When deploying web applications on IIS6 with Windows Authentication enabled, developers often encounter persistent credential prompts despite configuring Integrated Windows Authentication (IWA). This becomes particularly problematic in standalone server environments without Active Directory integration.

First, verify these essential settings in IIS Manager:

Authentication Methods:
- Anonymous Access: Disabled
- Integrated Windows Authentication: Enabled
- Basic Authentication: Disabled
- Digest Authentication: Disabled

The browser's security zone configuration significantly affects authentication behavior. For Internet Explorer (still relevant for legacy systems):

Tools → Internet Options → Security → Local Intranet → Custom Level:
- User Authentication → Logon: Automatic logon only in Intranet zone

Sometimes the issue requires modifying the metabase directly. Create a backup first, then try:

adsutil.vbs SET W3SVC/NTAuthenticationProviders "NTLM"

This forces NTLM authentication which works better in non-AD environments.

When standard configurations fail, consider these approaches:

1. Implement a custom HTTP module to handle authentication:
public class AutoAuthModule : IHttpModule
{
    public void Init(HttpApplication context)
    {
        context.BeginRequest += (sender, e) => 
        {
            var app = (HttpApplication)sender;
            app.Context.User = new WindowsPrincipal(WindowsIdentity.GetCurrent());
        };
    }
}

The double-hop authentication issue often causes repeated prompts. Verify these network elements:

• SPN (Service Principal Name) configuration
• Loopback check settings (DisableStrictNameChecking)
• Hosts file entries matching server names

Use these diagnostic tools to pinpoint the issue:

1. Fiddler to examine WWW-Authenticate headers
2. Wireshark to analyze NTLM handshake
3. Event Viewer for security audit failures
4. Failed Request Tracing in IIS

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When deploying ASP.NET applications on IIS6, many developers encounter this frustrating scenario: despite configuring Windows Authentication and disabling Anonymous access, the browser keeps prompting for credentials. This typically happens when:

• The server isn't domain-joined (standalone configuration)
• Client browsers aren't sending NTLM/Kerberos tokens automatically
• There's a mismatch between requested authentication protocols

First, verify these IIS6 settings in the Authentication Methods dialog:

<configuration>
  <system.web>
    <authentication mode="Windows"/>
  </system.web>
</configuration>

Then confirm these IIS Metabase settings (for IIS6):

adsutil.vbs SET W3SVC/NTAuthenticationProviders "NTLM"

For standalone servers, try these solutions:

// In web.config
<identity impersonate="true"/>

// For ASP.NET applications
protected void Application_AuthenticateRequest(Object sender, EventArgs e)
{
  if (Request.ServerVariables["LOGON_USER"] == "")
  {
    Response.StatusCode = 401;
    Response.End();
  }
}

Add your site to Trusted Sites zone with these registry settings:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap\Domains\yourserver.com]
"https"=dword:00000001
"*"=dword:00000001

If you later join a domain, set SPN properly:

setspn -A HTTP/webserver.domain.com domain\serviceaccount

• Use Fiddler to examine WWW-Authenticate headers
• Check IIS logs for 401 status codes
• Test with different browsers to isolate client-side issues