To increase the security of your ESXi hosts, you can put them in lockdown mode. In lockdown mode, operations must be performed through vCenter Server by default.
Normal Lockdown Mode and Strict Lockdown Mode
With vSphere 6.0, you can select normal lockdown mode or strict lockdown mode.
Normal Lockdown Mode
In normal lockdown mode, the DCUI service remains active. If the connection to the vCenter Server system is lost, and access through the vSphere Web Client is unavailable, privileged accounts can log in to the ESXi host's Direct Console Interface and exit lockdown mode. Only the following accounts can access the Direct Console User Interface:
Accounts in the Exception User list for lockdown mode who have administrative privileges on the host. The Exception Users list is meant for service accounts that perform specific tasks. Adding ESXi administrators to this list defeats the purpose of lockdown mode.
Users defined in the DCUI.Access advanced option for the host. This option is for emergency access to the Direct Console Interface in case the connection to vCenter Server is lost. These users do not require administrative privileges on the host.
Strict Lockdown Mode
In strict lockdown mode, which is new in vSphere 6.0, the DCUI service is stopped. If the connection to vCenter Server is lost and the vSphere Web Client is no longer available, the ESXi host becomes unavailable, unless the ESXi Shell and SSH services are enabled and Exception Users are defined. If you cannot restore the connection to the vCenter Server system, you must reinstall the host.
Lockdown Mode and the ESXi Shell and SSH Services
Strict lockdown mode stops the DCUI service. However, the ESXi Shell and SSH services are independent of lockdown mode. For lockdown mode to be an effective security measure, ensure that ESXi Shell and SSH services are also disabled. These services are disabled by default.
When a host is in lockdown mode, users on the Exception Users list can access the host from the ESXi Shell and through SSH if they have the Administrator role on the host. This access is possible even in strict lockdown mode. Leaving the ESXi Shell service and the SSH service disabled is the most secure option.
The Exception Users list is meant for service accounts that perform specific tasks such as host backups, and not for administrators. Adding administrator users to the Exception Users list defeats the purpose of lockdown mode.