Is it possible to redirect all other requests from example.com via PAC list?

Yes, you can redirect all requests for example.com through a PAC file by defining a specific rule in the FindProxyForURL function that returns a proxy address instead of DIRECT. This approach works universally across browsers because it relies on standard JavaScript logic within the PAC script rather than browser-specific UI quirks or unreliable blocking proxies.

Understanding PAC File Logic

A Proxy Auto-Configuration (PAC) file is essentially a JavaScript function named FindProxyForURL(url, host). The browser calls this function for every network request. To redirect traffic, your function must evaluate the URL and return a string instructing the browser how to handle the connection. Common return values include "DIRECT" for no proxy, "PROXY ip:port" for a specific proxy, or "SOCKS5 ip:port" for SOCKS traffic. The core challenge is ensuring that the rule for example.com takes precedence over any general catch-all rules. If you have a global proxy setting, you must explicitly override it for the target domain. This requires careful ordering of conditional statements within the function.

Constructing the Redirect Rule

To redirect all requests from example.com, you need to write a condition that checks if the host matches your target domain. It is crucial to use strict matching to avoid accidentally redirecting subdomains like api.example.com unless intended. Here is the basic structure for the PAC file:

function FindProxyForURL(url, host) {
    // Check if the host is exactly example.com
    if (shExpMatch(host, "example.com")) {
        return "PROXY 192.168.1.100:8080";
    }
    
    // Default to direct connection for all other sites
    return "DIRECT";
}

In this example, replace 192.168.1.100:8080 with the actual IP address and port of the proxy server you wish to use. If you want to redirect to a local listener or a different destination, adjust the return string accordingly. Note that using 0.0.0.0:0 as a proxy address is not a standard or reliable method for blocking or redirecting traffic in modern browsers; it often results in connection failures rather than clean redirections.

Handling Subdomains and Variations

If you also need to redirect subdomains such as www.example.com or mail.example.com, you should expand the condition. You can chain multiple if statements or use regular expressions if supported by your browser's PAC implementation. However, shExpMatch is widely supported and efficient.

function FindProxyForURL(url, host) {
    if (shExpMatch(host, "example.com") || 
        shExpMatch(host, "*.example.com")) {
        return "PROXY 192.168.1.100:8080";
    }
    return "DIRECT";
}

This ensures that any request originating from the example.com domain tree is routed through your specified proxy. Always test these patterns carefully, as overly broad wildcards can inadvertently affect performance or security for unrelated sites.

Configuring Google Chrome

Chrome supports PAC files through its system proxy settings or via command-line flags. For most users, configuring the system proxy is the most stable method. On Windows, go to Settings > Network & Internet > Proxy. Select "Use a setup script" and enter the URL where your PAC file is hosted. This URL can be an HTTP/HTTPS link to a web server or a local file path if configured correctly. For advanced users or those needing immediate testing without system-wide changes, you can launch Chrome with the --proxy-pac-url flag. This overrides the system settings for that specific instance.

chrome.exe --proxy-pac-url=http://localhost/pac.js

Replace http://localhost/pac.js with the actual location of your PAC file. This method is useful for debugging because it isolates the proxy configuration from other applications. Remember that local file paths (file://) are often restricted by security policies in modern browsers for PAC files due to cross-origin resource sharing concerns. Hosting the PAC file on a simple local HTTP server (like Python's http.server or Nginx) is a more reliable approach.

Troubleshooting Common Issues

If your redirection is not working, first verify that the PAC file is syntactically correct. Use a JavaScript linter or check the browser's developer tools under the Network tab to see if the PAC file is being fetched successfully. A 404 error here means the browser cannot read your rules. Next, ensure that your proxy server is actually running and accessible. If the proxy at 192.168.1.100:8080 is down, the browser will fail to connect, potentially timing out. You can test connectivity using tools like curl or telnet from the command line. Additionally, check for conflicting proxy extensions. Extensions like "Proxy SwitchyOmega" can override PAC settings if their profile is set to active. Disable any such extensions while testing your PAC file to ensure accurate results. For more details on managing proxy configurations, you might find Troubleshooting Slow VPS-based Proxy Connections helpful, as some principles apply to general proxy latency and connectivity issues.

Frequently Asked Questions

Can I use a local file:// URL for the PAC file?

Modern browsers restrict local file access for PAC scripts due to security policies. It is safer to host the PAC file on a local HTTP server and reference it via http://localhost. This avoids permission errors and ensures consistent behavior across different user accounts.

How do I debug my PAC file syntax?

You can validate your PAC file by checking the browser's event log or using online JavaScript validators. In Chrome, navigate to chrome://net-internals/#events and filter for "PROXY" to see if the PAC file is loaded and parsed correctly. Look for any syntax errors reported during the fetch phase.

Will this redirect HTTPS traffic as well?

Yes, the FindProxyForURL function processes both HTTP and HTTPS requests. However, note that for HTTPS traffic, the proxy must support tunneling (CONNECT method). If your proxy does not handle CONNECT requests, HTTPS connections may fail even if the PAC rule directs them there.

What happens if the proxy server is unreachable?

If the specified proxy is unavailable, the browser will typically retry the connection or fall back to a direct connection depending on its timeout settings. Some browsers offer automatic fallback options, but relying on this is not recommended. Ensure your proxy server is highly available if critical redirection is required.

Can I use regex instead of shExpMatch?

While shExpMatch is widely supported, some browsers allow the use of regular expressions via dnsResolve or custom functions. However, shExpMatch is generally faster and more compatible. Stick to simple pattern matching for best performance and compatibility across different browser versions.