Container DNS and service discovery in Docker - Step-by-Step Execution

Process Flow - Container DNS and service discovery

Container A starts

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Container A registers name in DNS

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Container B starts

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Container B queries DNS for Container A

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DNS returns Container A IP

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Container B connects to Container A using IP

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Service communication established

Containers register their names in an internal DNS. Other containers query this DNS to find IPs and connect, enabling service discovery.

Execution Sample

Docker

docker network create mynet

docker run -d --name web --network mynet nginx

docker run -it --rm --network mynet busybox nslookup web

Create a network, run a web container, then run a busybox container to look up the web container's IP via DNS.

Process Table

Step	Action	Command/Query	Result/Output
1	Create network	docker network create mynet	Network 'mynet' created
2	Start web container	docker run -d --name web --network mynet nginx	Container 'web' running with IP 172.18.0.2
3	Start busybox container	docker run -it --rm --network mynet busybox nslookup web	Query DNS for 'web'
4	DNS response	nslookup web	Name: web Address: 172.18.0.2
5	Connect to web	busybox connects to 172.18.0.2	Connection successful
6	Exit	busybox exits	Container removed

💡 Busybox container exits after DNS query and connection test

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Network 'mynet'	none	created	created	created	created
Container 'web' IP	none	172.18.0.2	172.18.0.2	172.18.0.2	172.18.0.2
DNS query result	none	none	query sent	172.18.0.2	172.18.0.2

Key Moments - 3 Insights

Why can the busybox container find the web container by name?

Because both containers are on the same user-defined network 'mynet', Docker automatically provides DNS resolution for container names within that network, as shown in execution_table step 4.

What happens if containers are on different networks?

Containers on different networks cannot resolve each other's names via DNS, so the busybox container would fail to find 'web' by name, unlike in step 4 where they share 'mynet'.

Why does the busybox container get removed after exit?

Because it was run with --rm flag, which tells Docker to remove the container after it stops, as noted in execution_table step 6.

Visual Quiz - 3 Questions

Look at the execution table, what IP address does the DNS return for the 'web' container at step 4?

A 127.0.0.1

B 192.168.1.1

C 172.18.0.2

D None

💡 Hint

Check the 'Result/Output' column at step 4 in the execution_table.

At which step does the busybox container send the DNS query for 'web'?

A Step 3

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Action' and 'Command/Query' columns in the execution_table.

If the network 'mynet' was not created, what would happen when running the web container?

A Container would fail to start

B Container would run but no DNS resolution for 'web'

C Container would run but busybox cannot find 'web' by name

D Container would run with default network and DNS resolution works

💡 Hint

Refer to variable_tracker and key_moments about network and DNS behavior.

Concept Snapshot

Docker containers on the same user-defined network can resolve each other's names via built-in DNS.
Create a network with 'docker network create'.
Run containers attached to this network.
Use container names to connect instead of IPs.
DNS resolves names to container IPs automatically.
This enables simple service discovery inside Docker networks.

Full Transcript

This visual execution shows how Docker containers use internal DNS for service discovery. First, a user-defined network 'mynet' is created. Then, a web container runs attached to 'mynet' and gets an IP address. Next, a busybox container runs on the same network and queries DNS for the name 'web'. The DNS returns the IP of the web container. Busybox uses this IP to connect to the web container successfully. Finally, busybox exits and is removed because it was run with the --rm flag. This demonstrates how containers discover each other by name using Docker's internal DNS on user-defined networks.