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HowTo Test or Check Reverse DNS on a Linux / Unix

September 8, 2019

I am a new Linux and Unix command line user. How do I test or check reverse DNS for given IP address under Linux, OS X, BSD, Unix-like or Windows XP/Server 2003 based systems?

Reverse DNS lookup (also known as rDNS) is a process to determine the hostname associated with a given IP address.

Typically, the DNS is used to determine what IP address is associated with a given hostname; so to reverse resolve a known IP address is to lookup what the associated hostname for it. A reverse lookup is often referred to simply as reverse resolving, or more specifically reverse DNS lookups.

[donotprint][/donotprint]The most common uses of the reverse DNS are:

  1. Anti-spam
  2. Network troubleshooting
  3. Avoid spammers and phishers using a forward confirmed reverse DNS etc

You can use standard UNIX / Linux utilities such as nslookup command, dig command or host command to find out reverse DNS of a given IP address.

Task: Find Reverse DNS for IP 75.126.43.235 under Linux/UNIX

Type the following host command:
$ host ip-address-here
$ host 75.126.43.235

Sample outputs:

235.43.126.75.in-addr.arpa domain name pointer cyberciti.org.

In this example output, IP 75.126.43.235 is reverse mapped to cyberciti.org. Here is another reverse lookups done using dig command:
$ dig -x ip-address-here
$ dig -x 75.126.153.206

Sample outputs:

; <<>> DiG 9.8.3-P1 <<>> -x 75.126.153.206
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39113
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;206.153.126.75.in-addr.arpa.	IN	PTR

;; ANSWER SECTION:
206.153.126.75.in-addr.arpa. 20975 IN	PTR	sxi.io.

;; Query time: 32 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Sat Feb  8 04:40:28 2014
;; MSG SIZE  rcvd: 76

Hiding additional display info while doing reverse ip lookup using dig command

You can only display the answer section of a reply with +answer option and clear all other display info with +noall option as follow:

dig +noall +answer -x 75.126.153.206

Sample outputs:

206.153.126.75.in-addr.arpa. 80127 IN	PTR	sxi.io.

Task: Find Reverse DNS for IP 75.126.43.235 under Linux/UNIX/Windows

nslookup works under Windows and UNIX like oses:
nslookup ip-address-here
nslookup 75.126.43.235

Output:

Server:         208.67.222.222
Address:        208.67.222.222#53

Non-authoritative answer:
235.43.126.75.in-addr.arpa      name = cyberciti.org.

Authoritative answers can be found from:

Demo: Reverse IP lookup on Linux, Unix, OS X and MS-Windows

Animated gif: host, dig, and nslookup command in action

Posted by: SXI ADMIN

The author is the creator of SXI LLC and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

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