DNS - Proxy question

I have a simple network with a domain controller that also does DHCP and DNS.
I am going to implement a SaaS application for web filtering.
Basically I set the DSL modem DNS settings to the filtering service DNS box.

What about users manually setting their DNS settings to an open DNS box.

My thought is this
1.  Users have to be local admins on their machine or this would be a moot point
2.  Group policy can be set to prevent access to the properties of the lan connection
Downside to this option is I would have to kill it for admins on the box, which includes me.  I guess I would exclude the policy from hitting my user account or domain admins.

Possible quick solution - create a firewall rule that only allows DNS requests from the internal DNS server.  Would this work?
DNS is set up to use forwarders that points to the DSL modem.
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tlovieConnect With a Mentor Commented:
That's what I would do, I would block at the firewall all the traffic outbound to DNS except from the machine you want to allow.  Then force everybody inside to use the DNS forwarders.
Ioannis_AvgerosConnect With a Mentor Commented:
Possible quick solution - create a firewall rule that only allows DNS requests from the internal DNS server.  Would this work?

Yes it would, that's what i would do without having to touch any policies on windows. This will also do the trick for non-domain computers as laptops or handheld devices that may use the network via wifi.

ryansotoAuthor Commented:
OK so the DNS answer appears to be solved but what about in the event a user sets his proxy address in the browser, to some outside proxy.
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tlovieConnect With a Mentor Commented:
You could block all outbound traffic to the common proxy ports (3128, 8080, 1080) and force them to go through an internal proxy, but this gets difficult unless you block all requests, since people in control of their own machines on the outside could run proxy services on arbitrary ports (443, 22, 80) that might fool your rules.
ryansotoAuthor Commented:
Im not so much worried about internal users setting up their own proxies outside.  More like the users trying to use internet explorer and putting in a proxy already existing on the internet.

Maybe a group policy to not allow access to the proxy settings?  What about for other browsers?
Man not easy :)
tlovie could be right, you could block all requests from inside apart from the machine that hosts the proxy server, then everyone would have to go throught that, and those who dont simply wont get access to the any web pages.
lancecurwensvilleConnect With a Mentor Commented:
Make OU's for your users;  Boss's, Network, Employees', etc.

GPO to disallow changes in setting to Local Area Connection Settings.  GPO to disallow changes to Internet Connection settings.  

Link those GPO's to the appropriate OU's.  Then you can set the people who are allowed to make changes on their own.
GPO's are found here:
LAN Settings:  User Config->Policies->Admin Templates->Network->Network Connections->Prohibit access to properties of LAN  <enable>
IE Proxy:  User Config->Policies->Admin Templates->Windows Components->Internet Control Panel->Disable the Connections Page  <enable>

Make your changes to your internal DNS/DHCP server.  As soon as users reboot, changes take place.

In reality, from a security standpoint, this is how it should be setup anyhow.  AD Group Policy is a powerful tool that most smaller networks don't utilize.  

hope this helps...
Addendum to earlier post...

RE:  other browsers..

If you are utilizing GPO's to restrict access, they should not have permissions/rights to install software, including other browsers; unless they are setup as administrators of local machines.  

If they are required to be local admins, short version, you are going to need to create an Acceptable Use Policy, and within this, have it stated that employees are not permitted to install software and/or browse inappropriate sites.  On your new webfilter, you should have the ability to browse logs to see who is going where, at that point, your company should have all the ammo it needs to either terminate or formally discipline that employee.
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