[dns-operations] Bailiwick stats? Idea for mitigation...

Roy Arends roy at dnss.ec
Sun Aug 10 14:32:05 UTC 2008

On Aug 10, 2008, at 1:02 AM, Brian Dickson wrote:

> Thinking along the lines of things that rdns servers can do  
> unilaterally to improve forgery resilience...
> Do we know what typical percentage of queries a rdns box is likely  
> to receive, of sub-domains of in-cache domains (which are not  
> themselves cached), versus all other queries?
> What I'm thinking is, when such a query is seen, even without  
> checking TXID/QID mismatches, just always require two identical  
> answers at each (external, non-cached) step of the recursive  
> resolution process, using UDP.

I proposed something similar: http://www.ops.ietf.org/lists/namedroppers/namedroppers.2008/msg01024.html

> With randomized ports per query, this effectively doubles the number  
> of entropy bits, albeit at a performance hit of 2x, but only for  
> those non-cached domains underneath cached domains.
> The question is, what is the *actual* performance penalty? If the  
> client-side percentage of such queries is low, like 20%, then the 2x  
> penalty would only add 20% to the load, whilst going a long way  
> towards making such attacks infeasible.
> How infeasible? If 1x birthday success to poison a cache takes 10  
> hours, using GigE LAN-connected servers, then 2x (using randomized  
> QID and source port) should take 2^32 times as long, or roughly 11M  
> years -- unless the attacker was *really* *really* *REALLY* lucky,  
> or had some way of reverse-engineering the PNRG state.
> Now that I put this down on e-paper, this seems pretty compelling.  
> Nowhere near the performance impact of TCP, and even not that much  
> logic or state required locally...
> Thoughts?
> Preaching to the choir?

The idea mentioned in the link above was actually only a small part of  
the solution. I could not comment further on the solution at the time  
without hinting what the kaminsky bug was.

So, what we were thinking of is doing the following: When a cached  
entry is about to be overwritten with something different, do an  
additional request. The material in that additional request has to be  
equal to that 'something different' before the cached entry is  


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