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Re: Tightened DNS security question re: DNS amplification attacks.
From: Mark Andrews <Mark_Andrews () isc org>
Date: Wed, 28 Jan 2009 10:36:29 +1100

In message <200901272116.n0RLGiJA002250 () ns1 konadogs net>, Nate Itkin writes:
On Tue, Jan 27, 2009 at 03:04:19PM -0500, Matthew Huff wrote:
< ... snip ... >
dns queries to the . hint file
are still occuring and are not being denied by our servers. For example:
27-Jan-2009 15:00:22.963 queries: client view
external-in: query: . IN NS +
< ... snip ... >
since you can't put a "allow-query { none; };" in a hint zone, what can I d
to deny the query to the . zone file?

AFAIK, that's about the best you can do with the DNS configuration. You've 
mitigated the amplification value, so hopefully the perpetrator(s) will drop 
you. If you're willing to keep up with the moving targets, the next level 
is an inbound packet filter. Add to your inbound ACL:

deny udp host neq 53 any eq 53

        Which pre-supposes that os not emitting queries of
        its own.

        BCP 140 looked at this problem and concluded that sending
        REFUSED was the best general guidance that can be given.
        While BCP 140 applies to recursive servers, returning REFUSED
        to queries which are not within the namespace served by
        authoritative servers is entirely consistant with BCP 140.

Also on this topic:
Coincident with this DNS DOS, I started seeing inbound PTR queries from 
various hosts on (which are blackholed by my DNS servers). 
They receive no response, yet they persist.  Anyone have thoughts on their 
part in the scheme?

        If you don't implement BCP 38 you don't block bogus traffic.

        Unless you are using then you aren't implementing
        BCP 38 either.  If you were you wouldn't be seeing queries from

Best wishes,
Nate Itkin

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews () isc org

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