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windows - DNS requests failing from computers that can ping DNS server

I have a situation where computers in some of our remote
offices from time to time lose the ability to use our DNS server (in head office) to
resolve hostnames. The offices are connected via VPN using Cisco ASA 5505 (VPNclient
config rather than Site to Site) connecting back to an ASA 5510 at head
office.



Ping to the IP address of the DNS server
works. But nslookup will get a "no response from server" message. Computers in other
locations can use DNS fine.



This is an
intermittent problem. One day/hour it works, another it doesn't. Other offices connected
in the same way work when another doesn't.



No
config changes have been made on routers around the time we see the
problem.



The DNS server that drops out for the
clients is on our private address space in the 172.16.0.0/12 network. The ASA 5505s and
the clients behind them are each in a 10.6.x.0/24 private address
space.




Each ASA 5505 has its DHCP
server configured to assign addresses and DNS config to clients behind them -these are
generally Windows XP clients, with a growing number of Windows 7. The primary DNS is our
one in the 172.16.0.0/12 space, the secondary is 8.8.8.8 to allow access to the internet
in case the VPN tunnel drops for any
reason.



Some users have reported that the
problem goes away after doing a repair connection in Windows XP. I think this could be
caused by the DNS cache being flushed as part of this - the Windows DNS cache makes the
intermittent problem look less so because it caches failed lookups as well as successful
ones. However, it is possible some other aspect of Windows is involved. Windows 7
clients have also had the same problem.



Any
pointers on deeper troubleshooting, or anyone else found this?

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