Time keeping is especially important for Active Directory and Kerberos. I encountered an error when I was attempting to ssh into one of my AD enabled ESX hosts. The SSH error was “Permission Denied”, however after inspecting the logs (/var/log/messages) I discovered that pam_krb5 was throwing “Clock skew too great” errors.
This was odd to me, as I know every one of the ESX servers has NTP configured. Apparently ntpd died at some point, which caused the clock to begin losing time. Once the time difference between the domain controller and the ESX host exceeded 300 seconds (5 minutes), ESX no longer allowed me to login using AD credentials.
The fix was somewhat easy…reset the clock. Since I was able to login to the console, I did so as root, and executed
ntpdate name.of.domain.controller, which forced it to sync the clock with the DC. After that was taken care of (which confirmed that it was ntp that broke), I went back to Virtual Infrastructure Client and reset the NTP settings for the host (it’s on the Configuration tab).