global forwarders - current BIND9 behaviour documentation

Ben Croswell ben.croswell at
Wed Jul 25 20:54:55 UTC 2012

All forwarders in the list will tried at least some. Every time the fastest
forwarder responds the srtt of the remaining forwarders are decayed.
Eventually they will be lower and get tried. If they are slower than the
original fastest their srtt go back up and the original will be used again.
It's the method for retrying a forwarder after it was set high due to a
timeout etc.

-Ben Croswell
On Jul 25, 2012 2:36 PM, "ip admin" <ipmanx at> wrote:

> Hi,
> anybody there who can provide a definitive answer on the current BIND 9.7
> (or higher) global forwarder behaviour?
> I did find the following info before on using multiple forwarders:
> My expectation based on that is that the fastest responding forwarder will
> basically always be used until a timeout may occur, i.e. when specifying
> three forwarders one will be the prefered one based on SRTT and the others
> are only used if the prefered one goes down.
> First of all when doing 'rndc dumpdb -all' I cannot find my forwarders' IP
> addresses in the named_dump.db at all as explained in the posting above
> (BIND 9.7.3-P3 on Linux), so I cannot verify the SRTTs. 'rndc stats' /
> named.stats does not show any info on the forwarders as well.
> Also by doing a tcpdump I can see that all three forwarders I have
> specified are constantly used. However it is not a real round-robin but
> roughly a 3:2:1 ratio instead (i.e. one receives approx 3 times the number
> of queries compared to the third one, the other one receives 2 times the
> number of queries compared to the 3rd one). In fact the 3:2:1 distribution
> reflects the response time I can manually determine by running dig against
> all forwarders - the one which responds quickest gets the most queries and
> the one which is slowest gets the fewest queries.
> My server receives quite a few queries (approx 10.000 within a minute).
> Any idea if the DNS-Server will send every 10th query or so the slower
> forwarders?
> I also tried to set the logging level to debug 10 for category resolver
> but no luck at all in finding out which forwarder is used (and why).
> So . . . if somebody could explain what the current behaviour is supposed
> to be that would be helpful.
> Regards
>  Tom
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