Docker NetworkingLesson 3.4

Docker DNS and service discovery between containers

embedded DNS server, container name resolution, network alias, --network-alias, DNS round-robin, service discovery pattern

How Containers Find Each Other by Name

When containers run on a custom bridge network, Docker runs an embedded DNS server at 127.0.0.11 inside each container. Any query for a container name is resolved to that container's IP automatically.

Name Resolution in Practice

docker network create backend

docker run -d --name redis --network backend redis:7

docker run -it --network backend alpine sh
# Inside the alpine container:
nslookup redis            # Resolves to redis container IP
wget -qO- http://redis:6379  # Can connect by name

Network Aliases

Aliases let multiple containers share one DNS name — useful for horizontal scaling or blue-green deploys:

docker run -d \
  --name api-v1 \
  --network backend \
  --network-alias api \
  my-api:1.0

docker run -d \
  --name api-v2 \
  --network backend \
  --network-alias api \
  my-api:2.0

Both containers answer to the hostname api. Docker performs DNS round-robin between them, distributing requests across both instances. This is the basis of Docker Compose's built-in service scaling.