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Re: nmap sending encapsulated packets
From: David Fifield <david () bamsoftware com>
Date: Wed, 2 Apr 2008 18:51:24 -0600

On Wed, Apr 02, 2008 at 06:51:28PM -0500, Kris Katterjohn wrote:
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Mike Lude wrote:
Now with almost any scan I do (for example, 
nmap -T Aggressive -O -v 192.168.155.22) 
it says that it can't find the host, and when I add the suggested -PN 
parameter it lists all ports as filtered, even though I have a 
perfectly accessible webserver running on the host to be scanned.

So, I break out wireshark to see what's going on, and trace what nmap 
is sending and what I am receiving at the host. Every single outgoing 
packet is encapsulated, with a protocol of 0xFF. Here's a hex dump of 
the first packet sent:

0000  9c f4 20 00 03 00 03 00  03 00 00 00 08 00 45 00   
0010  00 3c 94 91 00 00 80 ff  ed 8c c0 a8 9b 3d c0 a8   
0020  9b 16 45 00 00 28 71 62  00 00 2c 06 65 c9 c0 a8   
0030  9b 3d c0 a8 9b 16 d3 0d  00 50 79 f3 1a 0a 00 00   
0040  42 ba 50 10 04 00 4a 1a  00 00                     

This matches exactly with what I see on the host being scanned.


Hmm.. I don't know; hopefully somebody else on this list can say
something definitive.

Not just an unrecognized protocol, but if I'm not mistaken protocol 0xFF
should never actually be sent over a network.  I think the old hack
(1980's) for sending raw IP packets w/headers involved patching the
kernel and setting the socket() protocol field to 0xFF since it should
never be used for anything.

Wow, Kris, thanks for refreshing my memory. I've seen this before. I see
it when I use --send-ip on Windows. Mike, are you using that option?
Nmap prints a warning in that case. If you're not, it seems Nmap is
acting as if you are, and we need to check it out.

255 is the numerical value of IPPROTO_RAW, which is usually a special
code that means to create a raw socket. I guess when Microsoft disabled
raw sockets they just stopped giving this code its magical meaning, so
it's interpreted literally as 255.

David Fifield

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