PAC File Format

A PAC file , also known as a proxy auto configuration file, is a configuration file used by a web browser for automatically configuring a proxy server connection.

How does PAC extension work

Proxy server is an intermediate server in computer networks that acts as an intermediary between the user and the target server , allowing users to perform indirect requests (receiving and transmitting them through a proxy server) to other network services and receive responses.

Proxy auto config files contain JavaScript functions defining instructions on how to choose a proxy server. PAC files control loading of Internet traffic. This operation is essential either for commercial companies or common users. Each PAC file contains only one JavaScript function indicating the connection mode that the web browser must establish to access the specified URL.

Inside the PAC file function there are two input parameters. First is the URL being accessed. Second is the hostname extracted from the URL. Function inside the PAC file returns a string which specifies configuration. For example, if it returns command “DIRECT” it means that connection must be completed without using proxy. If all proxies are disabled and the DIRECT option was not specified, the browser will ask if it should temporarily ignore the proxies and try to establish direct connections.

Where you can find files in PAC format

Practically every program or application implementing connection with the Internet contains PAC files, because they are an essential part of web processes. You can find them in game, messengers, streaming services and other application folders.

How to open a PAC file

Proxy auto configuration files can be opened easily in text code editors like Notepad++. It is a free tool for opening and editing formats like this. But without special skills you can damage the work of the application if you made some changes in the PAC file. Also you can configure proxy directly in your browser in the special settings section.