After some googling I found this site with some good information. My proxy.pac looks as follows:
function FindProxyForURL(url, host)
{
if (isPlainHostName(host) || dnsDomainIs(host, ".begeleidingentraining.nl"))
return "DIRECT";
else
return "PROXY proxy.begeleidingentraining.nl:3128; DIRECT";
}
Place the proxy.pac file in the root of your webserver. Go to System Preferences -> Network -> Proxies, and select "Automatic Proxy Configuration." Enter the URL to your proxy.pac, and there you go!

