About DNS Aging and Scavenging
By Jithin on November 18th, 2016
Aging and Scavenging is a DNS server service which supports a mechanism for performing clean-up and removal of stale resource records which can accumulate in zone data over time. It helps to maintain the dynamic DNS environment by regular deletion of stale resource records from the DNS database. Some problems associated with stale records are: unnecessary space utilization long zone transfers, wrong resolution of the client query due to stale data, and accumulation of stale records on the DNS server can degrade its performance. These stale records problems can be resolved by the aging and scavenging features. Before using aging and scavenging features of DNS some conditions needed are:
1) Aging and scavenging features must be enabled on the DNS server and on the zone. By default, they are not enabled.
2) Resource records must be added dynamically to the zone or manually modified to be used in operations of aging and scavenging.
Aging is the process of identifying stale DNS records. It uses two intervals:
1) Non-Refresh interval
2) Refresh interval
This is the time period in which the resource records cannot be refreshed. It can be used to reduce the replication traffic in this time period to avoid the replication of the same information again.
This is the time period in which the resource records can be refreshed.
Resource record refresh: This is a DNS dynamic update without changing the hostname and IP address.
If the non-refresh interval and refresh interval are 7 days, then the resource records can be considered as stale if not refreshed after 14 days. If the non-refresh interval and refresh interval are elapsed, then the resource records can be refreshed as long as they are not removed from the DNS zone. Aging uses a resource record time-stamp to identify if the record is stale or not.
Resource records having timestamp zero: These records are static records that are not stale records.
Resource records having timestamp not equal to zero: These records are dynamic records which represent the hour of the last refresh date.
Scavenging is the process of removal and clean-up of stale resource records from the DNS zone. The stale resource records will be removed only if the scavenging is enabled on the resource record, where the resource record exists and at least one DNS hosting where the primary copy of the resource records exists.
Scavenging can be set in three places:
1) Individual record
If scavenging is set on zone it will work only for dynamic records. It will work for manual entries only if it’s enabled for the zone. Once scavenging is set on zone this will enable it on DNS servers. The DNS server where the scavenging option enabled is responsible to scavenge the record. The server will log a DNS event 2501 to indicate the number of scavenging record and it will log a DNS event 2502 if no record where scavenged.
Record timestamp+no refresh interval for zone+refresh interval for zone
If the sum of these values are greater the server time (current date and time on the DNS server) no action is taken and records are not deleted from the zone. If the sum is less than server time the records are deleted.
Aging and scavenging process for a sample record
Consider a DNS host “host-a.example.microsoft.com” register its host resource record on the DNS server where aging and scavenging are enabled. The DNS server set a time stamp for this record based on the current server time at the time of registration. The DNS server does not refresh the resource record for the duration of non-refresh interval. It can refresh the record before non-refresh interval if any update, such as the IP address of the host changes and it resets the time stamp accordingly. The DNS server refreshes the record after the non-refresh interval expires. During and after the refresh interval if any update comes it accepts and refresh the record. The server examines the subsequent scavenging and each record is compared to server time to determine whether the record should be removed and this is done by using scavenging formula.
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