A DNS resource record (RR) is a construct within which DNS servers store data about domain names. RRs contain a particular bit of information about a domain name, and each RR is matched to a domain name. A particular domain name can have more than one RR, but each RR is associated with only one domain name. The particular RR returned by a DNS server depends on what kind of request it receives. The simplest type of RR is a type A RR: This resource record holds a 32-bit IP address. When a node requests resolution of an FQDN, the server returns this value. Things can get more complicated, however. A domain name may be an alias for some other node. For example, the FQDN www.Internet-Standard.com might actually be an alias. The CNAME RR (for canonical name) provides the FQDN of the domain name to which the alias refers. Aliases can be used when a service is associated with a particular host name in a domain, but the service is actually provided by some other host. For example, most Web servers are hosted on FQDNs that look like this: www.mydomain.com. The owners of that domain might want to host their Web content on a host with a different name, perhaps at a hosting service. The CNAME RR would indicate the FQDN of the host on which the service is actually being hosted. Likewise, the owners of mydomain.com might also own mydomain.net and mydomain.org; these domains might also be aliased to the actual Web server domain. Several other RRs are defined for various different purposes; most of these are not immediately relevant to email. One RR type, however, is the MX (mail exchange) resource record. An MX record is quite different from an A record, mostly because the MX record includes a preference field. When more than one MX record is applicable to a particular domain, the preference field values of the records are used to determine the order in which the sender uses the different records. Each domain can have one or more MX RRs. Each MX RR contains two pieces of information: a preference (a 16-bit value) and a domain name (this should be a canonical domain name). The MX record indicates a host that handles email sent to the original domain. Thus, while you can address email to me at [email protected], the loshin.com domain has at least one MX RR directing that email addressed to the loshin.com domain actually be sent to some other mail server maintained by the provider that manages my domain. A domain can have more than one MX RR, and that’s where the preference value comes into play. When more than one MX RR is returned, the requesting node (an SMTP server) sorts the MX RRs in ascending order based on preference. An SMTP server must attempt to deliver a message to a host identified in an MX RR with a low-valued preference before attempting delivery to a node with a higher-valued preference.