This associates an alias for a host with its canonical hostname. This type of record is used to associate names in the in- addr.arpa domain with hostnames. This is used for reverse map- ping of IP addresses to hostnames. The hostname given must be the canonical hostname. MX This RR announces a mail exchanger for a domain.
Every mail exchanger has an integer preference associated with it.
A mail transport agent who desires to deliver mail to domain will try all hosts who have an MX record for this domain until it succeeds. The one with the lowest preference value is tried first, then the others in order of increasing preference value. This record provides information on the system's hardware and software. The hardware field identifies the hardware used by this host. There are special conventions to specify this.