Using a http proxy without manually configuring the browser to use proxy server
- Write a PAC file that mimics your requirements
- Host that file on a server somewhere on your LAN
- Configure your DHCP server to announce the location of that PAC file
- Configure the broswers on your LAN to use "Automatic Proxy Configuration Settings" ( In your browser options )
Some relevant links:
http://www-archive.mozilla.org/catalog/end-user/customizing/enduserPAC.html
http://en.wikipedia.org/wiki/Proxy_auto-config
http://technet.microsoft.com/en-us/library/dd361887.aspx