What is reverse DNS and why should you care?

06 March 2017 by

Reverse DNS (rDNS) lookup is the reverse of the usual “forward DNS lookup” where the DNS is queried for the IP address of a certain hostname. In reverse DNS, the DNS is queried for the hostname of a certain IP address. A reverse DNS lookup returns the hostname of an IP address.


Reverse DNS lookup tool

With this tool you can perform reverse DNS lookups. It also shows data about your current IP address. Type in the IP address you would like to test. Blog post continues below the form.

Results for your DNS lookup

IP address:
Organization:
Country:
City:

What is your IP address

IP address:
Organization:
Country:
City:

You can use the form above to make reverse DNS lookups. Type in an IP address (for example 8.8.8.8) and press enter and the tool will make a reverse DNS lookup and return the name record for that IP address. If you want to see similar data of your website visitors, sign up for free to Leadfeeder.

What is reverse DNS in laymans terms

Whenever we navigate to a certain webpage, say google.com, our browser checks for the IP address of that domain name and uses that address to find the correct page.

DNS, Domain Name System, is an address list of computers connected to the internet. If you want to go to Amazon, you’ll ask DNS where they are located. Just like you would use an address book in normal life (wait a minute, no one actually uses an address book these days, do they?).

Note that an address book only works in a one-dimensional way. It’s easy to find the address of John Smith from your address book but if you are given an address, it’s pretty hard to find out in your address book who lives there - even if you have their address stored somewhere in your address book.

Reverse DNS lookup (rDNS lookup) is exactly this. It’s the act of looking up internet hosts by their IP address. Hence the “reverse” part in the name. It’s the opposite or reverse way of using the internet’s address book, something your old address book can’t even do (try finding out who has the number 955-0455-0922 from a paper address book!)

While the internet overall can seem pretty anarchistic at times, this DNS address book is a stable system that everyone on the internet must agree upon. You can pretty much trust that if DNS says someone is google.com they pretty sure are them (and while this blind trust has led to some significant problems, it’s the best system we’ve got).

Reverse DNS returns hostnames for ip addresses Reverse DNS works the other way around than the usual forward DNS

Now, all internet connected devices have an IP address. The device you are reading this post has one and the computer that hosts this page has one. If they didn’t you couldn’t access this page. You

While not all, a clear majority of all IP addresses have a reverse DNS record. You can check yours with the tool above. So most computers have an entry of our address book. If we know an IP address, we can know who it is, just like you can see with the tool.

When your browser loaded this page, our server was aware of your IP address and your browser was aware of our IP address, thanks to DNS. This is how the internet works.

Leadfeeder is a tool that shows companies that have shown an interest to your services or products by visiting your website. With Leadfeeder companies can generate new leads every month with reverse DNS. Simple and effective! You can sign up to Leadfeeder for free here.

What are PTR-record and in addr arpa

DNS is defined as zones. Zone is a separate portion of the domain name space and was historically administred as one zone file. Most often a domain is one zone.

The owner of the zone maps different addresses to different domain names in their zone. For example, maps IP address 23.25.62.12 to point to hostname www in zone example.com. This is done with DNS records.

This would mean that writing www.example.com to would direct our browser to address 23.25.62.12. This is usually done with an A (or ALIAS) record and is the usual forward DNS.

How about reverse DNS? PTR record is the record for reverse DNS. Does the zone owner just add that IP address to their zone and all good? No. Reverse DNS works the other way around.

PTR record is stored in a special zone called .in-addr.arpa. This zone is adminstrated by whoever owns the block of IP addresses. In our case, the zone for the PTR record would be 12.62.25.23.in-addr.arpa.

The owner of the IP address is usually the ISP and if you want to add a PTR record to your IP address, you need to contact your ISP.

But why is this important?

What this means is that if you have a website, you know all the IP addresses that have visited that website. And with reverse DNS address book we can translate these IP addresses into hostnames.

While most IP addresses can be translated into hostnames, sometimes that hostname is not very useful. Because there’s a finite amount of IP addresses and seemingly unfinite amount of new mobile phones, laptops and refridgerators connecting to the internet most of the time normal people don’t own their addresses. They are merely using an IP address their Internet Service Provider has allocated them for a while.

That’s the case usually with mobile phones and home broadband connections. In those cases your hostname will be something like 62-78-145-65.bb.dnainternet.fi (that was mine when writing this). But there’s a lot you can still do with this address.

For example, that address tells you I’m from Helsinki, Finland and my ISP is DNA. You can even pinpoint a more accurate location inside Helsinki where the connection was made from. Nice, but there’s still no way you can identify me with this information.

Whois search results for an IP address A simple whois search can reveal more information about an IP address or a hostname

For B2C marketers, this is all you can do with reverse DNS. An IP address can give quite detailed location data and this can help you find good locations to expand your business, for example. But it’s all different with B2B marketing.

Most companies have their own IP addresses. This means you can use reverse DNS lookup to see which companies visit your website. In case of a larger company, you can even see from which of their offices the connection was made from - just like with my IP address above.

Using reverse DNS lookup for identifying the visitor is a great B2B marketing tactic. While it can’t tell you the name of the person who visited, most of the time just the name of the company can help a great deal (you probably wouldn’t want to show your name to every webpage you visit, would you).

Reverse DNS and email

Reverse DNS is one of the ways email servers use to verify that the sending server is not a malicious spammer. When someone sends email from address john.doe@example.com, the receiving server checks whether the IP address of that server has a reverse DNS record that is tied to example.com.

If the sending email server doesn’t have a reverse DNS record at all, that’s usually a sign of spam and the email sent from such server is rejected by most email servers.

If you are running your own mail servers, you must have a PTR record set up for your server. You can verify that it’s working with the tool above.

If the email server has a reverse DNS but it’s not for your domain, that’s not a problem. Just remember that in that case, it’s even more important to set up correct DKIM and SPF records.

Web analytics software and reverse DNS

Most web analytics software have reverse DNS built-in. Google Analytics, for example, shows this data under Audience - Technology - Network. If you are using some other analytics software, that probably has similar reports available somewhere too.

The problem, of course is that your report is going to be filled with home broadband and mobile phone visitors which are skewing your data and making it harder to find what’s useful from a sales and lead generation perspective.

It’s possible to create filters for Google Analytics but if you are really serious about discovering which companies visiting your website, try out a 14-day trial of Leadfeeder so you don’t have to do manual and time-consuming work in Google Analytics.

Leadfeeder uses machine learning to filter ISPs and non-relevant hostnames out of your data. It also integrates to other marketing and sales tools for better data sharing.

If you are currently only using web analytics to see statistical changes on your website try reverse DNS and see who really visits your website. It can help you focus on individuals and companies rather than masses.

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