I have a small home network with a Windows 2012 R2 Essentials DC that is also acting as a DNS Server. All the computers connect to this as primary. Physical machine. I also have a secondary VM that is Windows 2012 R2 Server that is a secondary DC and also secondary Domain Controller. If the primary is offline, all lookups on the net are really slow. Even if I reboot the computer, nslookup still shows the primary as the one being used. I get:

nslookup DNS request timed out

timeout ws 2 seconds

Default Server: Unknown


Secondary say is Any idea what's happening or is this normal?



EDiT: Even when I change the scope and set the workstations to look at the secondary DNS first, still have the same slowness, just nslookup responds quickly and correctly.

migrated from serverfault.com Aug 12 '16 at 22:31

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This is normal. Both the "slow" lookups (have to wait for a timeout) and nslookup vs system resolution behaving differently. Nslookup is a DNS server troubleshooting tool.

When you put in more than one IP address for DNS server on the client side of a Windows machine, it randomly chooses which it will use first and which to fall back on. It re-randomizes the order periodically (15 minutes usually). So when either DNS server is not responding, you'll have some clients "slow" and some fine.

Linux / Unix behaves differently, it always sends requests in the order you specify them. So if all your Linux machines point to your main DNS first, all of them will get "slow" when your main DNS is down.

Macs behave similarly to Linux, as it's based on FreeBSD (a Unix derivative, similar to Linux).

And, just for completeness, fall back is near instantaneous when the server that is supposed to be running DNS is up but DNS service is not running. Clients will get a port-unreachable ICMP response.

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