Disable SRTT and enable round-robin for forwarders

Stephen Wood stephen at seomoz.org
Fri Mar 22 22:45:22 UTC 2013

I have a question about how smooth round-trip-time (srtt) works. If I
declare 5 separate DNS servers to forward queries to and one of them is
much faster than the other one, naturally the bulk of all queries will go
towards that host because of its low curve, correct?

However, what happens when that "fast" host  timeouts during queries? Will
bind adjust the srtt appropriately and begin sending it less requests by
balancing them across the the slower dns servers?

I am asking because I am seeing elevations of failures during times of high
DNS load. One of the servers we forward to is Google, which is much faster
than all of the other ones, but Google enforces a query limit of about 500
qps from what I can tell from testing. I have a feeling -- though I could
certainly be wrong -- that we're breaching that limit and getting timeouts
but it's taking a moment for Bind to adjust the srtt slow down the traffic
to that DNS.

And a followup question, is there a simple configuration change I can do to
tell bind to ignore srtt values and simply round-robin across a list of DNS
that I provide?

Thank you in advance for your help.

Stephen Wood
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