DNS resolution between containers in Docker - Step-by-Step Execution

Process Flow - DNS resolution between containers

Start Docker Network

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Create Containers

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Containers Join Network

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Container A Queries DNS

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Docker Embedded DNS Resolves Name

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IP Address Returned to Container A

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Container A Connects to Container B Using IP

Docker creates a network where containers join and can resolve each other's names via embedded DNS, allowing communication by container names.

Execution Sample

Docker

docker network create mynet

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

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

docker exec containerA ping -c 1 containerB

Create a network, start two containers on it, then ping containerB from containerA using its name.

Process Table

Step	Action	Command/Query	Result	Notes
1	Create network	docker network create mynet	Network 'mynet' created	Custom bridge network allows DNS resolution
2	Start containerA	docker run -d --name containerA --network mynet nginx	containerA running	Attached to 'mynet'
3	Start containerB	docker run -d --name containerB --network mynet nginx	containerB running	Attached to 'mynet'
4	Ping containerB from containerA	docker exec containerA ping -c 1 containerB	Ping successful, IP resolved	DNS resolved 'containerB' to IP inside 'mynet'
5	Exit	Ping command ends	Ping command exits with success	Communication verified

💡 Ping command completes successfully, confirming DNS resolution and connectivity

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Network 'mynet'	None	Created	Created	Created	Created	Created
containerA status	None	None	Running	Running	Running	Running
containerB status	None	None	None	Running	Running	Running
DNS resolution in containerA	None	None	None	None	containerB -> IP	containerB -> IP

Key Moments - 2 Insights

Why can't containers resolve each other's names without a user-defined network?

By default, containers on the default bridge network cannot resolve names because Docker's embedded DNS only works on user-defined networks, as shown by step 1 creating 'mynet' enabling DNS.

What does the ping command inside containerA actually test?

It tests both DNS resolution of 'containerB' to its IP and network connectivity, as seen in step 4 where ping succeeds after DNS resolves the name.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the network 'mynet' created?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Check the 'Action' and 'Result' columns in the first row of the execution table.

According to the variable tracker, when does containerB start running?

A After Step 3

B After Step 2

C After Step 1

D After Step 4

💡 Hint

Look at the 'containerB status' row and see when it changes to 'Running'.

If containerA was not attached to 'mynet', what would happen to the ping command in step 4?

A Ping would succeed as usual

B Ping would fail due to no DNS resolution

C Ping would resolve IP but fail to connect

D Ping would connect but not resolve IP

💡 Hint

Refer to the key moment about DNS resolution requiring user-defined networks.

Concept Snapshot

Docker DNS resolution between containers:
- Create a user-defined network (docker network create)
- Run containers attached to this network (--network option)
- Containers can resolve each other by name via Docker embedded DNS
- Use commands like ping containerName inside containers to test
- Default bridge network does NOT support this DNS resolution

Full Transcript

This visual execution shows how Docker enables DNS resolution between containers by using a user-defined network. First, a network named 'mynet' is created. Then two containers, containerA and containerB, are started and attached to 'mynet'. When containerA runs a ping command to 'containerB', Docker's embedded DNS resolves 'containerB' to its IP address within the network. The ping succeeds, confirming both DNS resolution and network connectivity. Variables like container statuses and network state are tracked step-by-step. Key points include the necessity of a user-defined network for DNS resolution and that the ping command tests both name resolution and connectivity.