[DNSOP] UDP fragmentation vs multiple-responses and multi-qtypes

Ondřej Surý <ondrej.sury@nic.cz> Thu, 20 July 2017 16:41 UTC

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Date: Thu, 20 Jul 2017 18:41:33 +0200 (CEST)
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Subject: [DNSOP] UDP fragmentation vs multiple-responses and multi-qtypes
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multi-qtypes Security Considerations says:
>    The method documented here does not change any of the security
>    properties of the DNS protocol itself.

I don't think this statement is true.  Why?

a) DNS DDoS threats are real and there was a shift towards minimizing
   answers.  This goes in the reverse direction.  But you address this
   in both security considerations.

multiple-responses Security Considerations says:
> 
>    Additional records will make DNS responses even larger than they are
>    currently, leading to larger records that can be used in DNS
>    reflection attacks.  One could mitigate this by only serving
>    responses to EXTRA requests over TCP or when using Cookies [RFC5395],
>    although there is no easy way to signal this to a client other than
>    through the use of the truncate bit.

multi-qtypes Security Considerations says:
>    It should however be noted that this method does increase the
>    potential amplification factor when the DNS protocol is used as a
>    vector for a denial of service attack.


b) UDP fragmentations - it strongly increases the risk of UDP fragmentation
   which is strongly discouraged (SHOULD NOT) in BCP 145.

also multiple-responses Security Considerations says:

>    A malicious authoritative server could include a large number of
>    extra records (and associated DNSSEC information) and attempt to DoS
>    the recursive by making it do lots of DNSSEC validation.  However,
>    this is not considered a realistic threat; CPU for validation is
>    cheap compared to bandwidth.  This can be mitigated by allowing the
>    recursive resolver to ignore Additional records whenever it considers
>    itself under attack or its CPU resources are otherwise over-
>    committed.

It should be noted, that ECC validation is more CPU intensive than RSA, as
as such I find "CPU for validation is cheap compared to bandwidth" quite
bold claim that should come with some data.

Cheers,
--
 Ondřej Surý -- Technical Fellow
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