The server could not interpret the communication from the client. (The remote server returned an error: (503) Server Unavailable.)

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I have performed patching activity on my ESX servers. I have installed ESX patches using esxupdate and rebooted the ESX servers. After the reboot . The server came up and responding to ping reply. But i am unable to connect my ESX server neither by vSphere client nor with vCenter server.When i am trying to connect via vSphere Client , I am getting the below error message.

The server could not interpret the communication from the client. (The remote server returned an error: (503) Server Unavailable.)

As a first resort, I tried restarting My Management agents, vpxa agents and management network on ESX  servers mutiple times using the below commands

Service mgmt–vmware restart (VMware Management agents)
Service vmware-vpxa restart (Vcenter agent)
Service network restart (management network)

As a last resort, I have restarted my ESX server also. But still i am not able to connect my ESX server via vSphere client.This issue occurs not only for one ESX server but also for all of the ESX servers. In addition to that , I have  to install ESX 4.0 Update 4 on my ESX servers along with the multiple patches. I have placed my ESX host into maintenance mode(If you want to know how to place ESX server into maintenance mode, Please refer my Blog post on Maintanence Mode ) and installed  ESX 4.0 Update 4 and rebooted my ESX host . Once the server came up, I tried to install other patches using esxupdate utility. I  was getting another error

MaintenanceModeError(EsxupdateError):
description = ‘Maintenance mode is not enabled or could not be determined.’
errno = 18

I  have found the below error  in hostd.log under  /var/log/vmware in my ESX server
Connection to localhost:8307 failed with error N7Vmacore15SystemExceptionE(Connection refused)





I have referred the following KB Article 1014270 from VMware and Since, this occurs on all of the ESX servers in my environment, I suspect that the issue may be because of my configured DNS servers under /etc/resolv.conf entry . My ESX servers are completely  in DMZ environment and don’t have a network path to access the configured DNS server. So i  have Comment out the DNS server entries by placing a # in front of the DNS server IP address in /etc/resolv.conf and Saved the file. This prevents my ESX servers from attempting to contact the configured DNS servers and allows it to boot.



After comment out the DNS entries and i have restarted my management agent using the command “service mgmt–vmware restart”. Within a few seconds, My ESX server connected back to my vCenter server and also i am able to connect via vSphere Client.

Thanks for Reading !!!!