Windows 8.1 introduced significant changes in how different components handle Proxy Auto-Config (PAC) files. While traditional IE11 continues to support most legacy debugging methods, Modern UI apps (now called Universal Windows Platform apps) operate with a completely different network stack.

For IE11 debugging, we have several technical approaches:

// Example PAC function with debug output
function FindProxyForURL(url, host) {
    // Output to IE11's JavaScript console
    if (console && console.log) {
        console.log("PAC Evaluation - URL: " + url + ", Host: " + host);
    }
    
    // Alternative debug method using external logging
    try {
        var fso = new ActiveXObject("Scripting.FileSystemObject");
        var file = fso.OpenTextFile("C:\\pac_debug.log", 8, true);
        file.WriteLine((new Date()) + " - " + url + " | " + host);
        file.Close();
    } catch(e) {}
    
    return "DIRECT";
}

Key IE11 debugging techniques:

• Use F12 Developer Tools (Network tab shows proxy decisions)
• Check about:internet in IE11 address bar for connection information
• Monitor traffic with Fiddler or Wireshark

The Modern UI environment presents unique challenges:

// Modern UI compatible PAC debug approach
function FindProxyForURL(url, host) {
    // Windows Store apps can't use traditional alert() or console.log()
    // Instead, we need to use ETW (Event Tracing for Windows)
    try {
        var logMessage = "PAC: " + url + " | " + host;
        var etwProvider = new ActiveXObject("ETW.Provider");
        etwProvider.LogMessage(0, 0, 0, logMessage);
    } catch(e) {}
    
    // Return proxy configuration
    if (shExpMatch(host, "*.microsoft.com")) {
        return "PROXY proxy.example.com:8080";
    }
    return "DIRECT";
}

Essential tools for Modern UI PAC debugging:

• Windows Performance Analyzer (WPA) for ETW traces
• Network Isolation Tool (CheckNetIsolation.exe)
• Windows Event Viewer (Filter for Network-related events)

To identify differences between IE11 and Modern UI behavior:

// PAC function to detect caller type
function FindProxyForURL(url, host) {
    var callerType = "Unknown";
    
    // Detect IE11 vs. Modern UI
    try {
        if (typeof window === 'object' && window.navigator) {
            callerType = "IE11";
        } else {
            callerType = "ModernUI";
        }
    } catch(e) {
        callerType = "ModernUI";
    }
    
    // Return different proxies for testing
    if (callerType === "IE11") {
        return "PROXY ieproxy.example.com:8080";
    } else if (callerType === "ModernUI") {
        return "PROXY modernproxy.example.com:8080";
    }
    
    return "DIRECT";
}

For low-level debugging:

netsh trace start scenario=internetclient capture=yes persistent=no tracefile=C:\temp\net_trace.etl
netsh trace stop

Analyze the resulting ETL file with WPA to see:

• PAC file loading events
• Proxy resolution attempts
• Network stack behavior differences

When Modern UI apps fail to connect while IE11 works:

• Verify loopback is enabled (CheckNetIsolation LoopbackExempt)
• Check Windows Firewall rules for Modern apps
• Test with a simple PAC file that returns "DIRECT"
• Ensure the PAC file uses compatible JavaScript (no DOM methods)

For complex environments, consider:

// Example of conditional proxy selection
function FindProxyForURL(url, host) {
    // Bypass proxy for local addresses
    if (isPlainHostName(host) ||
        shExpMatch(host, "*.local") ||
        isInNet(dnsResolve(host), "10.0.0.0", "255.0.0.0") ||
        isInNet(dnsResolve(host), "192.168.0.0", "255.255.0.0")) {
        return "DIRECT";
    }
    
    // Modern UI specific handling
    try {
        if (typeof window === 'undefined') {
            return "PROXY modernproxy:8080; DIRECT";
        }
    } catch(e) {}
    
    // Default proxy for IE11
    return "PROXY defaultproxy:8080; DIRECT";
}

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Windows 8.1 introduced significant changes to how both IE11 and Modern UI apps handle PAC files. The traditional alert() debugging method that worked in Windows 7/IE10 no longer functions, creating new challenges for network administrators and developers.

For Internet Explorer 11, you can leverage these approaches:

// Example PAC function with logging
function FindProxyForURL(url, host) {
    // Log to console (viewable via F12 Developer Tools)
    console.log("Evaluating URL: " + url);
    
    // Return different proxies for testing
    if (shExpMatch(host, "*.internal.company.com")) {
        return "PROXY internal-proxy:8080";
    }
    return "DIRECT";
}

Key IE11 debugging methods:

• Use F12 Developer Tools (Network tab) to monitor proxy decisions
• Check about:internetsettings for active proxy configuration
• Enable script logging via registry (HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\Diagnostics)

Modern UI apps use a different network stack (WinINET vs. Windows.Web.Http) which handles PAC files differently:

// Example of checking network capabilities in a Modern app
Windows.Networking.Connectivity.NetworkInformation.getNetworkConnectivityProfile()
.then(function(profile) {
    if (profile.getNetworkConnectivityLevel() === 
        Windows.Networking.Connectivity.NetworkConnectivityLevel.none) {
        console.log("No connectivity detected");
    }
});

Troubleshooting steps for Modern apps:

• Verify PAC file accessibility (must be hosted on HTTP/HTTPS, not file://)
• Check Windows Firewall rules for Modern apps
• Use netsh winhttp show proxy to verify system-wide proxy settings
• Test with a simple PAC file that returns only "DIRECT"

For comprehensive debugging:

// PowerShell script to monitor proxy resolution
Get-ChildItem HKLM:\Software\Microsoft\Windows\CurrentVersion\Internet Settings\Connections | 
ForEach-Object { 
    Write-Host $_.PSChildName 
    Get-ItemProperty $_.PSPath 
}

Additional tools:

• Fiddler with WinConfig to examine Modern app traffic
• Process Monitor to track PAC file access
• Network Tracing (netsh trace start scenario=internetclient capture=yes)

Specific issues to check:

• PAC file encoding must be ANSI or UTF-8 without BOM
• Timeout handling differs between IE11 and Modern apps
• Authentication requirements may block Modern apps
• Check Group Policy settings affecting proxy configuration