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There are a considerable number of record classes available for DNS servers.
Many of them have specialized purposes - and you may never need them. On
the other hand, there are a few classes that are not only common - but critical
- and these classes will be discussed below.
The primary classes used by all DNS implementations are A and PTR.
- A is for "Address" - records of this class store a FQDN and an
associated IP. When a resolver or caching DNS server looks for "www.amfes.com"
- these are the kind of records searched.
- PTR is for "Pointer", the reverse of the A type - records of
this class store an IP and an associated FQDN. These are the records
searched during reverse-lookups.
Point of Possible Confusion: while a basic guideline may be to
keep the number of duplicate records to a minimum - there is nothing that
prevents DNS servers having duplicate records, as long as they do not conflict.
The secondary classes used by nearly all DNS servers are SOA, NS, and
MX.
- SOA is short for "Start Of Authority" - records of this type store
which DNS server is the ultimate authority for a given zone. There
should be only one SOA for a given zone. The SOA
record can also be used to set some default values for other records - more on
that later.
- NS is for "NameServer" - records of this type list the DNS servers
that are authoritative for a zone. Normally, there are at least two
entries but less than six. Some systems will complain about connecting
to networks with either a single nameserver or an excessive number of
nameservers.
- MX is for "Mail eXchanger" - records of this type list the SMTP
servers for a domain. More on this later.
There is another group of classes that you may need for some purposes -
SRV and TXT.
- SRV is for "Service" - some non DNS packages use these records to
advertise their presence. An example might be WPAD - a method for
autoconfiguring web browsers with a network's proxy server information.
- TXT is for "Text" - while you can place just about anything here,
this is again a record type used by some software for other purposes. An
example might be SPF - a proposed method for reducing spam.
These are all the record classes you need to setup a fully functional DNS
server. There are other classes - even one intended to store the
longitude/latitude of your server! - but unless you have a very specific
purpose, you'll probably never need them.
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