An additional level of complexity is introduced when resolvers violate the rules of the DNS protocol. A number of large ISPs have configured their DNS servers to violate rules (presumably to allow them to run on less-expensive hardware than a fully-compliant resolver), such as disobey TTLs, or indicate a domain name does not exist just because one of its name servers does not respond.

As a final level of complexity, some applications such as Web browsers also have their own DNS cache, in order to reduce use of the DNS resolver library itself, which can add extra difficulty to DNS debugging, as it obscures which data is fresh, or lies in which cache. These caches typically have very short caching times of the order of 1 minute. A notable exception is Internet Explorer; recent versions cache DNS records for 30 minutes.