Configuring DNS Records for AWS EC2 A Records, CNAME, and Subdomains

Configuring DNS Records for AWS EC2

After buying a domain, you need to configure DNS records to point it to your EC2 instance. This guide covers A records, CNAME records, and subdomain setup.

Prerequisites

• A registered domain name
• An EC2 instance with an Elastic IP
• Access to your DNS provider (Cloudflare, Namecheap, Route 53, etc.)

Step 1: Allocate an Elastic IP

Before configuring DNS, ensure your EC2 instance has a static IP:

# Via AWS CLI
aws ec2 allocate-address --domain vpc

# Associate with your instance
aws ec2 associate-address --instance-id i-xxxxx --allocation-id eipalloc-xxxxx

Or via AWS Console:

1. Go to EC2 → Elastic IPs
2. Click Allocate Elastic IP address
3. Select it, Actions → Associate Elastic IP address
4. Select your instance and associate

Why Elastic IP? The default public IP changes when you stop/start the instance. An Elastic IP is static it never changes.

Step 2: Create an A Record (Root Domain)

Point yourdomain.com to your EC2 IP:

On Cloudflare:

1. DNS → Records → Add Record
2. Type: A
3. Name: @ (represents root domain)
4. IPv4 address: 12.34.56.78 (your Elastic IP)
5. Proxy status: DNS only (gray cloud)
6. TTL: Auto
7. Save

On Namecheap (Advanced DNS):

1. Add New Record
2. Type: A Record
3. Host: @
4. Value: 12.34.56.78
5. TTL: 5 min
6. Save All Changes

Step 3: Create a CNAME for www

Point www.yourdomain.com to yourdomain.com:

On Cloudflare:

1. Add Record
2. Type: CNAME
3. Name: www
4. Target: yourdomain.com
5. Proxy status: DNS only
6. Save

Step 4: Create A Records for Subdomains

Point api.yourdomain.com to your EC2:

On Cloudflare:

1. Add Record
2. Type: A
3. Name: api
4. IPv4 address: 12.34.56.78
5. Proxy status: DNS only
6. Save

Repeat for other subdomains:

• admin → 12.34.56.78 (admin panel)
• app → 12.34.56.78 (app subdomain)

Step 5: Configure Nginx for Subdomains

Update your Nginx server blocks:

# API server block
server {
    listen 80;
    server_name api.yourdomain.com;
    location / {
        proxy_pass http://localhost:3000;
        # ... proxy headers ...
    }
}

# Frontend server block
server {
    listen 80;
    server_name yourdomain.com www.yourdomain.com;
    root /home/ubuntu/frontend/dist;
    location / {
        try_files $uri $uri/ /index.html;
    }
}

sudo nginx -t && sudo systemctl reload nginx

Step 6: Add SSL for Each Subdomain

# Single domain
sudo certbot --nginx -d yourdomain.com -d www.yourdomain.com

# API subdomain
sudo certbot --nginx -d api.yourdomain.com

# All at once
sudo certbot --nginx -d yourdomain.com -d www.yourdomain.com -d api.yourdomain.com

Step 7: Verify DNS

# Check A record
dig yourdomain.com A +short
# Should return: 12.34.56.78

# Check CNAME
dig www.yourdomain.com CNAME +short
# Should return: yourdomain.com.

# Check subdomain
dig api.yourdomain.com A +short
# Should return: 12.34.56.78

# Check from different DNS servers
nslookup yourdomain.com 8.8.8.8
nslookup yourdomain.com 1.1.1.1

Using AWS Route 53

If using Route 53 for DNS:

1. Go to Route 53 → Hosted zones
2. Create hosted zone for yourdomain.com
3. Update nameservers at your registrar
4. Create records:
   • Type A, Name: (empty for root), Value: your Elastic IP
   • Type A, Name: api, Value: your Elastic IP
   • Type CNAME, Name: www, Value: yourdomain.com

Common DNS Issues

DNS not propagating:

• Wait 5-60 minutes (TTL-dependent)
• Check with dnschecker.org for global propagation
• Ensure nameservers are correct at the registrar

Wrong IP returned:

• Check the A record value
• Clear local DNS cache: sudo dscacheutil -flushcache (Mac) or ipconfig /flushdns (Windows)

Subdomain not working:

• Ensure the A record for the subdomain exists
• Ensure Nginx has a server block with the correct server_name
• Check that the subdomain resolves: dig api.yourdomain.com

The Takeaway

Configuring DNS for EC2 involves: allocating an Elastic IP (static IP), creating A records for the root domain and subdomains (api, admin), creating a CNAME for www, configuring Nginx server blocks with matching server_name, and adding SSL with certbot for each domain. Verify with dig or dnschecker.org.