When building web applications that need to handle dynamic subdomains (like xyz.example.com, abc.example.com) while serving content from the root domain (example.com), a wildcard DNS configuration becomes essential. This is particularly common in SaaS platforms, multi-tenant applications, or user-customized portals.

Most shared hosting providers allow these basic DNS record types:

A (Address) Records
CNAME (Canonical Name) Records
MX (Mail Exchange) Records
AAAA (IPv6 Address) Records
TXT (Text) Records

1. Create a wildcard A record pointing to your server IP:

*.mydomain.com. 3600 IN A 192.0.2.1

2. Alternatively, set up a wildcard CNAME if your host supports it:

*.mydomain.com. 3600 IN CNAME mydomain.com.

In your Global.asax.cs or middleware:

protected void Application_BeginRequest(object sender, EventArgs e)
{
    var host = Request.Url.Host;
    if (host.Contains(".") && !host.StartsWith("www."))
    {
        string subdomain = host.Split('.')[0];
        // Process subdomain logic
        Context.RewritePath("/?subdomain=" + subdomain);
    }
}

For IIS hosts, web.config rewrite rules:

<rule name="Subdomain Redirect">
    <match url=".*" />
    <conditions>
        <add input="{HTTP_HOST}" pattern="^([^.]+)\.mydomain\.com$" />
    </conditions>
    <action type="Rewrite" url="/subdomainhandler?name={C:1}" />
</rule>

Use dig or nslookup to verify:

dig any-subdomain.yourdomain.com

Should return the same IP as your root domain.

Some shared hosts restrict wildcard DNS due to security policies. If unavailable:

• Request support to enable it
• Use a CNAME workaround if possible
• Consider upgrading to VPS for full DNS control

Wildcard DNS can expose your application to:

• Subdomain takeover risks
• Phishing vulnerabilities
• SEO duplicate content issues

Mitigate these with proper validation and canonical URL handling.

For high-traffic sites:

// Cache subdomain resolutions
MemoryCache.Default.Add(
    "subdomain_" + subdomain, 
    processedData,
    DateTime.Now.AddHours(1)
);

---

When working with dynamic subdomains in a shared hosting environment, you need to configure two key components:

1. DNS-level wildcard record (*.mydomain.com)
2. Application-level routing in ASP.NET

Most shared hosts support wildcard DNS through either:

# For IPv4:
*.mydomain.com. 3600 IN A 192.0.2.1

# Alternative CNAME approach:
*.mydomain.com. 3600 IN CNAME mydomain.com.

Check your host's DNS management panel for these options. Some hosts may restrict wildcard records, so you might need to:

• Contact support to enable wildcard subdomains
• Manually add common subdomains (www, xyz, abc) if wildcards aren't allowed

In your Global.asax or Startup.cs:

protected void Application_BeginRequest(object sender, EventArgs e)
{
    string host = Request.Url.Host;
    if (host.StartsWith("www."))
    {
        // Handle www subdomain
    }
    else if (host != "mydomain.com")
    {
        // Extract subdomain
        string subdomain = host.Split('.')[0];
        Context.Items["Subdomain"] = subdomain;
        
        // Optional: Rewrite URL internally
        HttpContext.Current.RewritePath(
            virtualPath: "/SubdomainHandler.aspx",
            query: $"sub={subdomain}",
            pathInfo: String.Empty);
    }
}

If you have access to web.config:

<system.webServer>
    <rewrite>
        <rules>
            <rule name="Wildcard Subdomain">
                <match url=".*" />
                <conditions>
                    <add input="{HTTP_HOST}" 
                         pattern="^([^\.]+)\.mydomain\.com$" />
                </conditions>
                <action type="Rewrite" 
                        url="/SubdomainHandler?sub={C:1}" />
            </rule>
        </rules>
    </rewrite>
</system.webServer>

Use these tools to verify your setup:

1. nslookup xyz.mydomain.com
2. curl -I http://test.mydomain.com
3. Browser developer tools (Network tab)

For high-traffic sites:

• Cache subdomain resolution results
• Implement proper 404 handling for invalid subdomains
• Consider database optimization for subdomain-specific content

When implementing wildcard DNS:

// Always validate subdomains
string safeSubdomain = Regex.Replace(subdomain, @"[^a-zA-Z0-9-]", "");
if (string.IsNullOrEmpty(safeSubdomain)) {
    throw new HttpException(400, "Invalid subdomain");
}