DNS & Service Discovery

Docker DNS & Service Discovery

Docker provides built-in DNS for container name resolution.

Automatic DNS

User-Defined Networks

# Create network
docker network create mynet

# Run containers
docker run -d --name web --network mynet nginx
docker run -d --name api --network mynet myapi

# Containers can resolve each other by name
docker exec api ping web
# Resolves to web container's IP

DNS Resolution Order

1. Container's /etc/hosts
2. Docker's embedded DNS server (127.0.0.11)
3. External DNS servers

Configuring DNS

Container-Level

# Set DNS servers
docker run --dns 8.8.8.8 --dns 8.8.4.4 alpine

# Set DNS search domain
docker run --dns-search example.com alpine

# Set hostname
docker run --hostname mycontainer alpine

Daemon-Level

// /etc/docker/daemon.json
{
  "dns": ["8.8.8.8", "8.8.4.4"],
  "dns-search": ["example.com"]
}

Docker Compose DNS

version: '3.8'

services:
  web:
    image: nginx
    # Automatically gets DNS name "web"
    networks:
      - backend

  api:
    image: myapi
    # Can resolve "web" hostname
    depends_on:
      - web
    networks:
      - backend

  db:
    image: postgres
    # Service name "db" is DNS resolvable
    networks:
      - backend

networks:
  backend:
    driver: bridge

Service Aliases

services:
  database:
    image: postgres
    networks:
      backend:
        aliases:
          - db
          - postgres
          - primary-db

# All these resolve to same container
ping database
ping db
ping postgres
ping primary-db

External DNS Resolution

# Verify DNS resolution
docker run --rm alpine nslookup google.com

# Check configured DNS
docker run --rm alpine cat /etc/resolv.conf

Troubleshooting

# Check DNS server in container
docker exec mycontainer cat /etc/resolv.conf

# Test DNS resolution
docker run --network mynet busybox nslookup web

# Debug with netshoot
docker run -it --network mynet nicolaka/netshoot
> dig web
> nslookup api

DNS Round-Robin

# Multiple containers with same alias
docker run -d --name web1 --network mynet --network-alias web nginx
docker run -d --name web2 --network mynet --network-alias web nginx

# "web" resolves to both IPs (round-robin)
docker run --rm --network mynet alpine nslookup web

Best Practices