Types of DNS Records

DNS (Domain Name System) is an internet system which translates domain names into IP addresses. This is the definition that you commonly see around the internet, but practically it does much more like resolving mail servers for a domain, mapping an alias name of a domain to another domain, inform mail servers which sources of emails they should trust when they receive email from a particular domain and the list goes on. To handle all these tasks DNS has certain records for each domain or zone as it is technically called. I’ll be listing out the most popular DNS records and some technical jargon related to DNS.DNS as a service running on the server listens on port number 53. All clients query it (or question it) on this port number and it answers these queries. Each domain name has some or all of the following domain records. You can use the DNS Lookup Tool to find the DNS records for a domain.

NS Record Name Server Record is mandatory for all domain. It specifies the authoritative Name Servers for a domain i.e. the server which contains DNS records for the domain. Examples ns1.example.com, ns2.example.com usually a minimum of two Name Servers exist, the first one is the primary (or master) Name Server containing all DNS records and the secondary (or slave) Name Server containing a read-only copy of the DNS records transferred from the primary. Whenever one fails another server takes over. Some domain may have as much as five Name Servers or sometimes even 13 !!! Specifying a NS record for a subdomain delegates all DNS records for that subdomain to the specified Name Servers. Picture this, NS records for example.com are ns1.webhost.com and ns2.webhost.com and NS records for sub.example.com point to ns1.anotherwebhost.com and ns2.anotherwebhost.com. In this scenario all DNS records for sub.example.com are handled by anotherwebhost.com.

A Record Address Record specifies an IP address for the domain name the IP address is of version 4 (IPv4). This is also a mandatory Record to be specified because even is you set NS records for your domain and leave the A record empty typing the domain name will show a “Page cannot be displayed” error. The reason is your browser cannot find the IP address of the web server inside which HTML files for the domain resides. Examples are, A record example.com is 192.0.32.10 and A record for sub.example.com is 200.123.34.87 (just random values). Even the www prefix for a domain (www.example.com) require an A record to work. As an alternative even a CNAME can be created which will be explained later in this article.

AAAA Record Similar to the above mentioned A record but is used for mapping hostnames to IPv6 addresses. Some examples, example.com mapped to fdda:5cc1:23:4::1f

CNAME Record Canonical Name record creates an alias name for a domain. As mentioned earlier www.example.com can be a CNAME for example.com. But CNAME can be used for advanced purposes too, while using your own domain with blogger you’ll have to create a CNAME www.example.com to point to ghs.google.com

MX Record Mail eXchanger Record specifies where a mail sent for a particular domain should go. This may be an external domain to an internal one. If you use specific mail hosting services like Windows Live Custom Domains the MX record should be set to xxxxxxxx.pamx1.hotmail.com else if your using your own hosting provider’s mail service the MX of example.com is something like mail.example.com. One important thing to note is that if you set your own domain’s subdomain as an MX you should specify A records for mail.example.com. If no MX record is set for a domain all emails destined for [users]@example.com will be delivered to the server at the IP address specified in its A record. But it is a good practice to set the MX records for a domain.

Priority for MX record Priority is an attribute set while specifying MX records. It is specified in numbers. When multiple MX records are specified the one with the lowest number set as the priority value has the highest priority and acts as the primary mail server and the other are backup servers.

TXT Record Text record contains a piece of text which specifies which are the mail servers authorized to send emails for the domain. A sample TXT record for example.com is “v=spf1 a mx -all”. The v=spf1 specifies the version of the Sender Policy Framework, the a and mx specify that the A records and MX records are authorized to send emails for example.com the -all says that if these conditions do not match the email can be rejected. While setting up your domain with Windows Live Custom Domains the TXT record should be specified as v=spf1 include:hotmail.com ~all this mean hotmail.com is authorized to send email for that domain.

SOA Record Start of Authority specifies information about the domain. Some of the information it specifies

• Serial Specifies a serial number based on the date of modification of any of the DNS records. It is used by secondary DNS servers to update themselves based on the changes made in primary servers.
• Primary Name Server the Primary or Master DNS server is specified here
• Email Email address of the domain administrator
• Refresh The time specified in seconds when the slave DNS servers should refresh themselves
• Retry In the event of a failure when trying to refresh, after how many seconds the slave servers should retry
• Expire The time specified in seconds for which the slave servers will keep the cached copy of the DNS records in case they cannot contact the Master DNS Servers