Configuring a Caching-Only Server

As mentioned earlier in the chapter, one of the roles a DNS server can play is that of a caching-only server . Caching-only servers are used to resolve recursive queries, cache the results, and return the results to the requesting client. The server is not authoritative for any zone and resolves requests from its cache or another DNS server. As a DNS server retrieves the results for name resolution requests , it caches the results in the cache.dns file.

If DNS servers did not cache results, users would see an increase in response time, as well as more network traffic. If a DNS server received a request to resolve www.bayside.net and two minutes later another client made the same request, it would have to go through the referral process twice to resolve the same name. Because DNS is used on the Internet and by Windows 2000, this name resolution scenario would be very inefficient. Instead, by caching the results the DNS server can use the resource records within the cache to respond to subsequent name resolution requests made after the initial name request.

The main advantage to implementing a caching-only server is that no traffic is generated from zone transfers. Because the server is not authoritative for a zone, nor does it maintain a copy of an existing zone, no traffic is generated. This is useful for remote office locations that are connected with slow WAN links that cannot support traffic generated by zone transfers. The main disadvantage is, when the server is first configured, the cache.dns file only contains entries for the root name servers so it will take some time to build up the cache file as requests are resolved.

Time to Live Property in Resource Caching

One important aspect of caching resource records is the Time To Live (TTL) property. Each resource record stored in the cache is assigned a TTL. This is a numeric value that determines how long the resource record can remain in the cache without requiring a refresh from the primary server. When the TTL for an object in the cache expires , the DNS server can no longer respond to queries for that hostname using the cached record. This ensures that information in the cache is fairly up to date. The default TTL for a zone is set to 3600 seconds, or 1 hour .

Installing a Caching-Only Server

When it comes to installing a caching-only server, you don't need to perform much configuration. After the DNS server service is installed using the steps outlined earlier in the chapter, verify that the root hints are configured correctly. Remember, because the server isn't authoritative for any zones, no zone information needs to be configured. You can verify the configuration of the root hints by right-clicking the DNS server within the DNS management console, clicking Properties, and selecting the Root Hints tab.

Root hints are used by DNS servers to discover other DNS servers authoritative for a domain. For example, for a DNS server to resolve a query for computer1.bayside.net , it must be able to locate the root DNS servers for the namespace. The DNS servers for the net domain can provide referrals to the DNS server authoritative for the bayside.net domain .