In experimenting with the program I described yesterday, I got to fiddling with the DNS resolver cache, called dnscache. Briefly, dnscache saves the results from recent DNS queries so that it doesn’t have to keep querying the DNS server. Considering that a DNS query can take 100 milliseconds or more to resolve, this can save considerable time. For example, for your browser to load this Web page, it has to make many different requests to my server: one for the base page, one for the stylesheet, one for each image, etc. It wouldn’t be uncommon to require a dozen separate requests to get all the resources that make up the page. If each resource required a separate DNS request, it would take more than a second just for DNS!

I got to wondering just how large the DNS cache is. A little bit of searching brings up any number of pages claiming that you can “speed up your connection” by tweaking the DNS resolver cache parameters. Specifically, they talk about changing registry keys for the cache hash table size, maximum time to live, etc. There’s even a Microsoft TechNet article describing these parameters for Windows Server 2003 (and, by extension, Windows XP). It’s interesting to note that the information on most of the pages claiming to speed things up conflicts rather badly with the information in the TechNet article.

After reading the tweaks and the TechNet article, I figured I’d give it a shot. I fired up the Registry Editor, made the changes, and … is it working? How can I tell? I tried browsing a few Web sites, but I couldn’t see any difference.

A little more searching and I found the command ipconfig /displaydns. This writes the contents of the DNS resolver cache to the console. A little work with the FIND utility, and I was able to count the number of entries in the cache. 34 on my Windows XP box. Interesting, considering that I set the CacheHashTableSize registry entry to over 7,000. I fiddled and tweaked, restarted the DNS Client service, flushed the cache, rebooted my computer, faced Redmond and cursed, and generally tried everything I could think of. No matter what settings I used, I always ended up with between 30 and 40 entries in my DNS cache.

On my Windows Server 2008 machine at the office, I always got between 270 and 300 entries, no matter what I tried.

So that leaves me with the following possibilities:

1. It’s not possible to change the size of the DNS resolver cache in Windows XP or Windows Server 2008.
2. It is possible, but the documentation is wrong.
3. The documentation is correct as far as it goes, but it’s incomplete.
4. The documentation is correct and complete, but I’m too dumb to make sense of it.
5. The documented registry entries actually changed the size of the cache, but ipconfig isn’t showing me all the entries that are in the cache.

At this point, all possibilities seem almost equally likely. I could do some indirect testing based on the amount of time it takes to resolve a series of DNS requests, but even that would be inconclusive. There are no documented API calls that allow me to examine the DNS cache or its size. (And the undocumented ones aren’t described well enough to be worth checking out.) My only means of seeing what’s in the cache is the ipconfig tool.

So I ask: does anybody know how to change the size of the Windows DNS resolver cache and prove that those changes actually work? Do I have to restart the DNS Client service? Reboot the machine? Set some super magic registry entry?

Any information greatly appreciated.