DNS Master File Format
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DNS servers answer queries from clients by sending reply messages containing resource records. We have already seen in this section the binary message formats used to encode these resource records. These message formats are great for transmitted messages, because they are compact and efficient, and computers have no problem reading fields very quickly and knowing how to interpret a particular string of ones and zeroes.
Humans, on the other hand, don't deal well with cryptic codes in binary. Before a resource record can be provided by a server, it is necessary for a human administrator to tell the server what those records are and what information they contain. To make this job easier, DNS includes a special text representation for zones and resource records. Administrators edit special master files that describe the zone and the records it contains; these files are then read into memory by the server's DNS software, and converted into binary form for responding to client requests. This is described in more detail in the overview topic on resource records.
Each master file consists of a simple flat text file that can be created with any sort of text editor. Each file contains a number of lines expressed using a simple set of syntax rules that describe a zone and the records within it. The basic syntactic rules for DNS master files are specified in RFC 1035, section 5.1. Certain DNS implementations use their own variations on the syntax in the standard, though they are all pretty similar.
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