DNS Time to live, aging and scavenging

The DNS settings looked at in this video determine how long DNS records remain on the DNS server or in the DNS cache. Check out http://itfreetraining.com for more of our always free training videos. DNS supports automatic removal of DNS records from the database if the appropriate options have been configured. This video looks at how to configure those options and the effects this can have on your network if the settings are not managed correctly.

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Time To Live (TTL)
Each DNS record has a time to live (TTL) setting. This value is configured by the DNS administrator. When a DNS record is stored in DNS cache, this DNS record can only be used for the time period stated in the time to live setting. When this expires, the DNS record must be obtained again. In the case of a work station, this means the working contacting a DNS server to obtain the DNS record again. In the case of a DNS server, the DNS server must obtain the DNS record again from another DNS server or from the authoritative DNS server for that DNS domain. This gives the administrator control over how long changes to a DNS records will take to have effect on the network. The lower the time to live setting is configured means changes to the DNS record will take less time to take effect on the network. This is because the DNS record will be discarded from the cache sooner and the DNS server will be forced to query a new copy of the DNS record. A lower setting does result in more DNS queries being sent to an authority DNS server which puts more load on the DNS server. A higher time to live setting means less queries to the authority DNS server, however this also means that changes to the DNS records take longer to take effect on the network.

DNS Record Aging
Each DNS record has a timestamp which indicates when the DNS record was created or when it was last updated. Dynamic updates allow a client to create their own DNS records on a DNS server. If dynamic updates are enabled, the client is able to update this timestamp. The DNS server, if configured, has the ability to go through and remove DNS records that have not been updated for a set time period. This has to be configured for the DNS server and is not configured by default. When a client starts up and dynamic updates are enabled on the DNS server, the client can request the timestamp to be updated. Depending on the settings on the DNS server will determine when and if the DNS client can update the DNS record.

Timestamp Updating
On a typical network, each time a client starts up that supports dynamic updates, it will attempt to register its DNS records in the DNS server. In most cases, even when using DHCP, the client will have the same IP address and computer name each day when it starts up. This is because DHCP has a lease time and these lease times generally are configured to span over several days. For this reason, typically the only change to the DNS record when the client attempts to update its DNS record is the timestamp. For this reason, if there are a lot of clients on the network, this means that there are a lot of DNS record changes where only the time stamp has changed. All these changes need to be replicated to the DNS server that is holding that zone file which, in a large company, can result in a lot of network traffic.

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References
"MCTS 70-640 Configuring Windows Server 2008 Active Directory Second edition" pg 482
"Time to live" http://en.wikipedia.org/wiki/Time_to_...
"Understanding aging and scavenging" http://technet.microsoft.com/en-us/li...
"Set Aging and Scavenging Properties for the DNS Server" http://technet.microsoft.com/en-us/li...