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Chapter 9: Domain Names, DNS and Subdomains

9.10 Subdomains

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Subdomains are free and unlimited. Each one is just another DNS record plus (usually) another server block or virtual host. For Mauli Dairy, subdomains like blog or api can be created the same way.

 example.com        ─┐
 www.example.com    ─┼─► A 13.233.10.25 (web-1) ─► server_name decides the folder
 blog.example.com   ─┘                              /var/www/example, /var/www/blog
 api.example.com    ───► A 13.234.55.60 (app server, Node on :3000 behind Nginx)
 shop.example.com   ───► CNAME my-alb-123.ap-south-1.elb.amazonaws.com

Step 1: DNS record (GoDaddy → DNS → Add New Record)

Type Name Value Use
A blog 13.233.10.25 Same server, different site
A api 13.234.55.60 Different EC2 instance (its own Elastic IP)
CNAME shop my-​alb-​123.​ap-​south-​1.​elb.​amazonaws.​com AWS load balancer / other hostname
A * 13.233.10.25 Wildcard: any undefined subdomain

In the Name field type only the subdomain part (blog), not blog.example.com. Otherwise you may create blog.example.com.example.com.

Step 2: a site for the subdomain on the server

sudo mkdir -p /var/www/blog
echo '<h1>Welcome to blog.example.com</h1>' | sudo tee /var/www/blog/index.html

# Nginx: blog.conf
server {
    listen 80;
    server_name blog.example.com;
    root /var/www/blog;
    index index.html;
}

# Apache: blog.conf
<VirtualHost *:80>
    ServerName blog.example.com
    DocumentRoot /var/www/blog
    <Directory /var/www/blog>
        Require all granted
    </Directory>
</VirtualHost>

Test, reload, then verify with dig blog.example.com +short and curl http://blog.example.com.

Subdomain for a backend app (reverse proxy)

server {
    listen 80;
    server_name api.example.com;
    location / {
        proxy_pass http://127.0.0.1:3000;        # Express / Flask app (Chapter 10)
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

(On CentOS remember sudo setsebool -P httpd_can_network_connect 1.)

Ravindra Bagale's Tip

Many students type the full blog.example.com into GoDaddy's Name field, and the record becomes blog.example.com.example.com. Write only blog in Name. And if you set up a reverse proxy for api, don't forget sudo setsebool -P httpd_can_network_connect 1 on CentOS.

Lab

Chala, blog subdomain sathi A record tumchya Elastic IP la add kara, server var /var/www/blog aani tyacha server block banva, aani reload kara. Mag dig blog.example.com +short aani curl http://blog.example.com ne "Welcome to blog" page disto ka te bagha.