What Is Reverse DNS?

The purpose of DNS is typically used to resolve a domain name to an IP address. This act is known as a forward resolution and is performed every time you visit a site on the internet. Reverse DNS (or rDNS), as its name implies, is a method of resolving an IP address back to a domain name.

Uses of Reverse DNS

The reason we use reverse DNS is the same as why we use the standard (forward) DNS. It is easier to remember and identify a domain name than a string of numbers. rDNS is less crucial than forward DNS, as forward DNS records are required to resolve a website. Domains will still load in the absence of a reverse DNS record.

Email Servers commonly use rDNS to block incoming SPAM messages. Many mail servers are set to automatically reject messages from an IP address that does not have rDNS in place. Although an rDNS record can block spam, it is not a reliable method and is used mostly as an extra layer of protection. It is also important to note that merely enabling rDNS can still result in rejected messages due to a variety of reasons.  Additionally, rDNS is also used in analytics and logging to help provide human-readable data rather than logs consisting entirely of IP addresses.

The Reverse DNS Lookup Process

Since forward DNS maps a hostname to an IP address, rDNS (or Reverse DNS) indicates that we are mapping the IP address of a server back to a hostname. Using rDNS, the IP address is reversed and then the in-addr.arpa is added to the end. For example, if we use the IPv4 address of 67.227.187.136, using rDNS, it would become 136.187.227.67.in-addr.arpa.

This method of reverse DNS resolution of an IP address uses a PTR record. The PTR record includes the “forward hostname” of the location where the IP is being utilized in the regular, or forward DNS mapping. If a domain has a PTR record, we can do an rDNS Lookup by using one of the methods noted below.

It should also be noted that the rDNS settings are not set by the domain’s nameservers specifically, but rather by the owner of the IP space through ARPA. This effectively pulls the PTR record from the in-addr.arpa zone file from one of it’s own designated nameservers.

Perform a rDNS Lookup

Numerous online tools can be used to perform an rDNS lookup. A few examples of these online tools are linked below:

https://mxtoolbox.com/ReverseLookup.aspx

https://www.whatismyip.com/reverse-dns-lookup/

https://www.iplocation.net/reverse-dns

You can also perform a rDNS lookup manually from the command line. In Linux, the command you would use is “dig” with the added “-x” flag. 

If you are on a Windows computer, you would typically use the “nslookup” command, though you could also use “ping -a”. An example of the Linux command and its output shown below:

dig -x 8.8.8.8

Output:

;<<>> DiG 9.9.4-RedHat-9.9.4-61.el7 <<>> -x 8.8.8.8
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 36810
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;8.8.8.8.in-addr.arpa. IN PTR

;; ANSWER SECTION:
8.8.8.8.in-addr.arpa. 21599 IN PTR google-public-dns-a.google.com.

;; Query time: 19 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Wed Jul 18 11:58:54 EDT 2018
;; MSG SIZE  rcvd: 93

You can see the full rDNS PTR record for that IP in the “ANSWER SECTION” leading 8.8.8.8 back to the Google subdomain, google-public-dns-a.google.com :

8.8.8.8.in-addr.arpa. 21599 IN PTR google-public-dns-a.google.com.

Set Up rDNS

Liquid Web makes it easy to set up and manage rDNS for your server IPs. Just follow the steps outlined in our Knowledge Base article below:

https://www.liquidweb.com/kb/using-manage-to-update-reverse-dns/

Setting up a reverse DNS record is straightforward and can be beneficial to ensure that an IP does indeed belong to the domain it claims. If you are unsure who your DNS provider is, follow our helpful guide in locating where you should add the rDNS record.