Common DNS Terms
■ A Record (Address Record): this type of
record directly maps a name to an IP address.
Originally, no two A Records were supposed
to point to the same IP address. (This is no
longer practical, but is considered “best
practice” where appropriate.)
■ Authoritative: a server is considered
authoritative when it is hosting the domain
in question itself rather than querying
another server for the information. A server
is considered authoritative by domain; it’s
not a boolean server setting like with DHCP.
The same server can be authoritative for one
domain, and not for another.
■ BIND (Berkeley Internet Name Domain):
the most common DNS server on the Internet.
■ Caching (or Namecaching): locally
stored copy of name resolution from an
authoritative DNS server. The caching
duration is based on the TTL settings from
the authoritative server (see TTL below).
■ CNAME (Canonical Name Record): this
creates an alias to another DNS entry that
inherits the properties of the original.
■ Forward Zone: a “zone” is used to define
the section of DNS space where a server is
responsible for mapping names to IP addresses.
■ Reverse Zone: a DNS server also can supply
reverse lookups, mapping names to queried
IP addresses. This often is used for security
to verify DNS information.
■ FQDN (Fully Qualified Domain Name):
this is the entire DNS name, including a
period at the end.
■ MX Record (Mail Exchange Record):
this specifies a mail route for a particular
domain. Multiple MX Records are possible
(and recommended!) with priority levels.
■ NS Record: declares what server serves a given
zone. This is where the server would declare
itself authoritative for a particular zone.
■ PTR Record (Pointer Record): a PTR record
often is called the reverse record, and it
associates an IP address with a domain name.
■ Propagation: the period of time between
when a DNS change is made on the
authoritative server and the time all servers
on the Internet have current information.
This propagation time can be several hours
or several days depending on the TTL
settings for a particular record.
■ Root Server: there currently are 13 root
servers on the Internet, which host the
top-level domains. Through very complex
routing and redundancy, these servers are
all over the globe and are placed with fault
tolerance in mind.
■ SOA Record (Start of Authority Record):
the first record in a zone file, containing
information about the zone itself, including
whether or not the server is authoritative.
■ SRV Record (Service Record): provides
information about what services are
available for a domain.
■ Top-Level Domain: any zone hosted by
the 13 root servers. These are domains like
com, edu, org, gov and country codes like
us, ca and uk.
■ TTL (Time To Live): this is a number set
by the authoritative server for a domain
that tells DNS servers how long to cache
information before querying again.