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Reverse DNS (rDNS) Explanation And It’s Operation

The domain name system (DNS) is one of the most important instances on the Internet. That is, without DNS and…

By TrendsTechBlog , in TECHNOLOGY , at May 13, 2020 Tags: ,

The domain name system (DNS) is one of the most important instances on the Internet. That is, without DNS and its union of name servers, using the global public computer network would be much more complicated. This registration system is precisely responsible for the management of project addresses on the World Wide Web: when a website is registered in DNS, an entry is created that contains both the domain address (easier to read for the user) like the linked IP address. Therefore, we can write the address example.es in the browser and access the corresponding website because a DNS server automatically translates these domain addresses into the corresponding IP addresses in the background.

What Is Reverse DNS

The DNS query that determines the IP address from a given domain or hostname is called Reverse DNS (rDNS) or Reverse DNS lookup. The requirement is that there is a so-called PTR (“pointer”) record linked to the name and that it allows this type of “reverse” search in the domain name system. Those responsible for organizing these entries, which are located in their namespace (domain), are various Internet providers. Modifications to this pointer are only possible after consultation with the provider.

When And For Who Is A Reverse DNS Lookup Useful

Like the standard search process, Reverse DNS provides not only the resolution of the name or IP address but also some additional information. In this way, in addition to the desired hostname, with a Lookup, you also obtain a geographical allocation of the IP and information about the responsible Internet service provider. This information is not particularly valuable for the analysis of private users since the real names generated automatically and linked to the IP addresses do not allow the personal identification that would be relevant for marketing purposes. However, user location data can be very profitable, since it allows conclusions to be drawn about the target markets they are targeting.

The information in Reverse DNS lookups is especially useful for B2B marketing: most companies have their IP address, so in many cases, it is possible to easily reconstruct when a particular company is using its website.

How Does Reverse DNS (rDNS) Work Exactly

Having a Reverse DNS lookup look up the IP address of a device or Internet server in the entire domain name system would be time-consuming. For reverse DNS lookups, a domain with three subdomains has been configured with in-addr.arpa (for IPv4 addresses) or ip6.arpa (for IPv6 addresses), allowing the resolution of the address in a maximum of three steps. The subdomains have the following structure:

  • RDNS 1 Subdomain: The subdomain immediately after in-addr.arpa or ip6.arpa represents the first component of the IP address.
  • RDNS 2 Subdomain: Level 2 contains the second component of the IP address.
  • RDNS 3 Subdomain: rDNS subdomain 3 contains the third component of the corresponding IP address.

The individual components of the IP address are arranged to the left of the main domain in reverse or analog order to their proximity.

   

Reverse DNS Lookup Explained In Concrete Examples

The simplest way to illustrate how Reverse DNS domains work is a short example search. Thus, the search for the IPv4 address 217.160.0.128 should look like this:

128.0.160.217.in-addr.arpa
When this search is started with a Reverse DNS lookup tool, the following result is obtained:

The hostname for 217.160.86.40 is 217-160-0-128.elastic-ssl.ui-r.com.
The domain is shown ( elastic-ssl.ui-r.com ) is operated by IONOS: specifically, it is a test website that is hosted on IONOS servers. This information can be provided by numerous Lookup tools.

The web project can also be accessed through the IPv6 address 2001: 8d8: 100f: f000 :: 2e3, for which the rDNS Lookup would look like the following:

3.e.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.f.f.0.0.1.8.d.8.0.1.0.0.2.ip6.arpa

Reverse DNS: The Best Tools For Reverse DNS Lookup

With nslookup, most Linux, Windows, and macOS systems already have a tool that allows you to perform any search in the domain name system. With this program, it is possible to carry out both standard DNS searches and Reverse DNS lookups, in which the local DNS server specified by the provider is contacted in a standard way. If necessary it is also possible to define another server name.

Alternatively, address searches can also be performed with rDNS via the web. There is a wide variety of lookup-based online tools that allow individual DNS server searches to be performed directly in the browser. Among them are the following three solutions:

  • MxToolBox: MxToolBox is a web service that offers different network diagnostic tools and Lookup. Although some of these offers are paid, the Reverse DNS check with SuperTool is always free. To use it, just select the “Reverse Lookup” option and type in the IPv4 or IPv6 address.
  • WhatIsMyIP.com: Unlike what the website name implies, at WhatIsMyIP.com it is not only possible to check what the IP address is, but also to perform rDNS searches. To do this, select the “Reverse DNS Lookup” item, enter the IPv4 or IPv6 address in the entry line and click on “Lookup”.
  • Debouncer: The Debouncer web service is specifically designed to check whether the domain itself or the mail server used regularly is on a spam blacklist. Through the “Reverse DNS check” option it is also possible to perform reverse DNS searches, although it is only possible to check IPv4 addresses.

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