For When You Can't Have The Real Thing
[ start | index | login ]
start > Windows > Client DNS failover behavior

Client DNS failover behavior

Created by dave. Last edited by dave, 31 days ago. Viewed 46 times. #1
[edit] [rdf]
labels
attachments
(2018-11-15)

How do Windows clients deal with having multiple DNS servers configured?

With one server:

Seconds since startAction
0Query DNS server
1If no answer, query DNS server
2If no answer, query DNS server
4If no answer, query DNS server
8If no answer, query DNS server
10If no answer, give up

A negative answer will count as an answer and stop the query at that point.

With two servers, A and B:

Seconds since startAction
0Query DNS server A
1If no answer, query DNS server B
2If no answer, query DNS server B
4If no answer, query both DNS servers
8If no answer, query both DNS servers
10If no answer, give up

With three servers, A and B and C:

Seconds since startAction
0Query DNS server A
1If no answer, query DNS server B
2If no answer, query DNS server C
4If no answer, query all DNS servers
8If no answer, query all DNS servers
10If no answer, give up

Takeaways:

  • The computer always queries the first DNS server in the list first. This means that our practice of putting the internal DNS server in clients (or dhcp scopes) first, with google as a fall-back, is valid -- google should only be queried if the internal server doesn't respond.
  • No single DNS resolution query should block for more than 10 seconds. This does not prevent calling programs from merely repeating their query, of course.
(>>Source which claims to be valid for Win7 through Win10. Article notes that if a computer has multiple interfaces defined that have their own DNS servers defined, resolution will be somewhat different and you should look at >>this article instead.
no comments | post comment
This is a collection of techical information, much of it learned the hard way. Consider it a lab book or a /info directory. I doubt much of it will be of use to anyone else.

Useful:


snipsnap.org | Copyright 2000-2002 Matthias L. Jugel and Stephan J. Schmidt