Introduced in Windows 2000, Dynamic DNS brought a process with it called DNS scavenging where it would not be needed in a perfect world -- but who lives in a perfect world? So before you spend time reading the rest of the article, let's see if it applies to you.
Have you ever pinged a machine by name, got a reply, but when you attempted to connect to it, you connected to a different machine name or could not connect at all? If you nodded your head in agreement or mumbled something about this happening to you, then this article may shed some light.
Still reading? Good. First, let me establish my bias in saying that all this information pertains to Active Directory Integrated Zones. That being said, let's establish some definitions before we continue.
* A -- This record maps the name of the machine (Host) to the IP address.
* PTR -- This record maps the IP address to the Host name.
Why scavenge? (I'm a pack rat!)
There are two parts of DDNS that we need to understand before we answer the question: DNS and DHCP.
This is where scavenging comes in. Scavenging will take stale records and delete them if they're beyond a set age. All records have an age. However, the age of a record is not considered until scavenging is turned on. Once scavenging is turned on, DNS does not calculate how old the record was prior to scavenging being enabled. (For more information on various triggers of the StartScavenging time frame, refer to the Microsoft DNS whitepaper.)
How do I use this scavenging stuff?
There are three different intervals that you need to understand before setting this up. These definitions come directly from the DNS GUI. Now that you're thoroughly confused, let's move on.
Did the italics help? If you're like me, your brain is twitching from all the circular circle wording words. To understand this a little better (without needing the mental capacity to solve a Rubik's Cube in two minutes), let's break d
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own what the definitions really mean.
We'll put all this together in an example that makes sense. In this scenario, the DNSClient does not re-register during the Refresh Interval period. Keep in mind, we are using a default of seven days.
If a client re-registers a record, then the No-refresh Interval would start all over again. In the scenario above, with the default settings of seven days, a record would have to be greater than 14 days old before DDNS would scavenge it. This might work if your DHCP lease times are eight days (default). Otherwise, you may need to set the intervals closer to your DHCP lease times. Also, keep in mind the Scavenging Period only runs on the interval specified, which is also by default of seven days.
Scavenging jobs will use processor time. However, the scavenging process is a low-priority thread of the DNS service. This is great in making sure that scavenging does not utilize all the processing capacity, but it is horrible if your DNS servers are heavily utilized. As a low-priority thread, on a highly utilized DNS server, there's a probability that the scavenging thread may never run. Also, if it attempts to run the scavenging process during a time when the DNS server is highly utilized, it will miss the scheduled interval. It will not attempt to start running over and over, but instead will wait until the next scheduled interval (remember the default is seven days). As of writing this article, I haven't found a setting that can be adjusted to change what hour the scavenging process starts.
For the advanced pack rat
As I alluded to earlier, the Scavenging Period setting only applies to an individual DNS server. Unlike the other settings, which are replicated by AD, this setting is specific to the DNS server in question. With this in mind, not enabling this setting means that no servers are scavenging records. Aging of records is taking place (No-refresh, Refresh), but nothing else is going on. This is good for a variety of reasons. First of all, you don't necessarily want ALL your DNS servers to scavenge. You only need one to do it. It'll replicate the record deletions to the other DNS servers. This also allows for some other configuration options.
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