Reverse IP Lookup

The reverse DNS lookup tool queries the specified IP address to convert it into a hostname. The hostname might be something like a regular domain or subdomain, for example, google-public-dns-a.google.com. This hostname is the Google hostname for the IP 8.8.8.8, which is Google’s public DNS IP address. If you enter your own IP address, it may show your ISP’s hostname, or if you query your server’s IP, it may display the domain name on which it is resolved.

DNS PTR Record

A PTR record, also known as a pointer record or reverse DNS record, is a type of DNS record used to store the domain or hostname associated with an IP address. It maps an IP address to a hostname.

PTR records, often referred to as reverse DNS records, are used for reverse IP lookups. You can obtain the associated domain or hostname using an IP address.

This is the opposite of the A record (for IPv4) or the AAAA record (for IPv6), which are known as forward DNS records.

Typically, a user wants to connect to a server using a known domain name but does not have the correct IP address. When a user enters a domain name in the browser, the DNS lookup process translates it into an IP address.

Reverse DNS lookup starts with an IP address and ends with the associated domain name or hostname, essentially the reverse process of forward DNS lookup.

How Are PTR Records Stored?

The structure of a PTR record is similar to other DNS record types. Different pieces of information are organized into specific fields.

<name> <ttl> <class> <type> <rdata>

• <name>: This field contains the IP address.
• <ttl>: Time to live. It is the duration in seconds for which the record remains valid. Once expired, it needs to be refreshed.
• <class>: Contains the abbreviation for the DNS record class used.
• <type>: Indicates the record type, in this case, PTR.
• <rdata>: Contains the resource data, which is the domain or hostname.

The syntax is similar to an A record, but the content differs. Importantly, for reverse mapping, the IP address is written in reverse order.

For IPv4

An A record must exist for every PTR record. A PTR record for an IPv4 address is split into segments, reversed, and then appended with .in-addr.arpa (the namespace for reverse DNS in IPv4).

For example, the PTR record for the IPv4 address 8.8.4.4 for the domain dns.google would be stored as 4.4.8.8.in-addr.arpa.

In the example above:

• 4.4.8.8.in-addr.arpa. is the record identifier for the PTR record corresponding to the A record 8.8.4.4.
• PTR is the DNS record type.
• dns.google is the record value, i.e., the domain or hostname associated with the IP address.
• 3600 is the TTL (time to live).

For IPv6

IPv6 addresses are structured differently from IPv4 addresses, and PTR records for IPv6 are stored in a separate namespace under .ip6.arpa. The IPv6 address is reversed and divided into four-bit segments, then appended with .ip6.arpa.

For example, the PTR record for the IPv6 address 2001:4860:4860::8844 for the domain dns.google would be stored as 4.4.8.8.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.6.8.4.0.6.8.4.1.0.0.2.ip6.arpa.

In this example:

• The long reversed IPv6 string is the record identifier for the PTR record corresponding to the A record 2001:4860:4860::8844.
• PTR is the DNS record type.
• dns.google is the record value, representing the domain or hostname associated with the IP address.
• 3600 is the TTL (time to live).

Primary Use of PTR Records

PTR records are used in reverse IP lookups and are mandatory for outgoing mail servers.

Some email servers, such as Gmail and Yahoo, employ spam filters that verify whether the forward and reverse DNS records of an IP address match. This ensures that the domain or hostname associated with an email is valid and that the corresponding IP address is authorized to send email.

If PTR records are missing or misconfigured, email delivery may suffer, as mail servers might block or reject emails from that domain. PTR records help ensure that emails originate from the stated source.

Do I Need PTR Records?

In short, yes. Email is a crucial part of business communications, and Google recommends using PTR records to improve email deliverability and maintain your reputation.

You definitely do not want your emails to end up in spam folders, as this can damage your credibility and cause recipients to question why your emails are not reaching them.

Can I Have Multiple PTR Records?

Typically, a PTR record can only point to a single hostname. If you require multiple PTR records for one IP address that point to different hostnames, it might work if you have several domains sharing the same A record. However, having multiple PTR records is generally discouraged since mail server software often expects a one-to-one correspondence: “One IP – one PTR.”

If multiple PTR records exist for one IP address, the server may randomly select one during a reverse lookup, which can cause issues with email verification and delivery.

How Long Does It Take for a PTR Record to Propagate?

This generally depends on how frequently the hosting provider updates the zone files. Even if you update PTR records via your hosting control panel and the provider updates their zone files immediately, propagation still takes time due to the DNS TTL.