Nmap Development mailing list archives
Re: Using TTL value of response packets on nmap port scans.
From: David Fifield <david () bamsoftware com>
Date: Thu, 12 Apr 2012 21:21:37 -0700
On Fri, Apr 13, 2012 at 07:11:34AM +0300, Otto Airamo wrote:
Hello, It seems that I was explaining in the details WHAT I would like from Nmap, but I was missing WHY this is needed. When port scan is executed over TCP, there is in most cases three relevant cases that may occur in each port that is scanned. 1) SYN-ACK packet is received. In most cases this means that port is open in target host. 2) No reply. In this case we do not know if target host is listening port. At least it is known that from this scope* this port is not accessible in target host. (*With scope I mean that either network firewall or personal firewall blocks incoming SYN/outgoing RST/ICMP error message. It is possible that port is open in target host and would answer with SYN-ACK if packet would come from, for example, same network or different IP address) Important point here is that WE DO NOT KNOW if port is actually open (from different scope)! 3) RST packet is received. Typically this means that port is closed in target host. If RST packet is generated by target host TCP/IP stack because port is closed, it does not matter what is the scope where we are performing scan. Result will remain same even if scan would be performed from same network/using any source IP address. At the same time RST packet may be generated by firewall located between scanner and target host. If scanner is not able to tell if RST packet is generated by target host or device between, result of the scan will fall into same category as case #2. (We do not know if port is open) If scanner would be able to detect difference between RST packet generated by target host TCP/IP stack and other cases, scanner would have better knowledge of target. Options are: a) Target host TCP/IP stack generated RST packet => we know that port is closed b) Somebody else than target host TCP/IP stack generated RST packet => we do not know if port is actually open even we do not have access from this scope. Now we have following network setup: scanner -- FW -- target Firewall is configured to RST connections to port 22. Some ports are allowd, mosts of the ports are dropped silently. Result of nmap could be: # nmap --reason target --top-ports=10 Interesting ports on target: PORT STATE SERVICE REASON TTL 21/tcp open ftp syn-ack 62 22/tcp refused ssh reset (ttl?) 63 23/tcp closed telnet reset 62 25/tcp filtered smtp no-response N/A 80/tcp open http syn-ack 62 110/tcp filtered pop3 no-response N/A 139/tcp filtered netbios-ssn no-response N/A 443/tcp filtered https no-response N/A 445/tcp filtered microsoft-ds no-response N/A 3389/tcp filtered ms-term-serv no-response N/A
Thanks for explaining what you mean. The output example especially makes it clear. I don't think we would add a TTL column to the output by default. The information is already in the XML for tools that want to use it. Also, by convention, states like "closed" are understood to mean that a RST was received, or whatever else is appropriate for the type of port scan. The --reason flag is for those who want to see the reason without any interpretation. The firewalk script also does what you describe in a thorough manner. http://nmap.org/nsedoc/scripts/firewalk David Fifield _______________________________________________ Sent through the nmap-dev mailing list http://cgi.insecure.org/mailman/listinfo/nmap-dev Archived at http://seclists.org/nmap-dev/
Current thread:
- Using TTL value of response packets on nmap port scans. Otto Airamo (Apr 11)
- Re: Using TTL value of response packets on nmap port scans. David Fifield (Apr 11)
- Re: Using TTL value of response packets on nmap port scans. Otto Airamo (Apr 12)
- Re: Using TTL value of response packets on nmap port scans. David Fifield (Apr 12)
- Re: Using TTL value of response packets on nmap port scans. Otto Airamo (Apr 14)
- Re: Using TTL value of response packets on nmap port scans. David Fifield (Apr 13)
- Re: Using TTL value of response packets on nmap port scans. Otto Airamo (Apr 12)
- Re: Using TTL value of response packets on nmap port scans. David Fifield (Apr 11)