DNS compression is actually described in section 4.1.4 of IETF RFC 1035: In order to reduce the size of messages, the domain system utilizes a compression

DNS compression is actually described in section 4.1.4 of IETF RFC 1035: In order to reduce the size of messages, the domain system utilizes a compression scheme which eliminates the repetition of domain names in a message. In this scheme, an entire domain name or a list of labels at the end of a domain name is replaced with a pointer to a prior occurance of the same name.