[Pdns-announce] Further DoS guidance, packages and patches available

bert hubert bert.hubert at netherlabs.nl
Thu Apr 3 08:17:43 UTC 2014

Hi everybody,

Sadly, further DoS attacks are plaguing the world of DNS, which is bad both
for the targets of those DoS attacks, but also for us DNS operators that
help originate them.  This post has guidance on how to make sure your
PowerDNS Recursor mitigates the current attacks.

If you are under attack and need help, we're there for you, and feel free to
contact us on powerdns.support at powerdns.com.

 * The attack

A current pattern of attacks is to register a domain with lots of
'nameservers', and then get botnets to create queries for those nameservers.
Resolvers around the world then barage these 'nameservers' with queries. And
of course, these nameservers aren't nameservers, they simply are targets of
a DoS attack.

By crafting things just so, this creates a powerful packet amplifier, with
one botnet packet turning into many many packets to the targets of the

Some further details are on

 * What we can do about it

Although PowerDNS already checks if a server might be down, and will limit
how often it queries it, this limitation does not stand up to the random
nature of these attacks.

DoS victim Paulo Anes has contributed a filter that can agressively filter
out queries to dead servers, it is described in https://github.com/PowerDNS/pdns/pull/1300
Thank you Paulo!

We've since deployed this filter in many places, and it works VERY well
against the current attacks. We recommend setting server-down-max-fails=32
for most servers, and server-down-max-fails=16 if under heavy attack.

Additionally, please make sure to read
for how to properly size your operating system for dealing with incoming

 * How can I get this filter?

The filter is not yet part of a released PowerDNS Recursor version. However,
the following PowerDNS Recursor packages/sources are of production quality,
and will in any case fare better than 3.5.3:


https://www.monshouwer.eu/download/3rd_party/pdns-recursor/git/ has
RHEL/CentOS native packages as well. 

 * When will PowerDNS do a release version with this filter?


 * Further questions

If you have further questions, or are currently under attack and need help,
please feel free to contact powerdns.support at powerdns.com.

Kind regards,


PowerDNS Website: http://www.powerdns.com/
Contact us by phone on +31-15-7850372

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