Network inspection and debugging in Docker - Step-by-Step Execution

Process Flow - Network inspection and debugging

Start Docker container

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Inspect container network settings

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Check container IP and ports

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Ping or curl from container or host

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Analyze network connectivity

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Use docker network commands for deeper debug

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Fix issues or adjust network config

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Test connectivity again

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End

This flow shows how to start a container, inspect its network, test connectivity, debug issues, and fix them step-by-step.

Execution Sample

Docker

docker run -d --name webserver -p 8080:80 nginx:alpine

docker inspect webserver

docker exec webserver ping -c 3 google.com

docker network ls

docker network inspect bridge

This sequence runs a webserver container, inspects its network, tests connectivity, and checks Docker networks.

Process Table

Step	Command	Action	Output/Result
1	docker run -d --name webserver -p 8080:80 nginx:alpine	Start nginx container detached with port mapping	Container ID printed, container running
2	docker inspect webserver	Show container details including network	JSON output with IP address, ports, network mode
3	docker exec webserver ping -c 3 google.com	Ping external site from container	3 packets transmitted, 3 received, 0% packet loss
4	docker network ls	List all Docker networks	bridge, host, none networks listed
5	docker network inspect bridge	Show details of default bridge network	JSON with connected containers, subnet, gateway
6	docker exec webserver wget -qO- http://localhost	Test webserver inside container	HTML content of nginx default page
7	docker stop webserver	Stop the container	Container stopped
8	docker rm webserver	Remove the container	Container removed

💡 Container stopped and removed, network inspection and debugging complete

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
container_status	none	running	running	running	running	running	running	stopped
container_ip	none	172.17.0.2	172.17.0.2	172.17.0.2	172.17.0.2	172.17.0.2	172.17.0.2	none
network_list	none	bridge, host, none	bridge, host, none	bridge, host, none	bridge, host, none	bridge, host, none	bridge, host, none	bridge, host, none

Key Moments - 3 Insights

Why do we use 'docker exec' to ping from inside the container instead of pinging from the host?

Because the container has its own network namespace and IP, pinging inside shows if the container can reach outside networks. See step 3 in execution_table where ping is run inside container.

What does the port mapping '-p 8080:80' do in the 'docker run' command?

It maps port 80 inside the container to port 8080 on the host, allowing access to the container's webserver via localhost:8080. This is shown in step 1 and confirmed in step 2 inspection.

Why inspect the 'bridge' network with 'docker network inspect bridge'?

Because the default container network is 'bridge', inspecting it shows connected containers and subnet info, helping debug network issues. See step 5 for details.

Visual Quiz - 3 Questions

Look at the execution table, what is the container IP address after step 2?

A 127.0.0.1

B 192.168.1.1

C 172.17.0.2

D None assigned

💡 Hint

Check the 'container_ip' variable in variable_tracker after step 2

At which step does the container stop running?

A Step 6

B Step 7

C Step 8

D Step 5

💡 Hint

Look at 'container_status' in variable_tracker and the 'Action' column in execution_table

If the ping command in step 3 failed, which command would you run next to check network settings?

A docker network ls

B docker rm webserver

C docker exec webserver curl http://localhost

D docker stop webserver

💡 Hint

Refer to step 4 in execution_table where network listing helps diagnose connectivity

Concept Snapshot

Docker Network Inspection & Debugging Cheat Sheet:
- Start container: docker run -d --name NAME -p hostPort:containerPort IMAGE
- Inspect container network: docker inspect CONTAINER
- Test connectivity inside container: docker exec CONTAINER ping TARGET
- List networks: docker network ls
- Inspect network details: docker network inspect NETWORK
- Stop and remove container: docker stop CONTAINER && docker rm CONTAINER
Use these steps to find and fix network issues in Docker containers.

Full Transcript

This visual execution shows how to inspect and debug Docker container networks step-by-step. First, we start a container with port mapping. Then we inspect its network settings to find the container IP and ports. Next, we test connectivity from inside the container using ping. We list all Docker networks and inspect the default bridge network to understand container connections. We also test the webserver inside the container with wget. Finally, we stop and remove the container to clean up. Variables like container status and IP are tracked through each step. Key moments clarify why we ping inside the container, the purpose of port mapping, and why inspecting the bridge network is important. The quiz questions reinforce understanding of container IP, stopping steps, and debugging commands. This process helps beginners visually follow Docker network inspection and debugging in a clear, stepwise manner.