BIND srtt algorithm not working as expected

Tony Finch dot at
Wed May 16 18:42:15 UTC 2018

Paul Roberts <paul at> wrote:
> This seems to imply on slide 16 that with lower query rates, BIND 9.8
> has a habit of sending fairly significant volumes to DNS servers with
> higher rtts. I am wondering if this is still the case in BIND 9.10 or
> 9.11 and whether there is anything that can be done about it?

The short answer is, 9.9 and later should be a lot better than 9.8.

There are a couple of obviously relevant entries in the CHANGES file:

Before the 9.6.0 release:

2423.   [security]      Randomize server selection on queries, so as to
                        make forgery a little more difficult.  Instead of
                        always preferring the server with the lowest RTT,
                        pick a server with RTT within the same 128
                        millisecond band.  [RT #18441]

Before the 9.9.0 release:

3024.   [func]          RTT Banding removed due to minor security increase
                        but major impact on resolver latency. [RT #23310]

f.anthony.n.finch  <dot at>
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