Currently Being ModeratedJan 17, 2013 6:12 AM (in response to chrisjako)
The best way to do this is to use a feature invented by Microsoft (gasp!). Microsoft invented something called 'Web Proxy Autodiscovery' (WPAD). With this setup on your network the computers both Windows and Mac can 'autodiscover' the presence of a proxy server and the settings needed to use it.
Originally, OS X could not use WPAD, however I (yes moi) was able to persuade Apple to add support for this and it is available in Snow Leopard (10.6), Lion, (10.7), and Mountain Lion (10.8). You can turn it on by opening System Preferences, clicking on the Network icon, selecting Ethernet interface on the left, unlocking the padlock at the bottom, then clicking on the Advanced... button on the right, and then going to the Proxies tab at the top. You need to turn on only 'Auto Proxy Discovery' option. You do not want to use Automatic Proxy Configuration.
There is one final step, WPAD can work in one of two different ways, it can either provide the settings via DHCP and point your computer to the address of the .pac (proxy auto config file), or it uses a sepcific webpage address based on your networks domain name (the DNS method). See http://en.wikipedia.org/wiki/Web_Proxy_Autodiscovery_Protocol
I strongly recommend using the DHCP method. This requires setting your DHCP server up to advertise via the DHCP option code 252
It is possible to setup Apple's own DHCP server with a DHCP option code 252 but you need to manually edit the /etc/bootpd.plist file to do this. This ability to define DHCP option codes using Apple's DHCP server was added by Apple again in response to one of my suggestions. I have also written a utility to make it easier to generate the code you need for this.
Most DHCP servers built-in to routers cannot do DHCP option codes but Linux, Unix, Windows Server, and as mentioned above Apple Server all can.
Currently Being ModeratedJan 17, 2013 9:58 AM (in response to John Lockwood)
Fantastic, my DHCP is now setup to our pac file on option 252 BUT..... I am a lazy admin, and cant be bothered to go round 40 machines to input this option in system preferences manually - on my windows network this can be easily done via GPO however i new to apple networks - can i role this out somehow via profile manager
Currently Being ModeratedJan 17, 2013 1:00 PM (in response to chrisjako)
You might be able to do this via Profile Manager (or MCX Managed Preferences in Workgroup Manager), however I do this by having the setting already set on a master image used to build each computer.
There is a commandline tool called networksetup which can set pretty much all the network settings and pretty much all the proxy settings, unfortunately the one it cannot do is turn on or off Auto Proxy Discovery. I am guessing Apple have not updated this utility to catch up with this new proxy feature. I will have to ask them to do so .
I also looked at the possibility of using the defaults command to write the setting to the /Library/Preferences/SystemConfiguration/preferences.plist file, unfortunately each network interface is under a GUID entry and not a simple name making this impossible as well.
UPDATE: While the commandline tool networksetup does not support doing this for Snow Leopard, it does for Mountain Lion, you would use it as follows.
networksetup -setproxyautodiscovery Ethernet on