I was getting the following warning quite often from DPM (both 2007 and 2010 versions).
Description: Unable to contact DPM protection agent.
The protection agent operation on <computer> failed because the service did not respond.
This was driving me crazy because everything looked to be running OK on both ends and it would eventually go away on its own. However one day right after receiving one of these warnings, I decided to log into the DPM server and see if it could see the problem computer. Sure enough, a ping failed. I also noticed that the ping command didn’t even return an IP address. Next I did a nslookup, which of course worked. So how can ping not find an IP address but nslookup can? I wish someone would enlighten me. However I know it has something to do with the DNS cache, and specifically the DNS Client service.
Restarting the DNS Client service made the ping work, and it also fixed the DPM warning. I got so annoyed by the warnings that I decided to disable the DNS Client service and I haven’t had any issues since…
Note I don’t believe this is a bug in DPM. In fact, since finding this I have also come across the same issue with Windows 7 and Windows Server 2008 (the DPM server is 2008 R2 btw). I have not been able to track down the source of the issue, but is seems like the DNS cache gets corrupted or something. I have even debated disabling this service on our SMB network, but I am afraid of the impact to our DNS server.