Windows Server 2003 has a new DNS feature called client screening, which allows the server to determine and remember if a given DNS server is accessible. If the server can't reach a given DNS server, it's marked as unreachable in its lookup cache and will not be used for further lookups until either the system is restarted or the cache expires or is manually cleared.
There are pros and cons to this approach. On the plus side, since a DNS server that's marked as unreachable won't be used in future DNS lookups, future lookups may take less time. On the downside, this can create problems for servers that are multi-homed (i.e., have more than one network adapter). A DNS server that's not reachable from one interface may be reachable from another -- and might be wrongly marked as unreachable.
Normally this should not create a problem. But if it does -- for instance, if you have a computer with multiple interfaces connected to the Internet -- there is a way you can disable it. Here's what to do:
Open the Registry on the affected computer and navigate to the value
Add a new DWORD value named ScreenUnreachableServers and set it to 0, and then stop and restart the DNS Client Service.
Note: You should not use this feature unless you are experiencing serious problems, such as lookup failures, and can't work around the problem
Why does this happen on a system-wide versus a per-adapter basis? My guess is that there's no way for the system to really know about changes to the topology of the network outside. If a DNS server goes unreachable for one adapter, there's no guarantee that it will not be unreachable for another as well, and so it's excluded.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators. He is also the author of the book Windows Server Undocumented Solutions.
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This was first published in May 2006