ESXi can automate whether services start based on the status of firewall ports.
Automation helps ensure that services start if the environment is configured to enable their function. For example, starting a network service only if some ports are open can help avoid the situation where services are started, but are unable to complete the communications required to complete their intended purpose.
In addition, having accurate information about the current time is a requirement for some protocols, such as Kerberos. The NTP service is a way of getting accurate time information, but this service only works when required ports are opened in the firewall. The service cannot achieve its goal if all ports are closed. The NTP services provide an option to configure the conditions when the service starts or stops. This configuration includes options that account for whether firewall ports are opened, and then start or stop the NTP service based on those conditions. Several possible configuration options exist, all of which are also applicable to the SSH server.
- Start automatically if any ports are open, and stop when all ports are closed: The default setting for these services that VMware recommends. If any port is open, the client attempts to contact the network resources pertinent to the service in question. If some ports are open, but the port for a particular service is closed, the attempt fails, but there is little drawback to such a case. If and when the applicable outgoing port is opened, the service begins completing its tasks.
- Start and stop with host: The service starts shortly after the host starts and closes shortly before the host shuts down. Much like Start automatically if any ports are open, and stop when all ports are closed, this option means that the service regularly attempts to complete its tasks, such as contacting the specified NTP server. If the port was closed but is subsequently opened, the client begins completing its tasks shortly thereafter.
- Start and stop manually: The host preserves the user-determined service settings, regardless of whether ports are open or not. When a user starts the NTP service, that service is kept running as long as the host is powered on. If the service is started and the host is powered off, the service is stopped as part of the shutdown process, but as soon as the host is powered on, the service is started again, preserving the user-determined state.