Many developers encounter this puzzling scenario: a website works with "www" but fails without it. This isn't magic - it's all about DNS configuration and web server setup. Let's dissect why u-psud.fr fails while www.u-psud.fr works perfectly.

The primary reason lies in how DNS records are configured. A website needs proper A records or CNAME records for both the naked domain (u-psud.fr) and the www subdomain (www.u-psud.fr). Many sites only configure the www version properly.

// Example DNS zone file entries
u-psud.fr.      IN  A       192.0.2.1
www.u-psud.fr.  IN  CNAME   u-psud.fr.

University websites often have complex infrastructure with:

• Missing A record for the naked domain
• Improper CNAME configuration
• Load balancers not configured for both variants
• SSL certificates not covering both domains

Let's examine what actually happens when you request both versions:

// Example curl commands to test
$ curl -I http://u-psud.fr/
HTTP/1.1 404 Not Found

$ curl -I http://www.u-psud.fr/
HTTP/1.1 200 OK
Server: Apache/2.4.41

For Apache servers, you need proper VirtualHost entries:

<VirtualHost *:80>
    ServerName u-psud.fr
    ServerAlias www.u-psud.fr
    DocumentRoot /var/www/html
    # Additional configuration...
</VirtualHost>

For Nginx, the configuration would be:

server {
    listen 80;
    server_name u-psud.fr www.u-psud.fr;
    root /var/www/html;
    # Additional configuration...
}

Best practice is to implement 301 redirects to standardize on one version (either www or non-www). Here's how to redirect naked to www in .htaccess:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^u-psud.fr [NC]
RewriteRule ^(.*)$ http://www.u-psud.fr/$1 [L,R=301]

Use these tools to verify your setup:

• dig u-psud.fr - Check DNS records
• curl -v http://u-psud.fr - View raw HTTP exchange
• SSL Checker - Verify certificate coverage

Remember that DNS changes can take up to 48 hours to propagate globally.

When you encounter a website that only works with "www" but fails on the naked domain (e.g., www.example.com vs example.com), this typically stems from DNS configuration choices made by the site administrators. Let's break down the technical reasons:

// Example of DNS records that might cause this behavior
$ dig u-psud.fr +short
→ No A record returned for naked domain

$ dig www.u-psud.fr +short
185.52.2.10  // Successful resolution

Here are the most frequent technical implementations that lead to this behavior:

• Missing A/AAAA record for the naked domain
• CNAME at root (technically invalid per RFC but sometimes used)
• Improper ALIAS/ANAME configuration
• Load balancer misconfiguration expecting specific host headers

Programmers can verify this using common network tools:

# Using nslookup (Windows/macOS/Linux)
nslookup example.com
nslookup www.example.com

# Using curl to test HTTP headers
curl -I http://example.com
curl -I http://www.example.com

For Apache administrators, here's how to ensure both domains work:

<VirtualHost *:80>
    ServerName example.com
    ServerAlias www.example.com
    Redirect permanent / http://www.example.com/
</VirtualHost>

For Nginx users:

server {
    listen 80;
    server_name example.com;
    return 301 $scheme://www.example.com$request_uri;
}

From an SEO perspective, it's crucial to:

• Choose one canonical version (www or non-www)
• Implement 301 redirects properly
• Ensure all backlinks use consistent format
• Verify in Google Search Console