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I'm writing a DNS resolver and am building a spreadsheet of when a DNS record should be cached, and not be cached... but it's harder than it first looks (to me at least).

Is there consolidated test cases for DNS caching that I can refer to? All the RFCs seem to have many revisions, and the requirements on caching are inconsistent at best. This is further complicated by the re-definitions of the SOA record.

An improperly cached DNS record can mean that I'm poisoning the cache with invalid records.

The most relevant example I can think of is where a fake referral is generated for an NS, A, or MX.

  1. When should validation of that referral be done?
  2. How is that referral validated?
  3. Should I traverse from the root servers again?
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See http://en.wikipedia.org/wiki/DNS_cache_poisoning and the many entries linked from there.

A short answer would be that you should only cache the answer when it comes from a server that you would normally trust for the the relevant domain.

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    Aye, there is the rub. I'm having trouble defining what 'normal' is. What are the conditions in which a record should be cached? – Christopher Dec 2 '10 at 4:08

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