What is PAC File?
A proxy auto-configuration script that tells a browser or operating system which proxy to use for each URL, or to connect directly.
A PAC file is a small JavaScript function, FindProxyForURL(url, host), served from a URL and referenced in the system's proxy settings as a setup script. For every request, the browser runs the function and follows its answer — a proxy address, a list of fallbacks, or DIRECT.
Companies use PAC files to send internal traffic direct and external traffic through a filtering proxy. Individuals rarely need one, and a leftover setup script is a common reason pages stop loading after uninstalling VPN or security software.
If a browser reports that a mandatory proxy configuration failed, the PAC file could not be downloaded or executed. Turning off the setup script, or fixing its URL, resolves it.
Related terms
Proxy Server
An intermediary server that forwards your internet requests to target websites and returns their responses, masking your real IP address and location in the process.
HTTP/HTTPS Proxy
A proxy that handles web traffic. HTTPS connections are encrypted end-to-end, while HTTP is unencrypted unless the destination site uses TLS.
ASN (Autonomous System Number)
A unique identifier for a network operator on the internet. Targeting by ASN lets you select IPs belonging to a specific ISP or carrier.
Backconnect Proxy
A proxy that connects you to a rotating pool through a single gateway endpoint, so the underlying IP changes automatically without you managing individual proxies.