DNS Capacity issue help -- Recursive Query -- it seems some packets are dropped by DNS

Tony Finch dot at dotat.at
Tue Apr 10 09:58:37 UTC 2018

PENG, JUNAN <jp2111 at att.com> wrote:

I need to start by saying that my load testing is very unscientific,
so I can only give you a few handwaving hints...

> I did recursive query capacity test.  I used traffic generator to place
> 15K QPS traffic to DNS 1 with FQDN1 (Note, FQDN1 can't be resolve by
> DNS1, it need to forward it to DNS2 and TTL is set to 0)

In my experience, 15kqps is easy to achieve with a hot cache, but if you
are forcing the resolver to make recursive queries you'll be sacrificing
a lot of the potential performance (but I can't give you numbers on how

Set the TTL to a non-zero value to get a more realistic test.

> Thing 1.  DNS query number is larger than response number between
> traffic generator and DNS1 .  About 15% traffic are dropped by DNS1 .

Are you hammering the same qname or a small number of qnames? If so I
would expect the server to drop queries while it is recursing - look at
the documentation for max-clients-per-query.

> Thing 2. DNS recursive query number between DNS1 and DNS2 is far less
> than query number between traffic generator and DNS1

That's kind of the point of a cache :-)

f.anthony.n.finch  <dot at dotat.at>  http://dotat.at/
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