Can I get a Witness…

October 12, 2010

Busting the Exchange Trusted Subsystem Myth….

The below is taken from the following Blog Post:

“….I’ve been building a brand new Exchange 2010 lab environment and ran smack into a myth that seems to be making the rounds among people who are deploying Exchange 2010. This myth gives bum advice for those of you who are deploying an Exchange 2010 DAG and not using an Exchange 2010 Hub Transport as your File Share Witness (FSW). I call it the Exchange Trusted Subsystem Myth, and the first hint of it I see seems to be on this blog post. However, that same advice seems to have gotten around the net, as evidenced by this almost word-for-word copy or this posting that links to the first one. Like many myths, this one is pernicious not because it’s completely wrong, but because it works even though it’s wrong.

If you follow the Exchange product group’s deployment assumptions, you’ll never run into the circumstance this myth addresses; the FSW is placed on an Exchange 2010 HT role in the organization. Although you can specify the FSW location (server and directory) or let Exchange pick a server and directory or you, the FSW share isn’t created during the configuration of the DAG (as documented by fellow Exchange MVP Elan Shudnow and the “Witness Server Requirements” section of the Planning for High Availability and Site Resilience TechNet topic). Since it’s being created on an Exchange server as the second member of the DAG is joined, Exchange has all the permissions it needs on the system to create the share. If you elect to put the share on a non-Exchange server, then Exchange doesn’t have permissions to do it.

Hence the myth:

** Add the FSW server’s machine account to the Exchange Trusted Subsystem group.
** Add the Exchange Trusted Subsystem group to the FSW server’s local Administrators group.

The sad part is, only the second action is necessary.

True, doing the above will make the FSW work, but it will also open a much wider hole in your security than you need or want….”

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