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Managing Aging and Scavenging

  • Статья

Applies To: Windows Server 2008

Aging and scavenging of stale resource records is a feature of Domain Name System (DNS) that is available when you deploy your server with primary zones. Aging and scavenging provides a mechanism for automatically removing resource records that been stored in a DNS zone so long that they are assumed to be no longer valid.

The most common use for aging and scavenging is to remove stale resource records that were dynamically registered, such as by DNS client computers when they were added to the network. For resource records that are not added dynamically to DNS zone data, a record time-stamp value of zero is applied, which prevents these records from aging or being removed during scavenging until you change the time-stamp value. For more information about dynamic registration of resource records, see Managing Dynamic Update for a Zone.

Removing stale resource records is important for two reasons:

  • A stale resource record can contain out-of-date information, either because the host is no longer present on the network or because another host is using the same IP address as the registered host. In either case, attempts to connect to the incorrect target host will fail, and incorrect resolution information will be cached by all DNS servers that were involved in resolving the query.

  • The accumulation of stale records can cause unnecessary growth in the DNS database, slowing the response time of the DNS server.

Where aging and scavenging is available, you can use the DNS Manager snap-in to perform the following related tasks for your DNS servers and any directory-integrated zones that they load:

  1. Enable or disable the use of scavenging at a DNS server

  2. Enable or disable the use of scavenging for selected zones at the DNS server

  3. Modify the no-refresh interval, either as a server default or by specifying an overriding value at selected zones

  4. Modify the refresh interval, either as a server default or by specifying an overriding value at selected zones

  5. Specify whether periodic scavenging occurs automatically at the DNS server for any of its eligible zones and how often these operations are repeated

  6. Manually initiate a single scavenging operation for all eligible zones at the DNS server

  7. View other related properties, such as the time stamp for individual resource records or the start scavenging time for a specified zone

By default, aging and scavenging features are disabled on all DNS servers and any of their zones. Before you use these features, configure the following settings for the applicable server and its directory-integrated zones:

  • Server aging/scavenging properties—for determining the use of these features on a server-wide basis.

    First, you need to enable aging and scavenging of stale resource records for use at the server. These settings are used to determine the affect of zone-level properties for any directory-integrated zones loaded at the server.

    For more information, see Set Default Aging and Scavenging Properties for Zones on the DNS Server.

  • Zone aging/scavenging properties—for determining the use of these features on a per-zone basis.

    When you set zone-specific properties for a selected zone, these settings apply only to the applicable zone and its resource records. Unless these zone-level properties are otherwise configured, they inherit their defaults from comparable settings that are maintained in server aging/scavenging properties.

    For more information, see Set Aging and Scavenging Properties for a Zone.

When the no-refresh interval is in effect for a specific resource record, attempts to dynamically refresh the time stamp of the resource record are ignored by the DNS server. This aspect of the aging/scavenging mechanism prevents unnecessary refreshes from being processed by the server for aged resource records. These early refresh attempts, if not handled in this way, might otherwise increase Active Directory replication traffic.

When the refresh interval is in effect for a resource record, attempts to dynamically refresh its time stamp are accepted and processed by the DNS server. When you set this interval, it is important that the length of time be greater than the maximum possible refresh period for any resource records that are contained within the zone. This period is equal to the maximum amount of time that it might take the record to be refreshed under normal network conditions, based on the specific source that is generating the record refresh.

For example, the following table shows default refresh periods for various services that are known to dynamically register and refresh records in DNS.

Service Default refresh period

Net logon

24 hours

Clustering

24 hours

DHCP client

24 hours

Note
The DHCP Client service sends dynamic updates for the DNS records. This includes both computers that obtain a leased IP address using Dynamic Host Configuration Protocol (DHCP) and computers that are configured statically for TCP/IP.

  •  

DHCP server

4 days (half the lease interval, which is 8 days by default)

Note

Refresh attempts are made only by the DHCP servers that are configured to perform DNS dynamic updates on behalf of their clients, for example Windows Server 2003 DHCP server and Windows Server 2008 DHCP server). The period is based on the frequency in which DHCP clients renew their IP address leases with the server. Typically, this occurs when 50 percent of the scope lease time has elapsed. If the DNS default scope lease duration of 8 days is used, the maximum refresh period for records that are updated by DHCP servers on behalf of clients is 4 days.

By default, the refresh interval is 7 days. In most instances, this value is sufficient and you do not have to change it unless any resource records in the zone are refreshed less often than every 7 days.

Although scavenging start time and other factors determine when zones and records are actually eligible for scavenging, you have the option to perform scavenging using either of two methods to initiate it:

  • Automatic scavenging: Specifies that aging and scavenging of stale records is to be performed automatically by the server for any eligible zones at a recurring interval that is specified as the scavenging period. When you use automatic scavenging, the default scavenging period is 1 day and the minimum allowed period is 1 hour.

    For more information, see Enable Automatic Scavenging of Stale Resource Records.

  • Manual scavenging: Specifies that aging and scavenging of stale records is to be performed as a nonrecurring operation for any eligible zones at the server.

    For more information, see Start Immediate Scavenging of Stale Resource Records.

You can, however, manually reset record properties to enable any statically entered records to qualify for the aging and scavenging process. If you do this, the record will be deleted based on the modified time-stamp value, at which point you might have to recreate a record if it is still needed.

For more information, see Reset Aging and Scavenging Properties for a Specified Resource Record.

Warning

Enabling aging and scavenging for standard primary zones modifies the format of zone files. This change does not affect zone transfer to secondary servers, but the modified files cannot be loaded by third-party DNS servers.

To complete this task, you can perform the following procedures: