Reverse DNS in Ruby?

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.

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.

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](

[To see links please register here]

)

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.