Enabling Aging
Aging
in DNS refers to the process of placing a timestamp on a dynamically
registered resource record and then tracking the age of this record. Scavenging
refers to the process of deleting outdated resource records on which
timestamps have been placed. Scavenging can occur only when aging is
enabled. Both aging and scavenging are disabled by default.
To
enable aging for a particular zone, you have to enable this feature
both at the zone level and at the server level. To enable aging at the
zone level, in the Zone Aging/Scavenging Properties dialog box, select
the Scavenge Stale Resource Records check box. To enable aging at the
server level, first open the Server Aging/Scavenging Properties dialog
box by right-clicking the server icon in the DNS console and then
clicking Set Aging/Scavenging For All Zones. Then, in the Server
Aging/Scavenging Properties dialog box, select the Scavenge Stale
Resource Records check box.
After aging
is enabled, a timestamp based on the current server time is placed on
all dynamically registered records in the zone. When the DHCP Client
service or DHCP server later performs a dynamic update of the records,
a timestamp refresh is attempted. Manually created resource records are
assigned a timestamp of 0; this value indicates that they will not be
aged.
Note
When aging and scavenging are enabled for a zone, zone files cannot be read by pre-Windows 2000 DNS servers. |
Modifying No-Refresh Intervals
The no-refresh interval
is the period after a times-stamp during which a zone or server rejects
a timestamp refresh. The no-refresh feature prevents unnecessary
refreshes from being processed by the server and reduces unnecessary
zone transfer traffic. The default no-refresh interval is seven days.
Modifying Refresh Intervals
The refresh interval
is the time after the no-refresh interval during which timestamp
refreshes are accepted and resource records are not scavenged. After
the no-refresh and refresh intervals expire, records can be scavenged
from the zone. The default refresh interval is seven days.
Consequently, when aging is enabled, dynamically registered resource
records can be scavenged after 14 days by default.
Tip
If
you modify the no-refresh or refresh interval, be sure to follow the
guideline that the refresh interval should be equal to or greater than
the no-refresh interval. |
Performing Scavenging
Scavenging
in a zone is performed either automatically or manually. For scavenging
to be performed automatically, you must enable automatic scavenging of
stale resource records on the Advanced tab of DNS server properties.
When this feature is not enabled, you can perform manual scavenging in
a zone by right-clicking the server icon in the DNS console tree and
then selecting Scavenge Stale Resource Records from the shortcut menu.
Start Of Authority (SOA) Tab
The Start Of Authority (SOA) tab, shown in Figure 5,
allows you to configure the SOA resource record for the zone. When a
DNS server loads a zone, it uses the SOA resource record to determine
basic, authoritative information about the zone. These settings also
determine how often zone transfers are performed between primary and
secondary servers.
Serial Number
The
Serial Number text box on the Start Of Authority (SOA) tab contains the
revision number of the zone file. This number increases each time a
resource record changes in the zone or when the value is manually
incremented on this tab by clicking Increment.
When
zones are configured to perform zone transfers, the master server is
intermittently queried for the serial number of the zone. This query is
called the SOA query. If, through the
SOA query, the serial number of the master zone is determined to be
equivalent to the local serial number, no transfer is made. However, if
the serial number for the zone at the master server is greater than
that at the requesting secondary server, the secondary server initiates
a transfer.
Primary Server
The
Primary Server text box on the Start Of Authority (SOA) tab contains
the full computer name for the primary DNS server of the zone. This
name must end with a period.
Responsible Person
When
this text box is configured, it contains a Responsible Person (RP)
resource record of the person responsible for administering the zone.
An RP resource record specifies a domain mailbox name for the
responsible person. The name of the record entered into this field
should always end with a period.
Refresh Interval
The
value you configure in the Refresh Interval field determines how long a
secondary DNS server waits before querying the master server for a zone
renewal. When the refresh interval expires, the secondary DNS server
requests a copy of the current SOA resource record for the zone from
its master server source, which then answers this SOA query. The
secondary DNS server then compares the serial number of the source
server’s current SOA resource record (as indicated in the master’s
response) with the serial number of its own local SOA resource record.
If they are different, the secondary DNS server requests a zone
transfer from the primary DNS server. The default value for this
setting is 15 minutes.
Tip
Increasing the refresh interval decreases zone transfer traffic. |
Retry Interval
The
value you configure in the Retry Interval box determines how long a
secondary server waits before retrying a failed zone transfer.
Normally, this time is less than the refresh interval. The default
value is 10 minutes.
