Setting Up Subdomain DNS Records

What are Subdomains?

Subdomains are prefixes added to your main domain (e.g., blog.example.com, api.example.com). Each can point to a different server or service.

Creating Subdomain Records

# Point to same server as main domain
blog.example.com.    IN  A      198.51.100.10
api.example.com.     IN  A      198.51.100.10

# Point to different server
staging.example.com. IN  A      198.51.100.20

# Use CNAME for third-party services
mail.example.com.    IN  CNAME  ghs.googlehosted.com.
shop.example.com.    IN  CNAME  shops.myshopify.com.

Wildcard Subdomains

# Match all subdomains not explicitly defined
*.example.com.    IN  A    198.51.100.10

Useful for SaaS platforms where each customer gets a subdomain (e.g., customer1.myapp.com).

Web Server Configuration

# Nginx — serve different content per subdomain
server {
    listen 80;
    server_name blog.example.com;
    root /var/www/blog;
}

server {
    listen 80;
    server_name api.example.com;
    location / {
        proxy_pass http://127.0.0.1:3000;
    }
}

SSL for Subdomains

# Individual certs
sudo certbot --nginx -d blog.example.com

# Wildcard cert (covers all subdomains)
sudo certbot certonly --manual --preferred-challenges dns -d "*.example.com"