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The transaction ID for DNS queries can take values from 1 to 65,536 and my computer pseudorandomly chooses one for each DNS request. If I sents 1,024 false replies per request, how many requests should I trigger to compromise the DNS cache with probability 99%? or as close tot hat as I can get. Thanks

I'm getting a result of .6 requests which doesn't seem right to me. Feel as though it should be around 30

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your calculation seems to be simplified. According to the formula given in RFC 5452, the number of fake packets 'F' that are candidates to be accepted is given by :

P_s = D * R * W / ( N * P * I )

where I is the number distinct IDs available (which is 65536 in your case), P is the number of ports used, N is the number of authoritative nameservers for a domain, R is the number of packets sent per second by the attacker, W is the window of opportunity, in seconds and D is the average number of identical outstanding queries of a resolver (typically 1, see Section 5).

If D = 1 (typical value according to the RFC), W = 0.1 (typical value according to the RFC), N = 2.5 (average value according to the RFC), P = 1 (assume only one port is used) and I = 65536 (provided in your question). Then P_s can be simplified into :

P_s = R / 1638400

Thus, plugging in the necessary values, for P_s to be 0.99, R should be greater than 1622016 packets/s. Since we used W = 0.1 and you send 1024 false replies per request, then you need about 1622016 * 0.1 / 1024 = 15.84 attempts.

(Hope my calculation is correct)

  • The final calculation comes out to 158.4 attempts not 15.84 – s4ndhyac Mar 1 at 6:14

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