So a little background.  After a power failure and realizing that my DNS server for my ESX/vSphere cluster was a virtual machine and that VMware ESX clusters and a lack of DNS don’t work well I decided to add to move DNS to a physical server in my environment. I loaded the DNS server on Windows 2008 SP1 and added a secondary IP address for the DNS server the ESX hosts were already using to that W2K8 server.  This server was already my VMware vCenter server and I didn’t want to change the IP address for the whole server.

So a few weeks go by and one of my ESX hosts is showing disconnected.  Reconnecting it or trying to add it as a new host don’t succeed and in looking at the error logs on my ESX host I see that the request to the host from the vCenter server is coming from the secondary IP address I added to the vCenter/DNS server…hmmmmm….

So I double-checked everything and even tried looking at the routes setup on the Windows 2008 server but to no avail I can’t change the source IP address it is using.  After much searching around the web I found an article from Microsoft saying that they changed the way source IP’s are selected between Windows 2003 and 2008….no kidding….lol

http://blogs.technet.com/networking/archive/2009/04/24/source-ip-address-selection-on-a-multi-homed-windows-computer.aspx

Basically Windows using the IP source address “closest” to the destination address, oh goody…

Well hang on, here’s the solution.  Update to Windows 2008 SP2 then download the hotfix in the below article and install it, reboot, remove the secondary IP address then add it using netsh as documented in the KB article.  Note…there is no hotfix for Windows 2008 R2….great…not

http://support.microsoft.com/kb/975808

Hope this helps someone and if you’re using more than 1 IP address on a Windows 2008+ host you should definitely be aware of this behavior/feature…. cough cough.  I love Microsoft most of the time but this one was a pain to find.