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

Process Flow - Network inspection and debugging

Start Docker container

↓

Inspect container network settings

↓

Check container IP and ports

↓

Ping or curl from container or host

↓

Analyze network connectivity

↓

Use docker network commands for deeper debug

↓

Fix issues or adjust network config

↓

Test connectivity again

↓

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.

Practice

1. What does the Docker command docker network ls do?

easy

A. Creates a new Docker network

B. Shows running containers

C. Deletes a Docker network

D. Lists all Docker networks on the host

Solution

1. Step 1: Understand the command purpose

   The command docker network ls is used to list networks, not containers or create/delete networks.

2. Step 2: Match command to action

   Since it lists networks, it shows all Docker networks on the host machine.

3. Final Answer:

   Lists all Docker networks on the host -> Option D

4. Quick Check:

   docker network ls = list networks [OK]

Hint: Remember: 'ls' means list, so it lists networks [OK]

Common Mistakes:

• Confusing network listing with container listing
• Thinking it creates or deletes networks
• Mixing up 'docker network ls' with 'docker ps'

2. Which of the following is the correct syntax to inspect details of a Docker network named my_network?

easy

A. docker network show my_network

B. docker inspect network my_network

C. docker network inspect my_network

D. docker network details my_network

Solution

1. Step 1: Recall Docker network inspection syntax

   The correct command to inspect a network is docker network inspect <network_name>.

2. Step 2: Verify options

   Only docker network inspect my_network matches the correct syntax exactly.

3. Final Answer:

   docker network inspect my_network -> Option C

4. Quick Check:

   docker network inspect = inspect network [OK]

Hint: Use 'docker network inspect' to see network details [OK]

Common Mistakes:

• Swapping order of 'inspect' and 'network'
• Using non-existent commands like 'show' or 'details'
• Confusing with container inspect syntax

3. You run docker exec container1 ping -c 2 container2 to test connectivity. What output indicates successful communication?

medium

A. ping: unknown host container2

B. 2 packets transmitted, 2 received, 0% packet loss

C. 0 packets transmitted, 0 received, 100% packet loss

D. Error: container2 not found

Solution

1. Step 1: Understand ping output

   Successful ping shows packets transmitted and received with 0% packet loss.

2. Step 2: Match output to success

   2 packets transmitted, 2 received, 0% packet loss shows 2 packets sent and received with no loss, indicating success.

3. Final Answer:

   2 packets transmitted, 2 received, 0% packet loss -> Option B

4. Quick Check:

   Ping success = packets received with 0% loss [OK]

Hint: Look for 0% packet loss to confirm connectivity [OK]

Common Mistakes:

• Confusing error messages with success
• Ignoring packet loss percentage
• Assuming any ping output means success

4. You try docker network inspect mynet but get an error: Error: No such network: mynet. What is the most likely cause?

medium

A. The network 'mynet' does not exist on the host

B. You need to run the command inside a container

C. Docker daemon is not running

D. You must use 'docker inspect network mynet' instead

Solution

1. Step 1: Analyze error message

   The error says 'No such network', meaning Docker cannot find a network named 'mynet'.

2. Step 2: Identify cause

   This usually means the network was not created or was removed.

3. Final Answer:

   The network 'mynet' does not exist on the host -> Option A

4. Quick Check:

   Error 'No such network' = network missing [OK]

Hint: Check network list before inspecting to avoid missing network errors [OK]

Common Mistakes:

• Thinking command must run inside container
• Assuming Docker daemon is stopped without checking
• Using wrong command syntax

5. You have two containers, webapp and db, connected to a user-defined network appnet. You want to debug why webapp cannot reach db. Which sequence of commands helps you inspect and test the network connectivity?

hard

A. docker network inspect appnet; docker exec webapp ping -c 3 db

B. docker ps; docker logs webapp; docker network ls

C. docker network create appnet; docker exec db ping -c 3 webapp

D. docker exec webapp ifconfig; docker network rm appnet

Solution

1. Step 1: Inspect the network configuration

   Use docker network inspect appnet to check if both containers are connected properly.

2. Step 2: Test connectivity from webapp to db

   Run docker exec webapp ping -c 3 db to verify if webapp can reach db by name.

3. Final Answer:

   docker network inspect appnet; docker exec webapp ping -c 3 db -> Option A

4. Quick Check:

   Inspect network then ping from source to target [OK]

Hint: Inspect network first, then ping from source container [OK]

Common Mistakes:

• Trying to ping from db to webapp instead of webapp to db
• Creating network again instead of inspecting existing
• Removing network before debugging