Windows 2000 Kerberos Clock Skew Tolerance

In Microsoft Windows 2000 networks, Kerberos version 5 serves as the primary security protocol for domain authentication, relying heavily on accurate time synchronization to prevent security breaches. This article examines the default clock skew tolerance configured in Windows 2000 domains, explains why this time threshold is critical for network security, and outlines how the setting operates within domain environments.

The Default Clock Skew Setting

The default Kerberos clock skew tolerance in a Microsoft Windows 2000 domain is 5 minutes (300 seconds).

If the system time on a client workstation differs from the system time on the Kerberos Key Distribution Center (KDC)—which runs on a domain controller—by more than five minutes, authentication fails. When this threshold is breached, the KDC rejects the authentication ticket and generates a KRB_AP_ERR_SKEW error.

Purpose of Clock Skew Tolerance

Kerberos relies on timestamps embedded within authentication tickets rather than sending passwords over the network. The clock skew limit serves two main purposes:

  • Replay Attack Prevention: Timestamps ensure that intercepted tickets cannot be captured by an attacker and replayed at a later time to gain unauthorized access.
  • Network Latency Accommodation: Strict time matching is impossible across distributed networks due to network latency, propagation delays, and minor hardware clock drift. The 5-minute window provides sufficient flexibility for operational discrepancies while keeping the attack surface small.

Time Synchronization and Management

To maintain this synchronization automatically, Windows 2000 introduced the Windows Time service (W32Time). By default, domain members synchronize their clocks hierarchically:

  1. Workstations and member servers synchronize with the authenticating domain controller.
  2. Domain controllers synchronize with the domain controller holding the Primary Domain Controller (PDC) Emulator Flexible Single Master Operation (FSMO) role.
  3. The PDC Emulator in the forest root domain is typically configured to synchronize with a reliable external Network Time Protocol (NTP) hardware source.

Configuring the Tolerance

While 5 minutes is the recommended default, administrators can modify this threshold using Group Policy. The setting is located at:

  • Computer Configuration > Windows Settings > Security Settings > Account Policies > Kerberos Policy > Maximum tolerance for computer clock synchronization

The policy allows values ranging from 1 to 99,999 minutes, though increasing the limit significantly weakens the protocol's defense against replay attacks.