"Automatic Proxy Configuration URL" or "Use Automatic Configuration script" at browsers

http://en.wikipedia.org/wiki/Proxy_auto-config

Both of those settings allow for a url to be specified for a file with a proxy configuration script.

The proxy configuration script is a file written in javascript to the netscape PAC syntax that tells the workstation what proxy servers to connect to, what domains or hosts to use proxies for, etc.

If this option is selected, the browser will try to download the file every time it starts up, and if it fails it will either use a previously downloaded file, or attempt to connect directly. Firefox will just attempt to coinnect directly.  This does not work for other applications on the workstation, just browsers. Occasionally Java will work using this, but not always.

If you do not have a web server hosting a PAC file, then you should not select that option.

The autodetect proxy configuration option in both firefox and ie will attempt to resolve a server named
" wpad " using the dns domain suffix list on the workstation, and if it resolves it, it will attempt to pull down a file using the url http://wpad.(domain suffix)/wpad.dat .

The wpad.dat file needs to be, you guessed it, a file configured in netscape PAC javascript format. This file isgenerated and placed automatically on servers running Windows Internet Proxy services .

For Mozilla there is an extension FoxyProxy http://foxyproxy.mozdev.org/ which can be used to switch from a proxy to a non proxied environment with a click of mouse without need to go all the time in config. You can also define patterns of URLs to use with one proxy or another (yes you can also specify multiple proxies).

Internet explorer 7 seems to implement something similar via a plugin called  ie7pro http://www.ie7pro.com/proxy-switcher.html . I have not tried this one so I do not know how this is working.

If you are looking for a permanent solution to switch proxy upon connection, you can get the p that you have and create something like an hastable (ie.e. at work I have 10.x.x.x while at home 192.x.x.x). You can create a script with AutoIT (http://www.autoitscript.com/) or a similar tool to modify the explorer proxy. The proxy setting for explorer are stored in the registry [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings] which can be manipulated easily. The script for AutoIT will require less than 20 lines.. (something like VBScript).

Finally if you want to set all the programs to use the proxy specified in Explorer, you can use the command proxycfg (http://msdn.microsoft.com/en-us/library/aa384069.aspx) which should come standard with winXP.

As you may guessed I struggled a bit with proxies...

Hope that this will help.
Davide

Hi I forgot to add the possibility of using the proxy.pac files which can do the same switching that I suggest to implement with AutoIT (switching proxy based on current local machine IP). You can find some good examples of configuring a pac file here : http://nscsysop.hypermart.net/proxypac.html . You can set up a proxy.pac that is switching proxy based on the destination url.

You can also add multiple proxy in the return function as in the code I attached. The multiple proxy will be tried one at once giving you the one that your machine can ping.

function FindProxyForURL(url, host) {
  var no_proxy = "DIRECT";
  if (isInNet(myIpAddress(), "10.0.0.0")) {
    return "PROXY 10.1.1.1:80"; PROXY 10.1.2.2:80; PROXY 10.3.2.3:8080";
  } else {
    return no_proxy ;
   }
}

                                              

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