Dev tools

Troubleshooting

Common PAC file bugs and how to fix them.

Symptom	Cause & Fix
myIpAddress() returns 127.0.0.1	The browser can't determine the machine's external interface IP. This often happens on machines with no active network adapters or unusual routing tables. Some browsers (especially Chrome) always return `127.0.0.1` in certain OS configurations. Workaround: use subnet detection via `dnsResolve()` on a known internal host instead, or avoid `myIpAddress()`-based logic in WPAD deployments.
HTTPS URL path not available	Chrome, Firefox, and all modern browsers strip the path and query from HTTPS URLs before calling `FindProxyForURL`. You receive only `https://hostname:443`. `shExpMatch(url, "/some/path")` will never match for HTTPS. Path-based routing only works for plain HTTP.
PAC file changes not picked up	Browsers cache PAC files aggressively — often for the entire browser session. To force a reload: restart the browser completely, or use a short `Cache-Control: max-age=300` header on your PAC file server. During development, use `pacparser` to test outside the browser.
isInNet() always returns false	You're passing a hostname to `isInNet()` that failed DNS resolution — `dnsResolve()` returned `""`, and `isInNet("", ...)` returns false. Always guard: `var ip = dnsResolve(host); if (ip && isInNet(ip, ...))`.
PAC file ignored entirely	Wrong MIME type. The server must respond with `Content-Type: application/x-ns-proxy-autoconfig`. A `text/plain` or missing content-type causes many browsers to silently reject the file.
alert() breaks PAC in IE/Edge	Internet Explorer and legacy Edge treat `alert()` as an invalid API in the PAC sandbox — calling it causes the entire PAC function to fail silently and the browser falls through to direct connection. Remove all `alert()` calls before deploying.
dnsDomainIs() misses apex domain	`dnsDomainIs("example.com", ".example.com")` returns `false` — the bare apex doesn't match the dotted suffix. Combine with an exact check: `host === "example.com" \|\| dnsDomainIs(host, ".example.com")`.
PAC works in Firefox but not Chrome	Chrome caches PAC results per-hostname for the duration of a network profile change, ignoring path-based differences. Chrome also resolves DNS from the host OS rather than the browser's DNS stack in some configurations. Test with `pacparser` to isolate PAC logic from browser-specific quirks.