[Pdns-users] DDoS attack with random A requests causes SQL backend overload

Mark Moseley moseleymark at gmail.com
Fri Jul 16 21:36:30 UTC 2021

On Fri, Jul 16, 2021 at 12:21 PM Peter van Dijk via Pdns-users <
pdns-users at mailman.powerdns.com> wrote:

> On Fri, 2021-07-16 at 12:08 +0200, Thomas Mieslinger via Pdns-users
> wrote:
> > Suggestions from older threads (Klaus Darrilon):
> > - Put that zone in a more efficent Backend (he suggested lmdb)
> Good idea.
> > - Put that zone in a more efficent Software (he suggested nsd) and use
> > dnsdist to route the traffic to the alternate Software
> Also a good idea.

Not to mention, you could use dnsdist (fronting a localhost pdns on an
alternate port, bound to to match that base domain name and
apply a rate limit against it sending queries for that domain to the

It's a couple of extra lines of Lua. I've not actually used the rate
limiting so I don't have a handy example but it'd be something like:

-- Match on .mydomain.com and rate limit it
addAction( RegexRule( "\\.mydomain\\.com$" ), QPSAction(
your_desired_rate_limit ) )
-- Fall through for everything else
addAction( AllRule(), PoolAction( "your_backend_pool" ) )

Lua regexes are a little weird. Definitely note the double escapes in the
regex. It doesn't treat the period as an escaped period in RegexRule if you
only have one "\" (and you'll want to tear out your hair when it matches
itsmydomain.com and blahblahmydomain.com).

I'm assuming that the domain under attack is one that you care about, vs a
purely garbage domain, in which case you could just block it at the dnsdist
level too.

> Very old suggestion:
> > - Use a firewall uint32 match to lock out queries to the attacked zone.
> Should work, bit more work to manage.
> > Crazy idea:
> > - enable DNSSec on that zone
> > - setup pdns recursor or similar add delegate the zone to it
> > - pdns-recursor should now be able to efficiently calculate the
> > NXDOMAINs based on NSEC/NSEC3 information
> Recursor can do that, but it cannot serve the zone to the world. It is
> not an Authoritative server.
> So, sadly, this suggestion does not work.
> Kind regards,
> --
> Peter van Dijk
> PowerDNS.COM BV - https://www.powerdns.com/
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> Pdns-users at mailman.powerdns.com
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