How do I use a PAC file?

Host the file on a web server or locally, then enter its URL under Automatic proxy configuration in your OS or browser network settings.

Can I put a proxy username and password in a PAC file?

No. PAC files only return proxy addresses. The browser prompts for credentials or uses saved ones, which is why IP allowlisting suits PAC setups best.

Web Proxy Auto-Discovery: a way for a network to tell browsers where its PAC file is, using DHCP or a wpad DNS name. It is common in corporate networks.

Does a PAC file support SOCKS5?

Yes. Return SOCKS5 host:port, or SOCKS host:port for older clients. Support differs slightly between browsers, so test the setup in each one you use.

Why is my PAC file being ignored?

Common causes are a syntax error, the wrong content type, a file:// URL the app doesn't accept, or a cached older version. Restart the browser after changes.

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What Is a PAC File (Proxy Auto-Config)?

A PAC file (Proxy Auto-Config) is a small JavaScript file with one function, FindProxyForURL, that a browser or operating system calls for every request to decide whether it goes direct or through a proxy, and through which one. It lets one setting route different sites through different proxies.

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how it works

How a PAC file works

You point your browser or OS at a PAC URL (Automatic proxy configuration). For each request it calls FindProxyForURL(url, host) and reads the returned string: DIRECT, PROXY host:port, SOCKS5 host:port, or a semicolon-separated list to try in order. Helper functions such as shExpMatch, dnsDomainIs and isPlainHostName make the rules readable.

Networks can also publish a PAC file for automatic discovery with WPAD, through DHCP or a wpad DNS name. Chrome passes only the scheme and host of https:// URLs to the function, so rules for secure sites should match on the host, not the path.

proxy.pacJavaScript

function FindProxyForURL(url, host) {
  // Local names and the intranet go direct
  if (isPlainHostName(host) || dnsDomainIs(host, ".corp.example")) {
    return "DIRECT";
  }
  // Only these sites use the proxy, falling back to direct
  if (host === "example.com" || dnsDomainIs(host, ".example.com")) {
    return "PROXY HOST:PORT; DIRECT";
  }
  return "DIRECT";
}

why it matters

Why PAC files matter for proxy users

A PAC file is the simplest way to send only some traffic through a proxy without touching each app: a research team can route one set of sites through a US proxy and leave everything else direct.

CleanProxies static ISP proxies support IP authentication for up to 20 IPv4 addresses, so a PAC rule like PROXY HOST:PORT works without a login prompt.

PAC file: what it means for you

PAC files cannot hold usernames and passwords. The browser asks for proxy credentials separately, so IP whitelisting is the smoother fit.
Serve the file with the application/x-ns-proxy-autoconfig content type; some clients ignore it otherwise.
Avoid dnsResolve() and isInNet() on every request: they trigger local DNS lookups for each host and slow browsing down.
Add ; DIRECT as a fallback only where leaking traffic outside the proxy is acceptable.

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