If you believe Microsoft’s description for the DNS Client service, it’s needed to resolve and cache DNS names. They emphasize that if the service is stopped, the computer will not be able to resolve DNS names and locate Active Directory domain controllers. Well, I can’t say anything about the Active Directory domain controllers, but as far as the DNS resolutions go, this descriptions appears to be plain wrong.
I first found out about this a while after installing a Windows XP machine, with SP1, if I recall that correctly (it doesn’t matter). I have my own DNS server running on a Linux machine, using PowerDNS and working as a “resolution proxy”. The same server machine also has a web proxy, a Socks proxy and a mail server setup, which I use from the client. Therefore, browsing the web, reading my email, using Messenger and so forth, I never noticed anything out of the ordinary, because all these services never required my client machine to do any DNS lookups itself.
The problems started when I tried to access news servers from the client machine. No proxy was used for that, so the client needed to resolve DNS names itself, and this behaved highly unreliably. One time it would work, the next time it wouldn’t. Then maybe it would work for quite a while, allowing me to read a newsgroup. Then, when I wanted to post, it wasn’t working once again. With a network analyzer and the log file of my own DNS server, I was able to find out what the Windows machine was doing: it was actually contacting the DNS server only every 15th time or so (not really a reproducible value). In that single instance, everything would work as expected. The client was reliably misbehaving in this way when I repeatedly executed nslookup and ping commands from the command line. After a long while of fiddling around, I found out about the DNS Client service. After an even longer time, I found that once I switched off that service, everything was completely normal, with the client contacting the DNS server for every single lookup.
So, does that service resolve DNS names? Well, sometimes. Does it cache DNS name resolutions? No, I’ve never seen that. I decided to blog about this because I wasn’t able to find any comprehensive information on this topic on the net. I did actually find a page at The Elder Geek where the service is described to behave in a similar way to what I’ve seen, so I know I’m not the only one. To this day I have no idea why that service is misbehaving for me. After I had initially thought it had something to do with my particular setup, I’ve since had reports from three different friends who have seen the same problem in their network setups, which are all distinctly different from mine. In each of these cases I was able to help them by suggesting they switch off the DNS Client service. The reason remains a mystery though…