hi,
i used nmap to os scan a system. like nmap -O 172.17.13.12.
but it replied by giving the following error message ,


"Unable to determine any DNS servers. Reverse DNS is disabled. Try using --system-dns"

then i tried like this , nmap --system-dns 172.17.13.12.. the result is ,


Starting Nmap 4.21ALPHA4 ( http://insecure.org ) at 2007-06-10 07:08 India Standard Time
Insufficient responses for TCP sequencing (0), OS detection may be less accurate
Insufficient responses for TCP sequencing (0), OS detection may be less accurate
Insufficient responses for TCP sequencing (0), OS detection may be less accurate
Interesting ports on 172.17.13.12:
Not shown: 1644 filtered ports, 57 closed ports
PORT STATE SERVICE
80/tcp open http
443/tcp open https
3389/tcp open ms-term-serv
MAC Address: 00:17:A4:CD:DA:F7 (Global Data Services)
Too many fingerprints match this host to give specific OS details
Network Distance: 1 hop

OS detection performed. Please report any incorrect results at http://insecure.org/nmap/submit/ .
Nmap finished: 1 IP address (1 host up) scanned in 173.766 seconds




Actually the machhine runs the windows OS. which is not detected.
Help me out how to os scan a pc.
what is this system dns. is it a proxy?
help me plz