How to build a web server farm with Caddy and Docker

You will deploy a scalable web server farm using Caddy as a reverse proxy and Docker for containerization. These steps target Ubuntu 24.04 with Docker Engine 27.x and Caddy 2.8.x.

Prerequisites

• Ubuntu 24.04 LTS server with root or sudo access
• Docker Engine 27.0 or newer installed
• Docker Compose plugin installed
• At least 2 GB of available RAM
• Static IP address or dynamic DNS provider for SSL certificates

Step 1: Install Docker and Docker Compose

Install the Docker Engine and Compose plugin to manage your containerized web farm. This ensures you have the necessary runtime environment to run Caddy and your backend applications.

apt update
apt install -y docker.io docker-compose-plugin

Reload the Docker daemon to apply the changes.

systemctl restart docker

Verify that Docker is running correctly.

docker info | grep "Server Version"

You will see output similar to this:

Server Version: 27.0.0

Step 2: Create the Docker Network

Define a bridge network to isolate your web farm containers from the host system. This network allows Caddy to communicate with backend application containers securely.

docker network create --driver bridge caddy-net

Confirm the network was created successfully.

docker network ls | grep caddy-net

The output should list your new network.

Step 3: Create the Caddy Dockerfile

Create a custom Dockerfile for Caddy to include your specific configuration. This file defines the container image and copies your Caddyfile into the container.

cat > /root/Caddyfile << 'EOF'
{
    admin off
}

example.com {
    reverse_proxy 172.18.0.2:8080
}

api.example.com {
    reverse_proxy 172.18.0.3:8081
}
EOF

Replace the domain names and upstream IP addresses with your actual values.

Step 4: Build the Caddy Container Image

Build the Docker image using the Caddyfile you just created. This process compiles your configuration into a container image that can be deployed to any host.

docker build -t caddy-web-farm /root/

Verify that the image was built successfully.

docker images | grep caddy-web-farm

You will see an image with the tag `caddy-web-farm` listed.

Step 5: Run the Caddy Container

Start the Caddy container using the custom image and the bridge network you created earlier. This launches your reverse proxy server and begins handling incoming HTTP traffic.

docker run -d --name caddy --network caddy-net -p 80:80 -p 443:443 -v /root/Caddyfile:/etc/caddy/Caddyfile caddy-web-farm

Check the container status to ensure it is running.

docker ps | grep caddy

The output should show the container status as `Up`.

Step 6: Configure Automatic SSL with Caddy

Configure Caddy to automatically obtain and renew SSL certificates using Let's Encrypt. This ensures your web farm is accessible over HTTPS without manual certificate management.

docker exec caddy caddy reload

Verify that the Caddy process is listening on port 443.

docker logs caddy | grep "tls"

You should see logs indicating that TLS certificates are being managed.

Step 7: Deploy Backend Application Containers

Create a simple backend container to simulate a web application behind the proxy. This demonstrates how the reverse proxy routes traffic to different services.

docker run -d --name backend-app --network caddy-net -p 8080:80 nginx:alpine

Map the backend container to a specific port on the host.

docker run -d --name backend-api --network caddy-net -p 8081:80 nginx:alpine

Verify that both backend containers are running.

docker ps | grep backend

The output should list both `backend-app` and `backend-api` as running.

Verify the installation

Test that your web server farm is functioning correctly. Access the root domain and the API subdomain to confirm that Caddy is routing traffic to the correct backend containers.

curl -k https://example.com

curl -k https://api.example.com

You will receive HTTP 200 responses from both endpoints.

Troubleshooting

Check container logs if you encounter connection errors or SSL failures. Use these commands to inspect runtime errors and network connectivity issues.

docker logs caddy

docker logs backend-app

docker logs backend-api

Inspect the Docker network to verify that containers can reach each other.

docker network inspect caddy-net

Restart the Caddy container if configuration changes require a reload.

docker restart caddy

Ensure that firewall rules allow traffic on ports 80 and 443.

ufw allow 80/tcp
ufw allow 443/tcp
ufw reload

Verify that your Caddyfile syntax is correct by testing it locally before deploying.

caddy validate /root/Caddyfile

If validation fails, correct the syntax errors in the Caddyfile and rebuild the image.