🧠 Dynamic Docker DNS with CoreDNS

This project provides a lightweight, automatic DNS service for Docker containers using CoreDNS and a Python event listener.
It dynamically maps container hostnames and IP addresses into CoreDNS, supports reverse DNS (PTR) lookups automatically, and allows you to specify for what interfaces you want to create DNS records.

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🚀 Features

✅ Automatic DNS Record Management

• Each Docker container gets a fully-qualified domain name (FQDN) automatically added to /etc/coredns/hosts.
• CoreDNS automatically provides forward (A) and reverse (PTR) resolution.
• Will add 0.0.0.0 entry for containers that are stopped.

✅ Event-Based Updates

• The DNS host file updates instantly on Docker events (start, stop, destroy, etc.).
• No need for polling — it’s lightweight and real-time.

✅ Multi-Network Support

• Supports one or more Docker network prefixes.
• Example: NETWORK_PREFIX=macvlan,dmz will include all containers in any network that starts with macvlan or dmz.

✅ Custom Domain

• Easily define your internal domain via the DOMAIN environment variable.
• Example: DOMAIN=docker.local.
• If no variable is entered the default value 'docker.local' will be used.

✅ Reverse DNS Support

• CoreDNS automatically provides reverse (PTR) lookups for all listed IPs.

✅ No DNS forwarding

• CoreDNS will not forward requests to upstream. Use this only for automatated Docker container resolving.

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🗂️ Project Structure

docker-coredns-dynamic/

├── Dockerfile # Builds the CoreDNS + Python container

├── Corefile # CoreDNS configuration

├── update_hosts.py # Watches Docker events and updates hosts file

└── docker-compose.yml # Example compose setup

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⚙️ Configuration

Environment variables:

Variable	Description	Example
DOMAIN	Default DNS suffix for containers	docker.local
NETWORK_PREFIX	Comma-separated list of Docker network name prefixes to include	macvlan, bridge

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🚀 How It Works

1. On startup, the Python script scans all Docker containers.
2. For each container connected to a macvlan*,bride* network, it writes an entry to /etc/coredns/hosts
3. CoreDNS reads this file using the hosts plugin and automatically serves both:

• Forward lookup: container.docker.local → 192.168.10.100
• Reverse lookup: 192.168.10.100 → container.docker.local

4. The script subscribes to Docker events (start, die, connect, etc.) and automatically updates the hosts file whenever containers change.

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🔧 Example Query

Forward lookup:

dig @192.168.10.10 container.docker.local

Reverse lookup:

dig -x 192.168.10.100 @192.168.10.10