[Snort-users] Bad Loop Back Traffic

Frank Knobbe frank at ...9761...
Tue Feb 24 11:27:06 EST 2004

On Tue, 2004-02-24 at 11:01, bclark at ...10956... wrote:
> I have also seen this type of traffic about 200,000 alerts last night.  I
> am not sure but I think it is a clients Windows machine.

The best summary of an explanation I have seen is attached below (From
Dan of the Incidents list).



-----Forwarded Message----- 
From: Dan Hanson <dhanson at ...35...>
To: incidents at ...35...
Subject: Administrivia: Are you seeing portscans from source
source port 80?
Date: Tue, 28 Oct 2003 08:59:56 -0700

I am posting this in the hopes of dulling the 5-6 messages I get every
that are reporting port scans to their network all of which have a
IP of and source port 80.

It is likely Blaster (check your favourite AV site for a writeup, I
summarize here).

The reason that people are seeing this has to do with some very bad
that was given early in the blaster outbreak. The advice basically was
that to protect the Internet from the DoS attack that was to hit
windowsupdate.com, all DNS servers should return for queries
windowsupdate.com. Essentially these suggestions were suggesting that
hosts should commit suicide to protect the Internet.

The problem is that the DoS routine spoofs the source address, so when
windowsupdate.com resolves to the following happens.

Infected host picks address as source address and sends Syn packet to port 80. (Sends it to itself) (This never makes it on the
you will not see this part)

TCP/IP stack receives packet, responds with reset (if there is nothing
listening on that port), sending the reset to the host with the spoofed
source address (this is what people are seeing and mistaking for

Result: It looks like a host is port scanning ephemeral posts using
packets with source address:port of

Solution: track back the packets by MAC address to find hte infected
machine. Turn of NS resolution of windowsupdate.com to

Hope that helps



Warning at the Gates of Bill:  
Abandon hope, all ye who press <ENTER> here...

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