I saw a post back in June and it just recently came up again:

http://www.securityartwork.es/2011/06/01/dns-port-forwarding-con-meterpreter/

It looked like a lot of hard work to set that up and I’m really lazy. I didn’t want to have to go through all that every time I got onto a new network. So, I made a very simple meterpreter post module to just call a Windows API key called ‘gethostbyaddr’ using Railgun.

TL:DR; You can download the post module here: ipresolver.rb

The function ‘gethostbyaddr’ (http://msdn.microsoft.com/en-us/library/ms738521(v=VS.85).aspx is pretty simple at first glance:

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struct hostent* FAR gethostbyaddr(
  __in  const char *addr,
  __in  int len,
  __in  int type
);

Give it an address, length and type and it gives you a hostname back… easy right?

Defining it, since it isn’t in the Railgun definitions is pretty simple:

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client.railgun.add_function('ws2_32', 'gethostbyaddr', 'DWORD', [
  ['PCHAR', 'addr', 'in'],
  ['DWORD','len','in'],
  ['DWORD','type','in']
])

First hurdle is to get your IP into ‘network byte order’. Rex (Metasploit’s API/Library) to the rescue.

http://dev.metasploit.com/documents/api/classes/Rex/Socket.html#M002073

The ‘addr_aton’ method does just that:

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nbi = Rex::Socket.addr_aton('192.168.1.100')
=> "xC0xA8x01d"

Make the call to the API and done right?

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result = client.railgun.ws2_32.gethostbyaddr(nbi.to_s, nbi.size,2)

Wrong, what you get back is a pointer to a mess, well lets get the mess (using a google IP for this example):

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struct = client.railgun.memread(result['return'],100)
=> "xA0x03fx00x88x0Efx00x02x00x04x00x8Cx0Efx00x00x00x00x00x94x0Efx00x00x00x00x00J}]cqw-in-f99.1e100.netx00@efx00x06x00bx00x13x01bx00xE0vvx00xvvx00xE8x0Efx00xEEx96x03x00x00bx00x00 x01x00x00x01x00x00x00x01x00x00x00x01x00x00x00"

Ya.. that…

I tried using the pointer at the 12th byte location and that worked most of the time, but failed bad on others:

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ptrptr = struct[12,4].unpack("V*")[0]
hostnameptr = client.railgun.memread(ptrptr,4).unpack("V*")[0]
hostname = client.railgun.memread(hostnameptr+nbi.size,59).split("")[0]

(there were a bunch more lines of error correcting but I’ll just wanted to show these as the actual methods used on a successful run)

But on other hosts the 12th byte came back with a pointer to all 0s, so there was no way to jump again (hostnameptr) to the actual hostname.

What I missed while trying to do things the “C” way was that the hostname was always pretexted with the IP address in network byte order… Hold up, I know it begins with something I already know, and ends in the standard “C” string terminator of a null byte. So all of that plus the crazy error correction became:

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struct = client.railgun.memread(result['return'],100)
hostname = struct.split(nbi)[1].split("")[0]

2 lines… work 100% of the time in my test cases.

That’s it, you can check out the download for the post module above.

I do however have 1 disadvantage over how the guys at SecurityArtWork did things. You can’t thread it. For whatever reason the API call that I am using uses the exact same memory space for each lookup. I tried putting threading in and what I got was a bunch of systems that resolved to the exact same thing.

If anyone knows a way to fix this I am all ears because right now the module is slow.