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DNS Specifications
Various objects and parameters in the DNS have size limits. They are listed below. Some could be easily changed, others are more fundamental.
- labels 63 octets or less
- names 255 octets or less
- TTL positive values of a signed 32 bit number.
- UDP messages 512 octets or less
For all parts of the DNS that are part of the official protocol, all comparisons between character strings (e.g., labels, domain names, etc.) are done in a case-insensitive manner.
NAME: an owner name, i.e., the name of the node to which this resource record pertains.
TYPE: two octets containing one of the RR TYPE codes.
CLASS: two octets containing one of the RR CLASS codes.
TTL: a 32 bit signed integer that specifies the time interval that the resource record may be cached before the source of the information should again be consulted. Zero values are interpreted to mean that the RR can only be used for the transaction in progress, and should not be cached. For example, SOA records are always distributed with a zero TTL to prohibit caching. Zero values can also be used for extremely volatile data.
RDLENGTH: an unsigned 16 bit integer that specifies the length in octets of the RDATA field.
RDATA: a variable length string of octets that describes the resource. The format of this information varies according to the TYPE and CLASS of the resource record.
###TYPE values
TYPE fields are used in resource records. Note that these types are a subset of QTYPEs.
TYPE: value and meaning
A 1 a host address
NS 2 an authoritative name server
CNAME 5 the canonical name for an alias
SOA 6 marks the start of a zone of authority
NULL 10 a null RR (EXPERIMENTAL)
WKS 11 a well known service description
PTR 12 a domain name pointer
HINFO 13 host information
MINFO 14 mailbox or mail list information
MX 15 mail exchange
TXT 16 text strings
###CLASS values
CLASS fields appear in resource records. The following CLASS mnemonics and values are defined:
IN 1 the Internet
CH 3 the CHAOS class