Module 10 Lesson 3: DNS and Service Discovery

In a dynamic environment, you can never rely on a container's IP address. Every time a container restarts, its IP might change. Service Discovery solves this by letting you use the container's Name instead.

1. The Embedded DNS Server

Every Docker container (on a user-defined network) has a DNS resolver at 127.0.0.11.

• When Container A tries to connect to db, it asks this internal server.
• The DNS server looks up the current IP of the container named db and returns it.

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2. Default Bridge vs. User-Defined Networks

This is a major point of confusion:

• Default Bridge: Does NOT support DNS discovery. You must use outdated and clunky --link flags to make containers talk to each other.
• User-Defined Bridge: Supports DNS discovery automatically.

Rule: Always create your own networks (docker network create ...) if you want your services to talk to each other.

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3. Aliases and Hostnames

You can give a container secondary "Nicknames" on a network.

docker run -d \
  --name app-v1 \
  --network my-net \
  --network-alias search-engine \
  elasticsearch

Now, other containers can reach this app using either app-v1 or search-engine. This is useful for "Blue-Green" deployments where you want to swap a new version into the search-engine name without changing the code in your other apps.

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4. Configuring External DNS

If you want your container to use a specific external DNS (like Google 8.8.8.8) instead of your local office's DNS:

• docker run --dns 8.8.8.8 my-image

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Exercise: The Discovery Lab

1. Create a network called lab-net.
2. Run an nginx container named web-server on that network.
3. Run an alpine container (interactive) on the same network.
4. Inside alpine, run ping web-server. Does it work?
5. Now, stop the web-server and start a new one with the same name.
6. Run ping web-server from alpine again. Did the IP address change? Did the name still work?
7. Check the file /etc/resolv.conf inside the alpine container. What does it say about the nameserver?

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Summary

Service discovery is the "Glue" of microservices. By relying on Docker's embedded DNS, you can build systems where containers find each other instantly, regardless of how many times they are moved, restarted, or scaled.

Next Lesson: Spreading the load: Load balancing with Nginx and Docker.