DNS resource records are contents of the DNS zone file. The zone file contains mappings between domain names and IP addresses in the form of text records. There are many types of the resource records. In this article, we are going to see DNS Resource Records in detail.
The common types of DNS Resource Records are given below. There are many other resources also.
Let’s see each record in detail.
Every zone file will have a SOA record. It will be present at the beginning of the zone. The SOA stands for Start of Authority. Normally, this type of record holds information about the zone itself and about other records. Each zone will be having only one SOA record.
The SOA record contains the following fields.
Eg: IN SOA nameserver.place.dom. postmaster.place.dom.
The NS record stands for nameserver record. This shows the authoritative servers the zone. They indicate primary and secondary servers for the zone specified in the SOA record. Zones can contain many NS records, but it should contain at least one NS record for a DNS zone.
For example, when the administrator on abc.com delegated authority for the noam.abc.com subdomain to noamdc1.noam.abc.com., the following line was added to the zone abc.com and noam.abc.com:
noam.abc.com. IN NS noamdc1.noam.abc.com.
The next resource record we are going to see is the A record. The A record stands for Address record. It maps a domain name to an IP address so that the resolver can request the corresponding IP address for the domain. As an example, the following A resource record, located in the zone abc.com, maps the FQDN of the server to its IP address.
abc.com IN A 172.16.48.1
The PTR record stands for the pointer record. It functions reversely as that of the A record. It maps a domain name to an IP address. We are familiar with the term reverse dns. This record is used to achieve the reverse dns. An example is given below.
220.127.116.11.in-addr.arpa. IN PTR abc.com.
CNAME Resource Records
The next is CNAME Resource Record. The CNAME stands for the Canonical name. The function of the CNAME record is to create an alias for the domain name. It is helpful to hide the implementation details of the network from the customers.
An example of the CNAME record is given below.
ftp.abc.com. IN CNAME ftp1.abc.com.
Once a DNS client queries for the A resource record for ftp.abc.com, the DNS server finds the CNAME resource record. Then it resolves the query for the A resource record for ftp1.abc.com, and returns both the A and CNAME resource records to the client. This is how CNAME record works.
MX Resource Records
The MX record stands for the mail exchange record. The mail exchange (MX) resource record specifies a mail exchange server for a DNS domain name. A mail exchange server is a host that will either process or forward mail for the DNS domain name. Processing the mail means either delivering it to the addressee or passing it to a different type of mail transport. Forwarding the mail means sending it to its final destination server. It will Simple Mail Transfer Protocol (SMTP) to another mail exchange server that is closer to the final destination, or queuing it for a specified amount of time.
Only mail exchange servers use MX records.
We can have multiple MX resource records for that domain. The following example shows MX resource records for the mail servers for the domain noam.abc.com.:
*. noam.abc.com. IN MX 0 mailserver1.noam.abc.com.
*. noam.abc.com. IN MX 10 mailserver2.noam.abc.com.
*. noam.abc.com. IN MX 10 mailserver3.noam.abc.com.
With MX records, we can have multiple mail servers in a DNS domain, and when a mailer needs to send mail to a host in the domain, it can find the location of a mail exchange server. Service (SRV) resource records enable you to specify the location of the servers for a specific service, protocol, and DNS domain. Thus, if you have two Web servers in your domain, you can create SRV resource records specifying which hosts serve as Web servers, and resolvers can then retrieve all the SRV resource records for the Web servers.
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