What Is Reverse DNS Lookup & What Does It Do?

What Is Reverse DNS Lookup and What Does It Do?

Reverse DNS is a process that takes an IP address as a query and returns the corresponding domain name.

This is the opposite of a regular DNS process that translates human-readable domain names (like www.example.com) into their corresponding IP addresses.

In this article, we’ll explore what is reverse DNS, how it works, and its importance in network operations.

Takeaways
  • Reverse DNS lookups convert IP addresses to domain names, critical for verifying email sender authenticity and preventing spam
  • Setting up reverse DNS requires ISP or hosting provider coordination to create PTR records in the DNS system
  • It serves as a diagnostic tool, translating IPs into readable domains for network troubleshooting and security

What Is Reverse DNS Lookup (rDNS)?

Reverse DNS lookup (rDNS) is a technique used to determine the domain name associated with an IP address.

When you query an IP address in a reverse DNS lookup, the query targets a special part of the domain name system (DNS) known as the PTR (pointer record).

The PTR record will respond with the domain name that corresponds to the query’s IP address. The process is important because it helps verify that the IP address is associated with a particular domain name. This practice helps improve security while aiding with email spam filtering.

Difference between Reverse IP Address and Reverse DNS

A reverse IP lookup is a broad term that indicates any search that includes an IP address.

It means that you can use IP addresses to search for other associated information like geographic location, ISP, and other data gathered from IP databases.

On the other hand, Reverse DNS lookup refers to the querying of a DNS server to find the domain name associated with an IP address.

Difference between a Forwarding and Reverse DNS Zone

A forwarding DNS Zone is responsible for mapping domain names to IP addresses, which includes A and AAAA records.

A Reverse DNS Zone, on the other hand, handles the mapping of IP addresses, domain names or hostnames. When an IP address is queried in a reverse DNS Zone, if a PTR record exists, it will respond with the hostname associated with that IP address.

What is Reverse DNS Used For?

Reverse DNS is used to verify that an IP address is associated with a hostname or a domain name. This verification process serves several purposes:

  • Email anti-spam: Helps reduce spam by ensuring that the email is not coming from a suspicious or unverified server
  • Network troubleshooting: This can help identify unfamiliar IP addresses and diagnose issues by providing more recognizable information about network traffic sources
  • Security: It can help determine whether an IP address is associated with any known malicious hostnames
  • Internet user verification: It serves as an additional form of user verification
  • Whitelisting/Blacklisting: Services that control access based on IP addresses may use reverse DNS to populate whitelists and blacklists.

How Does Reverse DNS Work?

Unlike a typical DNS query that looks for an IP address given a domain name, a reverse DNS lookup navigates the DNS hierarchy in the opposite direction.

When a reverse lookup is initiated, the IP address is processed in a format suitable for DNS querying.

Reverse DNS lookups for IPv4 addresses

For IPv4, this involves reversing the octets of the address and appending “.in-addr.arpa” to the end.

For example, with an IP address like 192.0.2.1, reverse DNS translates this to a query for the PTR record at “1.2.0.192.in-addr.arpa”.

The DNS server, upon receiving such a query, checks for a PTR record at that location. If such a record exists, the server will contain the hostname that corresponds to the IP address in question.

This is how the DNS system facilitates reverse mapping, translating numerical addresses back to more human-readable identifiers.

Reverse DNS Lookups for IPv6 Addresses

IPv6 addresses follow a similar pattern, but with a different domain (“.ip6.arpa”) and a more complex structure due to the hexadecimal and longer nature of IPv6 addresses.

Reverse DNS Example

The reverse DNS process starts by reversing the IP address into 89.67.45.123.

It then appends the suffix “.in-addr.arpa”, forming “89.67.45.123.in-addr.arpa”.

A DNS query is made for the PTR record at that address.

If there’s a PTR record configured, it will contain the FQDN, like “server.company.com”.

This FQDN is the domain name associated with the IP address, which the DNS system returns as the result of the reverse DNS lookup.

How to Do a Reverse DNS Lookup

To perform a reverse DNS lookup, follow these steps:

  1. Identify the IP Address: You will need the IP address for which you want to find the corresponding hostname or domain.
  2. Access Reverse DNS Lookup Tool: Use an online reverse DNS lookup tool. There are many available for free such as MxToolbox or YouGetSignal
  3. Input the IP Address: Enter the IP address into the tool’s search bar.
  4. Perform the Lookup: Click the “Reverse Lookup” button or equivalent to perform the search.
  5. Analyze the Results: The tool will display the domain name or hostname that is associated with the IP address you provided if there is a PTR record set up in the DNS.
  6. Verification (Optional): To verify the accuracy, you could perform a forward DNS lookup on the domain name to see if it resolves back to the original IP address.

Please note that not all IP addresses will have a PTR record, which is necessary for a reverse DNS lookup. If no record exists, the lookup will not return a domain name.

Final: What Is Reverse DNS Lookup and What Does It Do?

Image Source: Mx Toolbox (Screenshot)

Who Sets Up Reverse DNS?

Reverse DNS (rDNS) is set up by the organization that controls the IP address block, which is usually the Internet Service Provider (ISP) that owns the IP space.

This is because the ISP has control over the PTR records in the DNS zone files for the IP addresses they manage.

Large organizations that own their IP address blocks (like universities or corporations) can also manage their reverse DNS records if they have control over their IP space. Cloud and hosting providers, who manage IP ranges for their infrastructure, also set up reverse DNS for their customers.

In all cases, the entity that has the authority to modify the PTR records in the reverse DNS zone is considered the “parent account” for those IP addresses. They can delegate control to their customers or users, but initially, it is their responsibility to configure and maintain reverse DNS records.

How to Set Up Reverse DNS

Setting up your reverse DNS is a relatively quick and straightforward process. Follow the steps outlined below to set up rDNS.

Step 1: Collect Necessary Information

To collect the necessary information you may need to either contact your hosting provider or ISP, as they’re likely the ones owning your IP block. If that’s not the case, you can use other services like IP WHOIS Lookup.

Final: What Is Reverse DNS Lookup and What Does It Do?

Image Source: IP WHOIS Lookup (Screenshot)

Step 2: Define Reverse DNS Zone Name

Once you know who is the owner of the IP block, contact them and request a zone for your mail server’s IP address.

This zone usually ends with “in-addr.arpa” and boasts an IP block with the octets reversed. This is the official typing convention which is necessary to create your own reverse DNS zone. Here’s a guide on how to define your reverse DNS zone name.

  • Type in your network IP address
  • In case you use a classless address block, you will need to remove the netmask part of the address (this is usually a two-digit number behind the slash). Otherwise, if you use a full address block, omit the last octet
  •  Make sure that the remaining octets are reversed
  • Add “in-addr.arpa” to the end of the address
  • In case you use the classless address block and you remove the netmask portion of the address, you will need to add it to the address using the dash (-) sign

Step 3: Request Reverse DNS Delegation

When contacting your ISP, make sure to ask them to delegate reverse DNS to your DNS provider. They will likely ask you to provide your nameservers for your domain.

If you don’t know what your nameserver could be, you can use a tool called Dig Web Interface

Step 4: Create Reverse DNS Zone

Make sure to log in to your web hosting account and navigate to the DNS Menu in the control panel (cPanel). Search for “Manage DNS.” Depending on the domain registrar or hosting provider the steps to navigate the DNS management settings may differ.

Click “Add Domains” and enter the reverse DNS zone name that we created in step 2.

Step 5: Create a PTR Record

Within the reverse DNS zone, create a PTR record by entering the final digit of the IP address and the domain name that you want the IP to resolve to

Step 6: Take Your Time

Allow 24-48 hours for DNS changes to propagate throughout the Internet before the reverse DNS setup is fully functional​.

Final Word

Reverse DNS is a useful technique that helps us translate numerical IP addresses into the original domain name.

There are many benefits to using reverse DNS like added security, spam email mitigation, and more. If you want to set up a reverse DNS, you first need a website, so make sure to check our list of the best website builders and choose the best web hosting provider for your needs.

Next Steps: What Now?

Learn More About DNS

Frequently Asked Questions

Should I reverse DNS?

Yes, setting up reverse DNS is recommended, especially if you are operating a mail server, as it can significantly reduce the chances of your emails being marked as spam. It also helps in network troubleshooting and is generally considered a good practice for any public-facing server IP.

How to set up reverse DNS for IPv6?

To set up reverse DNS for an IPv6 address, you need to contact your ISP or hosting provider, as they typically manage the IP space and the corresponding reverse DNS records. You’ll provide them with your IPv6 address and the desired hostname. They will create a PTR record in the “ip6.arpa” zone, which involves reversing the segments of your IPv6 address and placing them in the reverse order, separated by periods, in the DNS zone file.

What is the reverse DNS lookup command on Linux?

On Linux, the dig command is commonly used for reverse DNS lookups. For an IPv4 address, you would use the command in the format dig -x . For an IPv6 address, the command structure is the same: dig -x . The -x option tells dig to perform a reverse lookup.

What is reverse DNS used for?

Reverse DNS is used for validating that an IP address is associated with a particular domain name. This is important for:

Email servers use reverse DNS to help identify and block spammers.

System administrators use it for network troubleshooting, logging, or security purposes.

Some services perform reverse DNS lookups to verify the identity or authenticity of connecting devices.

It’s also used in various internet standards and protocols to ensure proper operation and integrity of data exchange.

 

Sonja Vitas
Edited By:
Sonja Vitas
Content Editor

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