PAC File Tester guide

It is often named proxy.pac or wpad.dat. The browser calls the function for every URL it opens and connects as the returned string says.

function FindProxyForURL(url, host) {
  if (isPlainHostName(host)) return "DIRECT";
  if (dnsDomainIs(host, ".example.com")) return "DIRECT";
  return "PROXY proxy.example.net:8080; DIRECT";
}

• url is the full URL (for example http://www.example.com/a/b); host is just the host name (www.example.com, lower case, without the port)
• The return value is "DIRECT" (no proxy) or "PROXY host:port". Separate several with ; and the next one is used when the previous one is unavailable
• Some browsers also accept SOCKS host:port, HTTPS host:port and others

A Japanese version of the tool (pac-tester.html) is also available.

• Choose a .pac file with the Open button or drag and drop it onto the page
• Without http:// a URL line is treated as http. Lines starting with # are comments. The list loads from and saves to a .txt file
• Ctrl+Enter or the Evaluate button evaluates at once
• The coloured buttons above the results are the counts for each return value; click one to show only those rows. Filter by text, show only errors and warnings, or save as CSV
• Problems found in the script are listed under Syntax and logic checks. Click one to jump to the line

Evaluation settings

• Host-to-IP table: replaces DNS for dnsResolve, isInNet and isResolvable. One entry per line, as "host IP" or in hosts-file order "IP host". *.example.com 10.0.0.1 applies to every host under that domain
• IP address returned by myIpAddress(): test a PAC that picks a proxy per site by entering that site's client address
• Date and time: the date, time and UTC offset used by weekdayRange, dateRange and timeRange. Untick "Use the current date and time" to test any moment you like
• Strip the path and query from https URLs: matches Chrome and Edge (on by default)

1. Press Diff mode. The PAC box splits into Old PAC (current) and New PAC (changed). The new box starts with a copy of the old one
2. Put the PAC in use now in the old box and the changed PAC in the new box (Open and drag and drop work for each)
3. Both are evaluated against the same URLs. Only rows whose result changed are coloured and marked Changed. The number of changes appears above the results, and ticking Changed rows only hides the other rows

Differences in whitespace around ; in the return value don't count as changes. Errors and timeouts do. The CSV includes both old and new results.

Their behaviour follows the Firefox and Chromium implementations (both descend from Netscape's original code).

You can't use eval or new Function inside the PAC (blocked for safety). Calling new Date() directly gives your computer's clock, not the date and time in the settings.

• Full-width spaces and punctuation: after typing comments in Japanese, Chinese or Korean, a full-width space or bracket can slip into the code. JavaScript accepts a full-width space as whitespace, so browsers won't complain. Inside comments these are fine, inside strings they are a warning, in code they are an error
• Unbalanced brackets and quotes: mismatched { }, ( ), [ ] and unclosed quotes, with line numbers. Parser errors are also reported
• Missing final return: a URL that matches none of the if statements reaches the end of the function. The result is undefined, which browsers handle differently. End with return "DIRECT"; or similar
• Broken shExpMatch patterns: shExpMatch(host, ".example.com") (missing *, matches nothing), shExpMatch(host, "http://…") (host has no scheme), shExpMatch(url, "http://www.example.com") (missing the trailing / of the URL), patterns with capital letters and so on. All patterns are also listed
• Shadowed rules: if shExpMatch(host, "*.example.com") returns first, a later shExpMatch(host, "www.example.com") can never be reached. For simple sequences of if (shExpMatch(…)) return …; (including ||, dnsDomainIs and host == "…"), the checks report when an earlier pattern covers all of a later one. Same return value: info; different: warning
• Malformed return values: "PROXY proxy.example.net:8080, DIRECT" (the separator is ;), a bare "PROXY", a port out of range
• Helper misuse: the isInNet mask format, weekdayRange("mon") (lower case), timeRange(22, 6) and so on

• No network access: everything, including the parser library (acorn), is in one file, and it never connects to a server. The page's Content-Security-Policy (connect-src 'none' and more) also blocks loading from and sending to other sites
• Sandboxed execution: a PAC is a program, and one you received from someone else could contain harmful code. So the PAC doesn't run in the page. It runs in a Worker started inside an iframe with the sandbox attribute (a different origin from the page). It can't reach the page, its storage (localStorage and so on) or other tools on this site. The same network restrictions apply to the iframe and the Worker, so the PAC can't send data out
• Stoppable: if one URL takes longer than 0.2 seconds, the Worker is terminated and recreated. An infinite loop won't freeze the page
• Not saved by default: the PAC and test URLs are saved in the browser only if you tick the option to save them
• No ads or analytics in pac-tester-en.html (ads appear only on this guide and the download page)

Sample: the sample PAC is a generic example that uses only example domains (example.com, example.net, example.org, .example) and private or documentation IP addresses.