[Snort-users] Suspicious ICMP traces

Ofir Arkin ofir at ...949...
Tue Oct 23 09:48:08 EDT 2001


Demetri,

This not seems as a tunneled message. This is simply the echoed
information from the original offending packet, each and every ICMP
Error message carries with it (usually the IP header + 8 data bytes of
the offending packet.

Now, the type of message you are seeing is ICMP Port Unreachable -
Communication Administratively Prohibited. 

According to the trace you provided, IP 12.125.63.42 tried to
communicate with IP 192.168.75.5 (I bet this is a replaced IP, since
192.168.*.* is a reserved class B). From the trace provided we can see
that 12.125.63.42 tried to access port 137 on the target.

Some filtering device between the two prohibited the communication. It
can be a router or any other filtering device (even a firewall
configured to REJECT rather then DROP). This message notified the
sending side that this kind of communication is not allowed.

You can read more on this issue from my research paper "ICMP Usage In
Scanning" available from http://www.sys-security.com. Page 19: "The
Error message indicates that the destination system is configured to
reject datagrams from the sending system. This error is used when
datagrams based on some sort of criteria are being filtered by a
filtering device (firewall/router/other filtering devices) restrictions
or other security
measures. We can conclude that our Destination Host is up and running,
but we cannot reach it, since the filtering device is blocking our
packets, and is instructing us to stop sending datagrams."

I hope this helps you out.

Ofir Arkin [ofir at ...949...]
Founder
The Sys-Security Group
http://www.sys-security.com
PGP CC2C BE53 12C6 C9F2 87B1 B8C6 0DFA CF2D D360 43FA

-----Original Message-----
From: snort-users-admin at lists.sourceforge.net
[mailto:snort-users-admin at lists.sourceforge.net] On Behalf Of Demetri
Mouratis
Sent: ג 23 אוקטובר 2001 8:23
To: snort-users at lists.sourceforge.net
Subject: [Snort-users] Suspicious ICMP traces

Hello.  I'm interested in finding out what this packet trace might
represent.  I've done some reading on the subject and this looks like
some
kind of ICMP tunnel to me.  Specifically, I'm worrried that this might
be
a Loki type tunnel.

I'm not really sure so I thought I'd pass this along for second
opinions.  
One thing that raised my suspicions was that the ICMP packet seems to
contain a UDP datagram within it.  (Or am I jumping the gun on that?)

So, here is the relevant portion of alert:

[**] [1:485:1] ICMP Destination Unreachable (Communication
Administratively Prohibited) [**]
10/21-20:21:24.622037 12.125.63.42 -> 192.168.75.7
ICMP TTL:246 TOS:0x0 ID:64752 IpLen:20 DgmLen:56
Type:3  Code:13  DESTINATION UNREACHABLE: PACKET FILTERED
** ORIGINAL DATAGRAM DUMP:
192.168.75.7:137 -> 216.73.128.3:137
UDP TTL:112 TOS:0x0 ID:50100 IpLen:20 DgmLen:96
Len: 76
** END OF DUMP

I've got maybe 10,000 of these over a few day period.  I'm also seeing
portscans from 192.168.75.7 so I'm pretty sure something is not right
here.

Thanks in advance for any help you can provide.  


---------------------------------------------------------------------
Demetri Mouratis
dmourati at ...3878...


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