Some DNS Subtleties

Mail eXchange

the DNS provides the MX type of RR to discover where email is to be delivered. An MX RR specifies a primary mailhost, and lesser preferential hosts where mail for a specified domain is be delivered. For For example, ironbark has:

ironbark  IN  MX  10  ironbark
          IN  MX  20  redgum
          IN  MX  40  sheoak

Reverse lookups

a special domain (in-addr.arpa) and address format is used to map IP addresses to domain names, thus:

60.21.144.149.in-addr.arpa

This is called a PTR RR. Performing reverse lookups is much more difficult than normal forward address lookups.
 

CNAME

Often a host may be known by several names: names other than the official host name are called aliases, and a CNAME RR maps an alias name to a host's "real" name.
 

HINFO

describes some basic information about the type of CPU and the OS it is running. Rarely kept up-to-date.