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RE: UDP port scanning...
From: "Ofir Arkin" <ofir () packet-technologies com>
Date: Thu, 10 Feb 2000 14:21:15 +0200

Guys you really should read the CHANGES file :P

Fyodor have written there in the CHANGES for BETA 13:

"I made NMAP smarter about detecting filtering during UDP, XMAS , ...."

So i was curious and emailed him, and the answer was that if
above a certain number of ports ( very high, above 1000) no PORT
UNREACHABLE message was received than the UDP protocol must be

If a REJECT rule is defined for a UDP port this is another
indication for a port which is filtered since we will receive an
ICMP error message indicatig this.

A simplier method for checking if the UDP protocol is filtered
is to send a UDP datagram to a port which is certainly closed, aka
not running any service, port 0, port 65535 or which ever port you feel
will do the work and not be obvious for detection, than if you do not
receive the ICMP Port unreachable message from this closed port
you can assume a filtering device is filtering th Traffic with a DROP rule.

Ofir Arkin                      Tel: 972-3-5587001
Security Q&A Manager  Fax: 972-3-5587003
Packet Technologies     http://www.packet-technologies.com
                                   ofir () packet-technologies com

-----Original Message-----
From: antirez [mailto:antirez () invece org]
Sent: Thursday, February 10, 2000 12:35 PM
To: nmap-hackers () insecure org
Subject: Re: UDP port scanning...

On Thu, Feb 10, 2000 at 10:23:42AM +1100, Darren Reed wrote:
I must have written my original email in French - it seems like only 1 in
10 people (or therebaouts) actually understood what I wrote.  Or maybe the
1:10 ratio reflects the number of script kiddies on this list ;-)


In some mail from Simple Nomad, sie said:

Yes but if the firewall or router is simply dropping the packets (common
with filter-based rules) then all UDP ports will show up as open, when
fact they are not.

I think that a 'script kiddies' work arround may be:
if nmap found all UDP ports open print 'Warning, seems that all UDP ports
are open, maybe that outgoing type 3 ICMP or ingoing UDP packets are
filtered, bla bla'.


Salvatore Sanfilippo, Developer, Linuxcare, Inc.
+39.049.8024648 tel, +39.049.8036484 fax
antirez () linuxcare com, http://www.linuxcare.com/
Linuxcare. Support for the revolution.

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