bridge network.

Inside a Docker container, the network interface eth0 is created and assigned an IP from the Docker bridge network. The loopback lo is also present but wlan0 is not because containers do not share host Wi-Fi interfaces. The ip command is usually available in most base images or can be installed.

🧠 Conceptual

intermediate

Docker container port exposure

You run an nginx container with docker run -d -p 8080:80 nginx. What does this command do regarding container networking?

A Blocks all network traffic to container port 80

B Exposes container port 8080 to host port 80, allowing access via localhost:80

C Maps host port 8080 to container port 80, allowing access to nginx via localhost:8080

D Runs nginx on host network directly without isolation

💡 Hint

The -p flag maps host ports to container ports.

The -p 8080:80 option maps port 8080 on the host machine to port 80 inside the container. This means you can access the nginx server by visiting localhost:8080 on the host.

❓ Troubleshoot

advanced

Container cannot reach external internet

An nginx container cannot access the internet to download updates. Which Docker network setting is most likely causing this?

A Container has <code>--net=none</code> which disables all networking

B Container is running with <code>--network host</code> which disables internet access

C Container is using the default bridge network which blocks all outbound traffic

D Container is attached to a user-defined bridge network without proper DNS or gateway settings

💡 Hint

Check if the container has any network interfaces enabled.

The --net=none option disables all networking for the container, so it cannot reach the internet. User-defined bridge networks usually allow outbound traffic if configured correctly. The default bridge network allows outbound traffic by default. The --network host shares the host network and does not disable internet access.

🔀 Workflow

advanced

Connecting two containers on the same user-defined network

You have two containers, web and db, on the same Docker user-defined bridge network named mynetwork. How can web container reach db by hostname?

A Use <code>db</code> as the hostname in the <code>web</code> container to connect to the <code>db</code> container

B Use the IP address of <code>db</code> container only, hostnames are not resolved

C Use <code>localhost</code> inside <code>web</code> container to reach <code>db</code>

D Use the host machine IP address to connect from <code>web</code> to <code>db</code>

💡 Hint

User-defined bridge networks provide automatic DNS resolution between containers.

Docker user-defined bridge networks provide built-in DNS resolution, so containers can reach each other by container name as hostname. Using localhost refers to the container itself, not others. IP addresses can change, so hostnames are preferred. Host machine IP is not used for container-to-container communication on the same network.

✅ Best Practice

expert

Securing container network traffic

Which practice best improves security for container network traffic in a production environment?

A Use the default bridge network for all containers to reduce complexity

B Expose all container ports to the host to monitor traffic easily

C Disable Docker network isolation to simplify network setup

D Use encrypted overlay networks or VPNs between Docker hosts to protect container traffic

💡 Hint

Think about encrypting data in transit between containers on different hosts.

Encrypted overlay networks or VPNs protect container traffic between hosts by encrypting data in transit. Exposing all ports increases attack surface. Disabling network isolation reduces security. Default bridge network does not provide encryption or isolation between hosts.