/*
 * Standard proxy.pac automatic proxy configuration.
 *
 * sjoerd [@] warmetal.nl
 * 22 aug 2008
 *
 * Will not proxy for short hostnames for domain.local and raw URL http://10.*
 *
 */

function FindProxyForURL(url, host)
{
 // If no domain name in the host(no dots) are found go DIRECT
  if (isPlainHostName(host) || dnsDomainIs(host, ".domain.local"))
  {
     return "DIRECT";
  }
  else if (url.substring(0, 10) == "http://10." ||
           url.substring(0, 11) == "https://10." ||
           url.substring(0, 11) == "http://127." ||
           url.substring(0, 12) == "https://127." )
  {
     return "DIRECT";
  }
  else
  {
     return "PROXY proxy:8080";
  }
}

Note that you need a DNS record for the proxy

function FindProxyForURL(url, host)
{
  // our local URLs from the domains below don't need a proxy:
  if (shExpMatch(url, "*.wiki.5p-it.com*")) {
  return "DIRECT";
  } else {
  // All other requests go through port 3128 of proxy.5p-it.com.
  return "PROXY proxy.5p-it.com:3128";
  }
}

cat /etc/apache2/default-server.conf | grep -i root
DocumentRoot "/srv/www/htdocs"

Now you can enter an “automatic proxy configuration url” (firefox) or “Use automatic configuration script” (Internet explorer). The address would be:

http://proxy.5p-it.com/proxy.pac