The function dnsResolve (and similar other functions) performs a DNS lookup that can block your browser for a long time if the DNS server does not respond.
Caching of proxy auto-configuration results by domain name in Microsoft's Internet Explorer 5.5 or newer limits the flexibility of the PAC standard. In effect, you can choose the proxy based on the domain name, but not on the path of the URL. Alternatively, you need to disable caching of proxy auto-configuration results by editing the registry, a process described by de Boyne Pollard (listed in further reading).
It is recommended to always use IP addresses instead of host domain names in the isInNet function for compatibility with other Windows components which make use of the Internet Explorer PAC configuration, such as .NET 2.0 Framework. For example,
The current convention is to fail over to direct connection when a PAC file is unavailable.
Shortly after switching between network configurations (e.g. when entering or leaving a VPN), dnsResolve may give outdated results due to DNS caching.
For instance, Firefox usually keeps 20 domain entries cached for 60 seconds. This may be configured via the network.dnsCacheEntries and network.dnsCacheExpiration configuration variables. Flushing the system's DNS cache may, also, help, which can be achieved e.g. in Linux by sudo service dns-clean start.
Read more about this topic: Proxy Auto-config, The PAC File, Limitations