June 2018
Intermediate to advanced
368 pages
11h 1m
English
Content preview from Managing Mission - Critical Domains and DNS
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EDNS and large responses
EDNS stands for "extension mechanisms" for DNS, and it specifies methods to enable additional capabilities that were not present in the original DNS specification.
Originally, nameservers expected responses to fit within a 512-byte message, but, as DNS evolved, there became situations where this limit was exceeded. EDNSo can specify a response limit larger than the default.
The TC flag is set when the response is either larger than this EDNS0 receive buffer, or, in the absence of EDNS0, greater than the default of 512 bytes. This signals the client/resolver to retry the query over TCP.
For example, it is possible in this context to have "too much redundancy" in your nameserver delegation by simply adding so many nameservers ...
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