PTR Record – A DNS Pointer Resource Record

PTR Records are called as the Pointer Records. These are called as the Reverse DNS Records as it is used for reverse DNS lookups. The DNS best practices suggest that every host on the Internet that has a A record assigned should have a PTR record assigned to its IP address.

As long as there is an A record is available for the host, it should be reacheable on the internet. But, systems like an email or a webserver checks for a PTR record and might refuse connections as a measure of security and or against SPAM. For example, a mail server for example.com can refuse connection to a SMTP sending server for mail.test.com if the either the server did not have a PTR record or if the Reverse DNS lookup did not match the hostname and the IP Address

Syntax:

1.1.168.192.IN-ADDR.ARPA IN PTR www.itsyourip.com

Where,

The first 4 fields seperated by decimal (.) symbols refer to the IP address of the host in reverse order.

IN-ADDR.ARPA is a special Internet domain like a .com domain assigned by IANA specifically for Internet Address to Hostname mapping lookups as per RFC 1035.

The reason for reversing the IP Address on the record is to facilitate the delegation of zones based on the class (Class A or Class B or Class C) of the IP. For example, 10.IN-ADDR.ARPA can be the zone for ARPANET.

PTR records are hosted by the Network Owner. In other words, if you own a whole Class C address space then you are authoritative. However, if you have a few IP addresses from your Internet Service Provider (ISP) then it is very likely that your ISP is authoritative and hence they need to host your PTR records. You would obviously host your internet private network PTR records 🙂

The following is an excert from the RFC 1035 (Section 3.5) about the IN-ADDR.ARPA domain (ARPA is Address Routing Parameter Area):

The Internet uses a special domain to support gateway location and Internet address to host mapping.  Other classes may employ a similar strategy in other domains.  The intent of this domain is to provide a guaranteed method to perform host address to host name mapping, and to facilitate queries to locate all gateways on a particular network in the Internet.

Note that both of these services are similar to functions that could be performed by inverse queries; the difference is that this part of the domain name space is structured according to address, and hence can guarantee that the appropriate data can be located without an exhaustive search of the domain space.

The domain begins at IN-ADDR.ARPA and has a substructure which follows the Internet addressing structure.

Domain names in the IN-ADDR.ARPA domain are defined to have up to four labels in addition to the IN-ADDR.ARPA suffix.  Each label represents one octet of an Internet address, and is expressed as a character string for a decimal value in the range 0-255 (with leading zeros omitted except in the case of a zero octet which is represented by a single zero).

Host addresses are represented by domain names that have all four labels specified.  Thus data for Internet address 10.2.0.52 is located at domain name 52.0.2.10.IN-ADDR.ARPA.  The reversal, though awkward to read, allows zones to be delegated which are exactly one network of address space.  For example, 10.IN-ADDR.ARPA can be a zone containing data for the ARPANET, while 26.IN-ADDR.ARPA can be a separate zone for MILNET.  Address nodes are used to hold pointers to primary host names in the normal domain space.

Network numbers correspond to some non-terminal nodes at various depths in the IN-ADDR.ARPA domain, since Internet network numbers are either 1, 2, or 3 octets.  Network nodes are used to hold pointers to the primary host names of gateways attached to that network.  Since a gateway is, by definition, on more than one network, it will typically have two or more network nodes which point at it.  Gateways will also have host level pointers at their fully qualified addresses.

Both the gateway pointers at network nodes and the normal host pointers at full address nodes use the PTR RR to point back to the primary domain names of the corresponding hosts.

For example, the IN-ADDR.ARPA domain will contain information about the ISI gateway between net 10 and 26, an MIT gateway from net 10 to MIT's