Reverse DNS in Ruby?

Question

I'm in an environment with a lot of computers that haven't been properly inventoried. Basically, no one knows which IP goes with which mac address and which hostname. So I wrote the following:

# This script goes down the entire IP range and attempts to
# retrieve the Hostname and mac address and outputs them
# into a file. Yay!

require "socket"

TwoOctets = "10.26"

def computer_exists?(computerip)
 system("ping -c 1 -W 1 #{computerip}")
end

def append_to_file(line)
 file   = File.open("output.txt", "a")
 file.puts(line)
 file.close
end


def getInfo(current_ip)
 begin
   if computer_exists?(current_ip)
     arp_output = `arp -v #{current_ip}`
     mac_addr = arp_output.to_s.match(/..:..:..:..:..:../)
     host_name = Socket.gethostbyname(current_ip)
     append_to_file("#{host_name[0]} - #{current_ip} - #{mac_addr}\n")
   end
 rescue SocketError => mySocketError
   append_to_file("unknown - #{current_ip} - #{mac_addr}")
 end
end


(6..8).each do |i|
 case i
   when 6
     for j in (1..190)
       current_ip = "#{TwoOctets}.#{i}.#{j}"
       getInfo(current_ip)
     end
   when 7
     for j in (1..255)
       current_ip = "#{TwoOctets}.#{i}.#{j}"
       getInfo(current_ip)
     end
   when 8
     for j in (1..52)
       current_ip = "#{TwoOctets}.#{i}.#{j}"
       getInfo(current_ip)
     end
 end
end

Everything works except it does not find a Reverse DNS.

Sample output that I'm getting is this:

10.26.6.12 - 10.26.6.12 - 00:11:11:9B:13:9F
10.26.6.17 - 10.26.6.17 - 08:00:69:9A:97:C3
10.26.6.18 - 10.26.6.18 - 08:00:69:93:2C:E2

If I do nslookup 10.26.6.12 then I get the correct reverse DNS so that shows that my machine is seeing the DNS server.

I have tried Socket.gethostbyname, gethostbyaddr, but it doesn't work.

Any guidance will be much appreciated.

Solution 1

Today I also needed reverse DNS lookup and I've found very simple standard solution:

require 'resolv'
host_name = Resolv.getname(ip_address_here)

It seems it uses timeout which helps in rough cases.

Solution 2

I would check out getaddrinfo. If you replace the line:

host_name = Socket.gethostbyname(current_ip)

with:

host_name = Socket.getaddrinfo(current_ip, 0, Socket::AF_UNSPEC, Socket::SOCK_STREAM, nil, Socket::AI_CANONNAME)[0][1]

The getaddrinfo function returns an array of arrays. You can read more about it at:

Ruby Socket Docs

Solution 3

This also works:

host_name = Socket.getaddrinfo(current_ip,nil)
append_to_file("#{host_name[0][2]} - #{current_ip} - #{mac_addr}\n")

I'm not sure why gethostbyaddr didn't also work.

Comments

[akostadinov]

In fact this is not doing reverse lookup. You need to make the 7th parameter true: Socket.getaddrinfo(interesting_ip, 0, Socket::AF_UNSPEC, Socket::SOCK_STREAM, nil, Socket::AI_CANONNAME, true)