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How to Do a Reverse DNS Lookup

Performing a Reverse IP Lookup in Ruby, Python, and Bash

A reverse DNS lookup is sometimes called a reverse IP lookup because you start with an IP address and use it to lookup a hostname. For example, if you have an IP address like 66.249.64.10, a reverse DNS lookup will return the hostname crawl-66-249-64-10.googlebot.com.

You might also see reverse DNS lookups referred to as reverse DNS resolution, reverse PTR lookup, or reverse Pointer DNS lookup. They’re all synonymous.

Since reverse DNS lookups are a pretty common task, many languages have libraries that support lookups. We’ll cover performing a reverse IP lookup in 3 popular languages:

Reverse DNS Lookups in Ruby

Ruby has a number of gems which you can use, but I prefer resolv because it’s built into Ruby’s Standard Library, and it’s really easy to use.

Simply use the intuitive getname method:

Reverse DNS in ruby using ‘resolv’
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require 'resolv'

reversed_dns = Resolv.new.getname '203.208.60.1'

# "crawl-203-208-60-1.googlebot.com"

If resolv just isn’t for you, there are alternative gem that package powerful DNS tools together. For instance, dnsruby and net-dns are two great DNS libraries.

dnsruby is a cool gem because it’s built on top of resolv, but with some added functionality like the ability to do zone transfers or perform queries with retries across multiple nameservers.

Reverse DNS in ruby using ‘dnsruby’
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require 'dnsruby'

resolver = Dnsruby::Resolver.new.query '203.208.60.1'

# ;; Security Level : UNCHECKED
# ;; HEADER SECTION
# ;; id = 59252
# ;; qr = true    opcode = Query    aa = false    tc = false    rd = true
# ;; ra = true    ad = false    cd = true    rcode  = NOERROR
# ;; qdcount = 1  ancount = 1  nscount = 4  arcount = 5
# 
# OPT pseudo-record : payloadsize 1280, xrcode 0, version 0, flags 32768
# 
# ;; QUESTION SECTION (1  record)
# ;; 1.60.208.203.in-addr.arpa.   IN      PTR
# 
# ;; ANSWER SECTION (1  record)
# 1.60.208.203.in-addr.arpa.      86068   IN      PTR     crawl-203-208-60-1.googlebot.com.
# 
# ;; AUTHORITY SECTION (4  records)
# 60.208.203.in-addr.arpa.        86068   IN      NS      ns1.google.com.
# 60.208.203.in-addr.arpa.        86068   IN      NS      ns3.google.com.
# 60.208.203.in-addr.arpa.        86068   IN      NS      ns4.google.com.
# 60.208.203.in-addr.arpa.        86068   IN      NS      ns2.google.com.
# 
# ;; ADDITIONAL SECTION (5  records)
# ns2.google.com. 303789  IN      A       216.239.34.10
# ns3.google.com. 219381  IN      A       216.239.36.10
# ns4.google.com. 303744  IN      A       216.239.38.10
# ns1.google.com. 303769  IN      A       216.239.32.10

reversed_dns = resolver.answer[0].domainname.to_s

# "crawl-203-208-60-1.googlebot.com"

net-dns is also a great gem and is regularly maintained. net-dns is a close port of perl’s Net::DNS and it shares almost all the same powerful functionality. So if you’re familiar with the perl library, this might be the gem for you.

Reverse DNS in ruby using ‘net-dns’
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require 'net/dns/resolver'

resolver = Net::DNS::Resolver.new.search '203.208.60.1'
# ;; Answer received from 10.44.225.18:53 (89 bytes)
# ;;
# ;; HEADER SECTION
# ;; id = 26855
# ;; qr = 1opCode: QUERYaa = 0tc = 0rd        = 1
# ;; ra = 1ad = 0cd = 0rcode = NoError
# ;; qdCount = 1anCount = 18nsCount = 0arCount = 0
# 
# ;; QUESTION SECTION (1 record):
# ;; 1.60.208.2083.in-addr.arpa.   IN      PTR     
# 
# ;; ANSWER SECTION (1 record):
# 1.60.208.203.in-addr.arpa.   47300   IN   PTR   crawl-203-208-60-1.googlebot.com.
reversed_dns = resolver.answer[0].ptr
# "crawl-203-208-60-1.googlebot.com."

Pythonic Reverse IP Lookups

For python, the fast and simple approach is to use the already built in socket library. Using the aptly named gethostbyaddr method, the lookup is fairly easy:

Reverse DNS in python using ‘socket’
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import socket

reversed_dns = socket.gethostbyaddr('203.208.60.1')
# ('crawl-203-208-60-1.googlebot.com', ['1.60.208.203.in-addr.arpa'], ['203.208.60.1'])
reversed_dns[0]
# 'crawl-203-208-60-1.googlebot.com'

However, there are specific packages that you can install that will give you some added power if you need it. For instance dnspython is an awesome library that is regularly maintained.

Reverse DNS in python using ‘dnspython’
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from dns import reversename, resolver

rev_name = reversename.from_address('203.208.60.1')
reversed_dns = str(resolver.query(rev_name,"PTR")[0])
# 'crawl-203-208-60-1.googlebot.com.'

Bash: Command Line Reverse DNS Lookups

Reverse DNS lookups are probably the easiest from the command line. Most linux platforms support this right out of the box. Here we’re using my terminal app on Mac OS X; and it couldn’t be simpler. Just use the host command followed by an IP address.

Reverse DNS lookup on the Command Line
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~$ host 66.249.66.1
1.66.249.66.in-addr.arpa domain name pointer crawl-66-249-66-1.googlebot.com.

You can also quickly do forward DNS lookups using the same host command:

Forward DNS lookup on the Command Line
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~$ host crawl-66-249-66-1.googlebot.com
crawl-66-249-66-1.googlebot.com has address 66.249.66.1