What is Docker DNS? | DevOps Dictionary

Docker DNS — How Containers Find Each Other by Name

What Is Docker DNS in Simple Terms?

When a container wants to reach another container, it needs to know its IP address. But container IPs change every time a container restarts. Docker's embedded DNS server solves this — it automatically assigns each container a DNS hostname matching its container name, and updates the DNS record every time the container restarts with a new IP.

Containers use Docker DNS automatically on user-defined networks. On the default bridge network, there is no DNS — containers are on their own.

◈ DIAGRAM

+------------------------------------------+
| Container: payment-api                   |
| Needs to reach: postgres                 |
|                                          |
| /etc/resolv.conf:                        |
|   nameserver 127.0.0.11  <- Docker DNS   |
|                                          |
| Query: postgres -> 127.0.0.11 -> 172.18.0.3|
| postgres restarts -> new IP 172.18.0.4   |
| Query: postgres -> 127.0.0.11 -> 172.18.0.4|
| Same name, automatically updated IP      |
+------------------------------------------+

How Docker DNS Works

Bash

# Create user-defined network
docker network create payment-network
 
# Run containers on it
docker run -d --name api --network payment-network node:20-alpine sleep 3600
docker run -d --name db  --network payment-network postgres:15 \
  -e POSTGRES_PASSWORD=secret
 
# Check DNS config inside the container
docker exec api cat /etc/resolv.conf
# nameserver 127.0.0.11    <- Docker's embedded DNS
# options ndots:0
 
# Resolve db by name
docker exec api nslookup db
# Server:   127.0.0.11
# Name:     db
# Address:  172.18.0.3
 
# Restart db — it gets a new IP
docker restart db
docker exec api nslookup db
# Name:     db
# Address:  172.18.0.4    <- updated automatically

DNS in Docker Compose

YAML

version: "3.8"
services:
  payment-api:
    environment:
      DB_HOST: postgres    # service name = DNS hostname
      REDIS_HOST: redis    # works automatically in Compose
  postgres:
    image: postgres:15
  redis:
    image: redis:7-alpine
# Compose auto-creates a user-defined network
# All services resolve each other by service name

Network Aliases

Bash

# Give a container additional DNS names
docker run -d \
  --name postgres-primary \
  --network app-network \
  --network-alias db \
  --network-alias database \
  postgres:15 -e POSTGRES_PASSWORD=secret
 
# Now reachable by three names:
# postgres-primary, db, database
docker exec api ping db        # works
docker exec api ping database  # works

Default Bridge Has No DNS

Bash

# Default bridge — no DNS
docker run -d --name web nginx
docker run -d --name api node:20-alpine sleep 3600
 
docker exec api nslookup web
# ping: bad address 'web' <- FAILS — no DNS on default bridge
 
# Fix: create user-defined network
docker network create app-network
docker network connect app-network web
docker network connect app-network api
docker exec api nslookup web  # now works

COMMON MISTAKE / WARNING
**Common Mistake:** Hardcoding container IP addresses in application config. Container IPs change on every restart. Always use container names (on user-defined networks) or service names (in Compose) — Docker DNS resolves them to the current IP automatically.