Debugging network issues in Docker - Step-by-Step Execution

Process Flow - Debugging network issues

Start: Identify network problem

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Check container network status

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Ping test between containers

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Inspect Docker network settings

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Check firewall and host network

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Fix configuration or restart network

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Verify connectivity restored

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End

This flow shows the step-by-step process to find and fix network problems in Docker containers.

Execution Sample

Docker

docker network ls

docker exec container1 ping -c 3 container2

docker network inspect bridge

docker network disconnect bridge container1

docker network connect bridge container1

docker exec container1 ping -c 3 container2

This sequence lists networks, tests connectivity, inspects network details, reconnects a container, and retests connectivity.

Process Table

Step	Command	Action	Result	Notes
1	docker network ls	List all Docker networks	Shows default and user networks	Confirms networks exist
2	docker exec container1 ping -c 3 container2	Ping container2 from container1	3 packets transmitted, 3 received	Connectivity OK at start
3	docker network inspect bridge	Inspect default bridge network	Shows containers connected and settings	Check IPs and links
4	docker network disconnect bridge container1	Disconnect container1 from bridge	No output	Simulate network issue
5	docker exec container1 ping -c 3 container2	Ping container2 from container1	3 packets transmitted, 0 received, 100% packet loss	Connectivity lost
6	docker network connect bridge container1	Reconnect container1 to bridge	No output	Restore network connection
7	docker exec container1 ping -c 3 container2	Ping container2 from container1	3 packets transmitted, 3 received	Connectivity restored

💡 Connectivity restored after reconnecting container1 to the bridge network

Status Tracker

Variable	Start	After Step 4	After Step 6	Final
container1 network connection	Connected to bridge	Disconnected from bridge	Reconnected to bridge	Connected to bridge
ping result	Success (3/3)	Fail (0/3)	Not tested	Success (3/3)

Key Moments - 3 Insights

Why does ping fail after disconnecting container1 from the bridge network?

Because container1 is no longer connected to the network that container2 is on, so it cannot reach container2. See execution_table step 5.

Why do we inspect the bridge network before disconnecting?

To confirm which containers are connected and their IP addresses, helping to understand the network setup. See execution_table step 3.

What confirms that the network issue is fixed?

The ping command succeeds again after reconnecting container1 to the bridge network. See execution_table step 7.

Visual Quiz - 3 Questions

Look at the execution table, what is the ping result at step 5?

A 3 packets transmitted, 3 received

B 3 packets transmitted, 0 received, 100% packet loss

C No packets transmitted

D Ping command not run

💡 Hint

Check the 'Result' column for step 5 in the execution_table.

At which step is container1 disconnected from the bridge network?

A Step 2

B Step 3

C Step 4

D Step 6

💡 Hint

Look at the 'Command' and 'Action' columns to find when disconnect happens.

If container1 was not reconnected at step 6, what would be the ping result at step 7?

A 3 packets transmitted, 3 received

B 3 packets transmitted, 0 received, 100% packet loss

C Ping command would fail to run

D Ping would succeed partially

💡 Hint

Refer to the ping result at step 5 when container1 was disconnected.

Concept Snapshot

Debugging Docker network issues:
1. List networks with 'docker network ls'.
2. Test connectivity using 'docker exec <container> ping <target>'.
3. Inspect network details with 'docker network inspect <network>'.
4. Disconnect and reconnect containers to fix issues.
5. Verify connectivity restored by pinging again.

Full Transcript

This visual execution shows how to debug network issues in Docker. First, we list networks to confirm their presence. Then, we test connectivity between containers using ping. We inspect the network to understand container connections. Next, we simulate a network problem by disconnecting a container from the network, causing ping to fail. Finally, we reconnect the container and verify that connectivity is restored by a successful ping. Variables like container network connection and ping results change accordingly, helping us track the problem and fix.