A PAC file contains a set of rules coded in JavaScript which allows a web browser to determine whether to send web traffic direct to the target server or via a proxy server. PAC files can control how a web browser handles HTTP, HTTPS, and FTP traffic. The complexity of the PAC file and the JavaScript it contains will vary based on the complexity of the network routing required. They can be setup to do failover to alternate IP addresses, and therefore servers, in the event of a route to a preferred server not working. As such, PAC files can be used to approximate a function of a modern load balancer.