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With nothing on the port:
nmap 127.0.0.2 -p 23 -sU
PORT STATE SERVICE
23/udp closed telnet
09:57:52.335826 IP 127.0.0.1.42869 > 127.0.0.2.23: UDP, length 0
09:57:52.335870 IP 127.0.0.2 > 127.0.0.1: ICMP 127.0.0.2 udp port 23
unreachable, length 36
- -------------
With the port set to DROP:
nmap 127.0.0.2 -p 23 -sU
PORT STATE SERVICE
23/udp open|filtered telnet
10:05:14.032653 IP 127.0.0.1.57201 > 127.0.0.2.23: UDP, length 0
10:05:15.034157 IP 127.0.0.1.57202 > 127.0.0.2.23: UDP, length 0
A closed port gets an ICMP response. No response yields "open|filtered".
Arturas Zalenekas wrote:
UDP has a timeout. If NMAP doesn't get a response (doesn't metter is it
UDP or ICMP protocol), it will mark the port as closed. That is a sort
descrition, how NMAP determins, if the UDP port is open or closed.
The time windows, how NMAP has to scan a specific protocol, can be set.
There is an default value for UDP protocol.
Actually, everything is described in the manuals, so why actually are you
asking !? The manual is more then self explaining.
If you don't understand these options or the use for these options, feel
free to ask, but first read the man pages please.
Kind regards,
Arturas Zalenekas
Network Security Engineer and Analyst
On Wed, April 12, 2006 20:26, Alice Bryson wrote:
Yes, i agree that.
How about UDP, if an udp port firewalled, how does NMAP know it?
2006/4/13, Nathaniel Hall <nathaniel.d.hall@gmail.com>:
I am assuming you are using a DROP rule on your firewall. NMAP knows
that if it does not receive a response for a TCP connection then it is
firewalled. Dropping traffic at a firewall violates RFC and makes it
much easier to know when there is a firewall between the scanner and the
end host. I recommend using REJECT
-A INPUT -j REJECT --reject-with icmp-host-unreachable
That will conform to RFC (I'm pretty sure) and will make it harder to
detect a firewall with NMAP.
Alexey Eremenko wrote:
Hi all !
I know that "nmap" can show open ports. But nmap also shows my
firewalled ports !
How?
Since some servers (like apache) are firewalled with iptables, how can
nmap know wherever
my system run the service with open port, filtered port or doesn't run
it at all ?
--
Nathaniel Hall, GSEC GCFW GCIA
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Gregory Boyce | gboyce@akamai.com
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Akamai Technologies | 617-444-3041
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