A (Address Records) type Resource Record

An “A” (Address) type Resource Record is an “IPv4 host record” and it is the most common type. An “A” type is simply a static mapping of a hostname to an IPv4 address.

Address Resource Records are placed in a zone file stored on an authoritative nameserver so that when the nameserver is queried with a hostname, it can answer with the correct IP address.

Address resource records provide the base hostname to IP Address resolution service provided by a DNS nameserver.
An Internet Address record is configured the same way for all hosts; however, additional records may be necessary if the host being resolved provides additional services such as DNS or E-mail.

 
Sender Policy Framework (SPF), DomainKeys (DK), and DomainKeys Identified E-mail (DKIM)

http://www.linuxnix.com/2012/02/domain-serverdns-evolution.html

http://www.linuxnix.com/2008/06/dns-server-syntax-checking.html
http://www.linuxnix.com/2009/11/how-to-log-dns-server-activity.html

http://www.inetdaemon.com/tutorials/internet/dns/index.shtml

http://www.inetdaemon.com/tutorials/internet/dns/servers/caching.shtml
Linux
The client resolver is called ‘nscd’ (name service cache daemon). To wipe the cache, restart the daemon service with the following command:
/etc/rc.d/init.d/nscd restart

http://www.linuxnix.com/2009/05/dns-troubleshooting-in-linux.html

http://www.linuxnix.com/2009/11/how-to-log-dns-server-activity.html

http://www.linuxnix.com/2009/11/surendras-dig-notes.html
http://www.linuxnix.com/2009/05/dns-troubleshooting-in-linux.html
http://www.linuxnix.com/2013/11/tricky-question-what-protocol-tcp-or-udp-dns-server-uses-when-a-client-queries.html

https://kb.iu.edu/d/aauw