How to configure an authoritative time server in Windows Server

Windows includes W32Time, the Time Service tool that is required by the Kerberos authentication protocol. The purpose of the Windows Time service is to make sure that all computers that are running Microsoft Windows 2000 or later versions in an organization use a common time.

To guarantee appropriate common time usage, the Windows Time service uses a hierarchical relationship that controls authority, and the Windows Time service does not permit loops. By default, Windows-based computers use the following hierarchy:

• All client desktop computers nominate the authenticating domain controller as their in-bound time partner.
• All member servers follow the same process that client desktop computers follow.
• All domain controllers in a domain nominate the primary domain controller (PDC) operations master as their in-bound time partner.
• All PDC operations masters follow the hierarchy of domains in the selection of their in-bound time partner.

In this hierarchy, the PDC operations master at the root of the forest becomes authoritative for the organization. We highly recommend that you configure the authoritative time server to gather the time from a hardware source. When you configure the authoritative time server to sync with an Internet time source, there is no authentication. We also recommend that you reduce your time correction settings for your servers and stand-alone clients. These recommendations provide more accuracy and security to your domain.

Configuring the Windows Time service to use an internal hardware clock

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

To configure the PDC master without using an external time source, change the announce flag on the PDC master. The PDC master is the server that holds the forest root PDC master role for the domain. This configuration forces the PDC master to announce itself as a reliable time source and uses the built-in complementary metal oxide semiconductor (CMOS) clock. To configure the PDC master by using an internal hardware clock, follow these steps:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config\AnnounceFlags
3. In the right pane, right-click AnnounceFlags, and then click Modify.
4. In Edit DWORD Value, type A in the Value data box, and then click OK.
5. Quit Registry Editor.
6. At the command prompt, type the following command to restart the Windows Time service, and then press ENTER:
   net stop w32time && net start w32time

Note The PDC master must not be configured to synchronize with itself. For more information about why the PDC master must not be configured to synchronize with itself, visit the following Web site to view Request For Comment (RFC) 1305:If the PDC master is configured to synchronize with itself, the following events are logged in the System log:

Event Type: Information
Event Source: W32Time
Event Category: None
Event ID: 38
Computer: ComputerName
Description: The time provider NtpClient cannot reach or is currently receiving invalid time data from NTP_server_IP_Address. For more information, see Help and Support Center at http://support.microsoft.com.

Event Type: Warning
Event Source: W32Time
Event Category: None
Event ID: 47
Computer: ComputerName
Description: Time Provider NtpClient: No valid response has been received from manually configured peer NTP_server_IP_Address after 8 attempts to contact it. This peer will be discarded as a time source and NtpClient will attempt to discover a new peer with this DNS name. For more information, see Help and Support Center at http://support.microsoft.com.

Event Type: Error
Event Source: W32Time
Event Category: None
Event ID: 29
Computer: ComputerName
Description: The time provider NtpClient is configured to acquire time from one or more time sources, however none of the sources are currently accessible. No attempt to contact a source will be made for 15 minutes. NtpClient has no source of accurate time. For more information, see Help and Support Center at http://support.microsoft.com.

When the PDC master runs without using an external time source, the following event is logged in the Application log:

Time Provider NtpClient: This machine is configured to use the domain hierarchy to determine its time source, but it is the PDC emulator for the domain at the root of the forest, so there is no machine above it in the domain hierarchy to use as a time source. It is recommended that you either configure a reliable time service in the root domain, or manually configure the PDC to synchronize with an external time source. Otherwise, this machine will function as the authoritative time source in the domain hierarchy. If an external time source is not configured or used for this computer, you may choose to disable the NtpClient.

This text is a reminder to use an external time source, and it can be ignored.

Configuring the Windows Time service to use an external time source

To configure an internal time server to synchronize with an external time source, follow these steps:

1. Change the server type to NTP. To do this, follow these steps:
   1. Click Start, click Run, type regedit, and then click OK.
   2. Locate and then click the following registry subkey:
      HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Parameters\Type
   3. In the right pane, right-click Type, and then click Modify.
   4. In Edit Value, type NTP in the Value data box, and then click OK.
2. Set AnnounceFlags to 5. To do this, follow these steps:
   1. Locate and then click the following registry subkey:
      HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config\AnnounceFlags
   2. In the right pane, right-click AnnounceFlags, and then click Modify.
   3. In Edit DWORD Value, type 5 in the Value data box, and then click OK.
3. Enable NTPServer. To do this, follow these steps:
   1. Locate and then click the following registry subkey:
      HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpServer
   2. In the right pane, right-click Enabled, and then click Modify.
   3. In Edit DWORD Value, type 1 in the Value data box, and then click OK.
4. Specify the time sources. To do this, follow these steps:
   1. Locate and then click the following registry subkey:
      HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Parameters
   2. In the right pane, right-click NtpServer, and then click Modify.
   3. In Edit Value, type Peers in the Value data box, and then click OK.
      
      Note Peers is a placeholder for a space-delimited list of peers from which your computer obtains time stamps. Each DNS name that is listed must be unique. You must append ,0x1 to the end of each DNS name. If you do not append ,0x1 to the end of each DNS name, the changes made in step 5 will not take effect.
5. Select the poll interval. To do this, follow these steps:
   1. Locate and then click the following registry subkey:
      HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient\SpecialPollInterval
   2. In the right pane, right-click SpecialPollInterval, and then click Modify.
   3. In Edit DWORD Value, type TimeInSeconds in the Value data box, and then click OK.
      
      Note TimeInSeconds is a placeholder for the number of seconds that you want between each poll. A recommended value is 900 Decimal. This value configures the Time Server to poll every 15 minutes.
6. Configure the time correction settings. To do this, follow these steps:
   1. Locate and then click the following registry subkey:
      HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config\MaxPosPhaseCorrection
   2. In the right pane, right-click MaxPosPhaseCorrection, and then click Modify.
   3. In Edit DWORD Value, click to select Decimal in the Base box.
   4. In Edit DWORD Value, type TimeInSeconds in the Value data box, and then click OK.
      
      Note TimeInSeconds is a placeholder for a reasonable value, such as 1 hour (3600) or 30 minutes (1800). The value that you select will depend upon the poll interval, network condition, and external time source.
   5. Locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config\MaxNegPhaseCorrection
   6. In the right pane, right-click MaxNegPhaseCorrection, and then click Modify.
   7. In Edit DWORD Value, click to select Decimal in the Base box.
   8. In Edit DWORD Value, type TimeInSeconds in the Value data box, and then click OK.
      
      Note TimeInSeconds is a placeholder for a reasonable value, such as 1 hour (3600) or 30 minutes (1800). The value that you select will depend upon the poll interval, network condition, and external time source.
7. Quit Registry Editor.
8. At the command prompt, type the following command to restart the Windows Time service, and then press ENTER:
   net stop w32time && net start w32time

Troubleshooting

For the Windows Time service to function correctly, the networking infrastructure must function correctly. The most common problems that affect the Windows Time service include the following:

• There is a problem with TCP/IP connectivity, such as a dead gateway.
• The Name Resolution service is not working correctly.
• The network is experiencing high volume delays, especially when synchronization occurs over high-latency wide area network (WAN) links.
• The Windows Time service is trying to synchronize with inaccurate time sources.

We recommend that you use the Netdiag.exe utility to troubleshoot network-related issues. Netdiag.exe is part of the Windows Server 2003 Support Tools package. See Tools Help for a complete list of command-line parameters that you can use with Netdiag.exe. If your problem is still not solved, you can turn on the Windows Time service debug log. Because the debug log can contain very detailed information, we recommend that you contact Microsoft Product Support Services when you turn on the Windows Time service debug log.

For a complete list of Microsoft Product Support Services phone numbers and information about support costs, visit the following Microsoft Web site:Note In special cases, charges that are ordinarily incurred for support calls may be canceled if a Microsoft Support Professional determines that a specific update will resolve your problem. The usual support costs will apply to additional support questions and issues that do not qualify for the specific update in question.

NTP supports several different packet types. Typically, NTP clients and Simple Network Time Protocol (SNTP) clients send client mode request packets to an NTP server. The NTP server responds with a server mode packet. To configure the W32time service to send symmetric active mode packets instead of client mode packets to an NTP server, type the following command at a command prompt:Note Use the 0x8 flag to force W32time to send normal client requests instead of symmetric active mode packets. The NTP servier replies to these normal client requests as usual.

Reliable time source configuration

A computer that is configured to be a reliable time source is identified as the root of the Windows Time service. The root of the Windows Time service is the authoritative server for the domain and typically is configured to retrieve time from an external NTP server or hardware device. A time server can be configured as a reliable time source to optimize how time is transferred throughout the domain hierarchy. If a domain controller is configured to be a reliable time source, the Net Logon service announces that domain controller as a reliable time source when it logs on to the network. When other domain controllers look for a time source to synchronize with, they select a reliable source first, if one is available.

Manually-specified synchronization

With manually-specified synchronization, you can designate a single peer or list of peers that a computer obtains time from. If the computer is not a member of a domain, it must be manually configured to synchronize with a specified time source. By default, a computer that is a member of a domain is configured to synchronize from the domain hierarchy. Manually-specified synchronization is most useful for the forest root of the domain or for computers that are not joined to a domain. When you manually specify an external NTP server to synchronize with the authoritative computer for your domain, you provide reliable time. However, to provide high accuracy and security to your domain, we recommend that you configure the authoritative computer for your domain to synchronize with a hardware clock.

Without a hardware time source, W32time is configured as a NTP type. You must reconfigure the MaxPosPhaseCorrection and MaxNegPhaseCorrection registry entries. The recommended value should be 15 minutes or even lower, depending on time source, network condition, and security requirement. This requirement also applies to any reliable time source that is configured as the forest root time source in the time sync subnet. For more information about these registry entries, see the "Windows Time service registry entries" section in this article.

Note Manually-specified time sources are not authenticated unless a specific time provider is written for them, and these time sources are therefore vulnerable to attacks. Also, if a computer synchronizes with a manually-specified source instead of its authenticating domain controller, the two computers might be out of synchronization. This scenario causes Kerberos authentication to fail and could also cause other actions that require network authentication to fail, such as printing or file sharing. If only the forest root is configured to synchronize with an external source, all other computers within the forest remain synchronized with each other. This configuration makes replay attacks difficult.

All available synchronization mechanisms

The "all available synchronization mechanisms" option is the most valuable synchronization method for users on a network. This method enables synchronization with the domain hierarchy and may also provide an alternative time source if the domain hierarchy becomes unavailable, depending on the configuration. If the client cannot synchronize time with the domain hierarchy, the time source automatically falls back to the time source that is specified by the NtpServer setting. This method of synchronization is most likely to provide accurate time to clients.

Windows Time service registry entries

The following registry entries are located under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\:

For more information about Windows Time service, click the following article numbers to view the articles in the Microsoft Knowledge Base: For more information about the Windows Time service in a Windows Server 2003-based forest, visit the following Microsoft Web site: