1. Check external DNS configuration to ensure that proper servers are configured and functioning correctly.
2. Check external firewall configurations to ensure proper ports are open.
3. Verify that the local domain controllers are configured and functioning properly.
4. Verify all system hardware including the system clock, virtual machine clocks, and domain controllers have accurate time settings.
5. Check for recent system changes that could affect the authentication process.
6. Validate system logs to check availability, functionality, and performance related to authentication.