GPO Application Precedence - "Just Because You Can" Edition

This one really gets back to my roots as a big fan of everything related to Active Directory and Group Policies. Someone had a question yesterday about GPO application that, I admit, gave me pause. It would have been an excellent question for an MCITP exam or a sysadmin interview.

It's also an example of a GPO strategy that might be too complicated for its own good.

The basic behavior of Group Policy application order is well-known by almost every Windows admin. Let's review:

  • Local policies are applied first.
  • Then policies linked at the site level.
  • Then policies linked at the domain level.
  • Then GPOs linked to OUs in order such that policies linked to "higher" OUs apply first, and the policies linked "closest" to the object go last.
  • If multiple GPOs are linked at the same level, they go from the bottom-up. (AKA by Link Order)
  • Last writer wins, i.e., each subsequent GPO overwrites any conflicting settings defined in earlier GPOs. Settings that do not conflict are merged.
  • Enforce (formerly known as No Override,) Block Inheritance and Loopback Processing can be used at various levels of the aforementioned hierarchy in various combinations to augment the behavior of GPO application.
So that seems like a pretty simple system, but it's just flexible enough that you can get into some confusing situations with it. For instance, take the following OU structure:
(OU)All Servers
       +--(OU)Terminal Servers

The Terminal Servers OU is a Sub-OU of the All Servers OU. Now, let's link two different policy objects to each of the OUs:

(OU)All Servers [Servers_GPO]
       +--(OU)Terminal Servers [TS_GPO]

So using what we know, we assume that a computer object in the Terminal Servers OU will get all the settings from Servers_GPO, and then it will receive settings from TS_GPO, which will overwrite any conflicting settings from Servers_GPO.

Now let's put the Enforced flag on Servers_GPO:

(OU)All Servers [Servers_GPO-ENFORCED]
       +--(OU)Terminal Servers [TS_GPO]

Now the settings in Servers_GPO will win, even if they conflict with settings in TS_GPO. But let's go one step further. What happens if you also Enforce TS_GPO?

(OU)All Servers [Servers_GPO-ENFORCED]
       +--(OU)Terminal Servers [TS_GPO-ENFORCED]

Which GPO will win?  Had I been taking a Microsoft exam, I might have had to flip a coin. I have to admit, I had never considered this scenario. If neither policy was enforced, we know TS_GPO would win. If Servers_GPO was enforced and TS_GPO was not enforced, then we know Servers_GPO would win. But what about now?

And furthermore, why would anyone want to do that? I can't explain what goes on in some administrator's heads when they're planning these things out, but luckily I did have Technet at my disposal:

You can specify that the settings in a GPO link should take precedence over the settings of any child object by setting that link to Enforced. GPO-links that are enforced cannot be blocked from the parent container. Without enforcement from above, the settings of the GPO links at the higher level (parent) are overwritten by settings in GPOs linked to child organizational units, if the GPOs contain conflicting settings. With enforcement, the parent GPO link always has precedence. By default, GPO links are not enforced.

So with that, we should be able to surmise that the parent GPO - Servers_GPO - will win. A little testing confirmed it - the higher-level GPO takes precedence over a lower-level GPO even when they're both enforced.

I might call this one of those "just because you can, doesn't mean you should" sort of administrative practices.

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