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When a DNS resolver queries for a domain, the response contains additional records. The question is: "is the DNS resolver required to follow those instructions, or are they optional"?

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2 Answers 2

The resolver is free to discard those records and check them itself, and this is sometimes useful for security. Google do this.

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How would this work in the case the additional records is telling you the server is valid only for AAAA records? If a resolver ignores it and queries that server it could fail on a A record, as that server only support AAAA –  GeorgeU Nov 19 '10 at 16:07
    
Google only discard records that they can verify themselves - you can consider doing that. –  fahadsadah Nov 19 '10 at 16:26
    
well struggling with a DNS vendor, so trying to get an understanding of what the DNS resolvers out there do.... –  GeorgeU Nov 19 '10 at 18:12
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@GeorgeU: given the nature of the Net, resolvers do what their developers decided or gave the user the ability to configure. The official RFC contains a section "5.4.1. Ranking data" that may be of interest (faqs.org/rfcs/rfc2181.html). –  Zayne S Halsall Nov 20 '10 at 9:21
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Records in the Additional Section are completely optional, and these days generally ignored for the most part.

In particular, several of the weaknesses in the DNS protocol were caused by giving too much credence to those records.

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