What is PTR record
PTR records (Pointer Records) are used for reverse DNS lookups. An IP address can be associated to domain/hostname. This is opposite to what an A/AAAA record does for forward lookup zones. PTR records go into reverse DNS domain which end with '.in-addr.arpa' or '.ip6.arpa' for IPv4 and IPv6 address block respectively. It is used to verify if a server name is actually associated with the IP address. If a PTR record for an IP address points to a hostname and the A/AAAA record for that hostname points back to the same IP address, it is forward confirmed.
PTR record format
A typical PTR record looks like the following in standard BIND format:
$ORIGIN 66.23.109.in-addr.arpa. 96 3600 IN PTR mail.myshop.com.
$ORIGIN 0.8.1.0.22.214.171.124.ip6.arpa. 3.c.126.96.36.199.0.0 3600 IN PTR mail.myshop.com.An anatomy of the PTR Record
96 3600 IN PTR myshop.com.looks like the below:
|Host Label||TTL||Record Class||Record Type||Pointer Name|
It defines the hostname of a record and whether the hostname will be appended to the label. Fully qualified hostnames terminated by a period will not append the origin.
The time-to-live in seconds. This is the amount of time the record is allowed to be cached by an outside DNS server.
Mainly 3 classes of DNS records exist:
- IN (Internet) – default and generally what internet uses.
- CH (Chaosnet) – used for querying DNS server versions.
- HS (Hesiod) – uses DNS functionality to provide access to databases of information that change infrequently.
The record format is defined using this field. Common record types are A, AAAA, CNAME, CAA, TXT etc. In the case of a PTR record, the record type is PTR.
The server name associated with the IP address.
PTR record glossary
Berkeley Internet Name Daemon - is the most commonly used DNS software on the Internet and Dynu observes BIND format.
Reverse DNS (rDNS) is a method of resolving an IP address into a domain name, just as the domain name system (DNS) resolves domain names into associated IP addresses.