Maximum tolerance for computer clock synchronization
Applies To: Windows Server 2008, Windows Server 2008 R2
This security policy reference topic for the IT professional describes the best practices, location, values, and security considerations for this policy setting.
Reference
This security setting determines the maximum time difference (in minutes) that Kerberos V5 tolerates between the time on the client clock and the time on the domain controller that provides Kerberos authentication.
To prevent "replay attacks," the Kerberos v5 protocol uses time stamps as part of its protocol definition. For time stamps to work properly, the clocks of the client and the domain controller need to be in sync as much as possible. In other words, both computers must be set to the same time and date. Because the clocks of two computers are often out of sync, administrators can use this policy setting to establish the maximum acceptable difference to the Kerberos protocol between a client clock and domain controller clock. If the difference between a client computer clock and the domain controller clock is less than the maximum time difference that is specified in this policy, any time stamp that is used in a session between the two computers is considered to be authentic.
Important
Changes to this setting are not persistent on computers running Windows Server 2003 and Windows XP. If you configure this setting and then restart the computer, this setting reverts to the default value. Beginning with Windows Server 2008 and Windows Vista, you can persist changes if you select the option Define this policy in the database. Selecting this option does not change current computer time settings.
The possible values for this Group Policy setting are:
A user-defined number of minutes from 1 through 99,999
Not defined
To prevent replay attacks, the Kerberos protocol uses time stamps as part of its definition. For time stamps to work properly, the clocks of the client computer and the domain controller need to be synchronized as closely as possible. Because the clocks of two computers are often out of sync, administrators can use this policy setting to establish the maximum acceptable difference to the Kerberos protocol between a client computer clock and domain controller clock. If the difference between a client computer clock and the domain controller clock is less than Maximum tolerance for computer clock synchronization, any time stamp that is used in a session between the two computers is considered authentic.
Best practices
- It is advisable to set Maximum tolerance for computer clock synchronization to a value of 5 minutes.
Location
GPO_name\Computer Configuration\Windows Settings\Security Settings\Account Policies\Kerberos Policy\
Default values
| Server type or GPO | Default value |
|---|---|
Default Domain Policy |
5 minutes |
Default Domain Controller Policy |
Not defined |
Stand-Alone Server Default Settings |
Not applicable |
DC Effective Default Settings |
5 minutes |
Member Server Effective Default Settings |
Not applicable |
Client Computer Effective Default Settings |
Not applicable |
Operating system version differences
There are no differences in this policy in Windows operating systems beginning with Windows Server 2008.
Security considerations
This section describes how an attacker might exploit a feature or its configuration, how to implement the countermeasure, and the possible negative consequences of countermeasure implementation.
Vulnerability
To prevent "replay attacks" (which are attacks in which an authentication credential is resubmitted by a malicious user or program to gain access to a protected resource), the Kerberos protocol uses time stamps as part of its definition. For time stamps to work properly, the clocks of the client computer and the domain controller need to be closely synchronized. Because the clocks of two computers are often not synchronized, administrators can use this policy to establish the maximum acceptable difference to the Kerberos protocol between a client computer clock and a domain controller clock. If the difference between the client computer clock and the domain controller clock is less than the maximum time difference specified in this setting, any time stamp that is used in a session between the two computers is considered to be authentic.
Countermeasure
Configure the Maximum tolerance for computer clock synchronization setting to 5 minutes.
Potential impact
None. This is the default configuration.
Additional references
The following links provide additional information about topics that relate to securing domain controllers: