***A-Records (Host address)
The A-record is the most basic and the most important DNS record type.
It is used to translate human friendly domain names such as "www.simpledns.com" into IP-addresses such as 188.8.131.52 (machine friendly numbers).
A-records are the DNS server equivalent of the hosts file - a simple domain name to IP-address mapping.
A-records are not required for all computers, but are needed for any computer that provides shared resources on a network.
***MX-Records (Mail exchange)
MX-records are used to specify the e-mail server(s) responsible for a domain name.
Each MX-record points to the name of an e-mail server and holds a preference number for that server.
If a domain name is handled by multiple e-mail servers (for backup/redundancy), a separate MX-record is used for each e-mail server, and the preference numbers then determine in which order (lower numbers first) these servers should be used by other e-mail servers.
If a domain name is handled by a single e-mail server, only one MX-record is needed and the preference number does not matter.
When sending an e-mail to "firstname.lastname@example.org", your e-mail server must first look up any MX-records for "xyz.com" to see which e-mail servers handles incoming e-mail for "xyz.com".
This could be "mail.xyz.com" or someone else's mail server like "mail.isp.com".
After this it looks up the A-record for that e-mail server name to connect to its IP-address.
IMPORTANT: An MX-record must point to the name of a mail server - not directly to the IP-address.
Because of this, it is very important that an A-record for the referenced mail server name exists (not necessarily on your DNS server, but wherever it belongs), otherwise there may not be any way to connect to that e-mail server.
Do not point an MX-record to a CNAME-record. Many e-mail servers don't understand this. Add another A-record instead.
***PTR-Records (domain name pointer)
PTR-records are used to map IP addresses to domain names (reverse of A-records).
The name of a PTR-record is the IP address with the segments reversed and with "in-addr.arpa" appended to the end.
As an example, looking up the domain name for IP address "184.108.40.206" is done with a query for the PTR-record for "220.127.116.11.in-addr.arpa"
***CNAME-Records (Canonical name for an alias)
CNAME-records are domain name aliases.
Computers on the Internet often performs multiple roles such as web-server, ftp-server, chat-server etc.
To mask this, CNAME-records can be used to give a single computer multiple names (aliases).
For example the computer "computer1.xyz.com" may be both a web-server and an ftp-server, so two CNAME-records are defined:
"www.xyz.com" = "computer1.xyz.com" and "ftp.xyz.com" = "computer1.xyz.com".
Sometimes a single server computer hosts many different domain names (take ISPs), and so CNAME-records may be defined such as "www.abc.com" = "www.xyz.com".
The most common use of the CNAME-record type is to provide access to a web-server using both the standard "www.domain.com" and "domain.com" (with and without the www prefix).
This is usually done by creating an A-record for the short name (without www), and a CNAME-record for the www name pointing to the short name.
CNAME-records can also be used when a computer or service needs to be renamed, to temporarily allow access through both the old and new name.
A CNAME-record should always point to an A-record and never to itself or another CNAME-record to avoid circular references.
***NS-Records (Authoritative name server)
NS-records identify the DNS servers responsible (authoritative) for a zone.
A zone should contain one NS-record for each of its own DNS servers (primary and secondaries).
This is mostly used for zone transfer purposes (notify messages).
These NS-records have the same name as the zone in which they are located.
But the more important function of the NS-record is delegation.
Delegation means that part of a domain is delegated to other DNS servers.
For example all ".com" sub-names (such as "simpledns.com") are delegated from the "com" zone (hosted by the "InterNIC").
The "com" zone contains NS-records for all ".com" sub-names (a lot!).
You can also delegate sub-names of your own domain name (such as "subname.yourname.com") to other DNS servers.
You are in effect the "InterNIC" for all sub-names of your own domain name (if you have a really cool domain name, you might even be able to sell sub-names for profit).
To delegate "subname.yourname.com", create NS-records for "subname.yourname.com" in the "yourname.com" zone.
These NS-records must point to the DNS server responsible for "subname.yourname.com" for example "ns1.subname.yourname.com" - or a DNS server somewhere else like "ns1.othername.net".
An NS-record identifies the name of a DNS server - not the IP-address.
Because of this, it is important that an A-record for the referenced DNS server exists (not necessarily on your DNS server, but wherever it belongs), otherwise there may not be any way to connect with that DNS server.
If an NS-record delegates a sub-name ("subname.yourname.com") to a DNS server with a name in that sub-name ("ns1.subname.yourname.com"), an A-record for that server (""ns1.subname.yourname.com") must exist in the parent zone ("yourname.com").
This A-record is called a "glue record", because it doesn't really belong in the parent zone, but is necessary to locate the DNS server for the delegated sub-name.
***SOA-Records (Start of authority)
A zone contains exactly one SOA-record, which holds the following properties for the zone:
Name of primary DNS server
The domain name of the primary DNS server for the zone.
The zone should contain a matching NS-record.
• E-mail address of responsible person
The e-mail address of the person responsible for the zone.
The standard for this is the "hostmaster" user name - such as "email@example.com".
• Serial number (see Zone Transfers)
Used by secondary DNS servers to check if the zone has changed.
If the serial number is higher than what the secondary server has, a zone transfer will be initiated.
This number is automatically increased by Simple DNS Plus when changes are made to the zone or its records (but not until you click the Refresh button, when another zone is selected, or when the "Edit DNS Records" window is closed).
Unless you have a specific reason for changing this number, it is best to let Simple DNS Plus manage it.
You should never decrease a serial number.
• Refresh Interval (see Zone Transfers)
How often secondary DNS servers should check if changes are made to the zone.
• Retry Interval (see Zone Transfers)
How often secondary DNS server should retry checking if changes are made - if the first refresh fails.
• Expire Interval (see Zone Transfers)
How long the zone will be valid after a refresh.
Secondary servers will discard the zone if no refresh could be made within this interval.
• Minimum (default) TTL
Used as the default TTL for new records created within the zone.
Also used by other DNS server to cache negative responses (such as record does not exist etc.).
A SOA-record is automatically created when you create a new zone.