Doublt about DNS Client...

Chris Buxton cbuxton at
Mon Mar 10 16:34:17 UTC 2008

The RFC's don't say, if I recall correctly.

Some stub resolvers (the most common DNS clients) try again for up to  
75-81 seconds. Some retry only a few times, maybe as few as 2 queries  
total. The only constant (or near constant) seems to be that the delay  
between the first query and the second is 5 seconds.

Some versions of nslookup time out after 2 seconds.

Of course, the actual application that requests a name lookup from the  
stub resolver often will only wait 5 seconds. Or it might wait until  
the stub resolver gives up. It depends on the application.

However, there's a difference between not getting an answer and  
getting a negative answer. In the case of a negative answer, the name  
server responded with a final answer. Thus, the client (stub resolver,  
nslookup, dig, whatever) stops right there - it does not try a  
different name server to try to get a positive answer. A negative  
answer is an answer.

Chris Buxton
Professional Services
Men & Mice

On Mar 10, 2008, at 2:13 AM, Raghavendra wrote:

> Hi All,
> Can anybody tell me what is the maximum polling time and polling  
> retries restricted by DNS client RFC?
> I saw in some dns client codes they poll after every 5 seconds and  
> for 4 times in worst case this will turn out to be 20 seconds  
> waiting time and in best case 5 seconds.
> Where as in windows nslookup it takes less than a seconds to reply  
> for dns both positive and negative response cases.
> Just want to know is these values left to coder's choice? or RFC has  
> restriction.
> -Raghu

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