In late November of 2009 VMWare released the U1 packages for vSphere (ESX 4) and vCenter 4 (VirtualCenter). U1 stand for Update 1 and you can compare it to a service pack on the Windows side of things. Update 1 for vSphere fixed several bugs. Update 1 for vCenter added Windows 7 and Windows 2008 Server R2 compatibility. So far so good. I deployed the U1 package for ESX 4 via the Update Manager with no issues. However, U1 for vCenter surprised me with a noce problem.
After applying U1 for VirtualCenter 2 hosts in a HA cluster did not play nice. I was not able to enable HA (High Availability) for this cluster on these 2 hosts. The event log inside vCenter did not provide me with much information, however the vmkernel log pointed towards something in regards to name resolution. I checked the DNS settings and name resolution via DNS – no issues here. Then I checked for the hosts file to see if something was messed up.
Well, the hosts files (/etc/hosts) were missing. Let me rephrase that – they were not missing, but they were renamed to hosts.old. I have yet to find the reason why this happened or why the vcenter agent would do it. However, after I renamed these files back to just “hosts” and then initiated the “Reconfigure for HA” inside VirtualCenter these 2 hosts came back just fine and joined the HA setup. I am working with ESX since the good old 2.5.x days and back then it was very important to have a custom configured hosts file. With 3.5.x that requirement disappeared (if I remember this correctly), however I have always used custom hosts files in case DNS craps out on me and this incident now shows that there is a large dependency on having at least a basic hosts file on an ESX 4 vSphere host to use features like HA (High Availability).
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