Internet Architecture Board - IAB Documents: "A brief primer on DNS wildcards
The DNS 'wildcard' mechanism has been part of the DNS protocol since the original specifications were written twenty years ago, but the capabilities and limitations of wildcards are sufficiently tricky that discussions of both the protocol details of precisely how wildcards should be implemented and the operational details of how wildcards should or should not be used continue to the present day. This section attempts to explain the essential details of how wildcards work, but readers should refer to the DNS specifications ([RFC 1034] et sequentia) for the full details.
In essence, DNS wildcards are rules which enable an authoritative name server to synthesize DNS resource records on the fly. The basic mechanism is quite simple, the complexity is in the details and implications.
The most basic and by far the most common operation in the DNS protocols is a simple query for all resource records matching a given query name, query class, and query type. Assuming (for simplicity) that all the software and networks involved are working correctly, such a query will produce one of three possible results:
success
If the system finds a match for all three parameters, it returns the matching set of resource records;
no"
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