Re: [Full-Disclosure] msblast DDos counter measures (More Insight Maybe?)

[Vladimir Parkhaev <vladimir@arobas.net>]

Quoting Christopher Lyon (cslyon@netsvcs.com):
> Look at these traces to see what it is doing. Note the source and
> destination ports and addresses.  
> 
> WINDOWSUPDATE.COM set to resolve normally
> 19:48:23.963345 192.168.187.171.1823 > 204.79.188.11.http: S
> 886046720:886046720(0) win 16384
> 
> It is allowed to resolve normally and the source is just what we think.
> 192.168.x.x with the x's random numbers. This is what we all know and
> can prove. 

Yeah, OK. That is a SYN packet.


> 
> 
> WINDOWSUPDATE.COM set to 127.0.0.1
> 19:39:56.131653 localhost.localdomain.http > 192.168.83.210.1269: R
> 0:0(0) ack 68419585 win 0
> 
> Now look at the source, the source is 127.0.0.1 and the destination is
> the 1921.68.x.x with the x's being random numbers. That is what I am
> saying is different. Also note that the dst port is 80. 

Yeah, OK. That is a RST packet! You are confused.

Lemme have a second go at it:
Your box 192.168.187.171 (infected).
You set windowsupdate.com to 127.0.0.1
Your infected box sends SYN to itself (dst=127.0.0.1) port 80,
and randomly selected src in 192.168.x.y range and port. You do 
not see this packet, it does not go on the wire. Next your PC 
replies with a RST packet, the one you posted 
(19:39:56.131653 localhost.localdomain.http > 192.168.83.210.1269: R)
                                                                  ^^^
                                                          RST packet!
because there is webserver listening on port 80 ( if there was, you  would have
seen SYN/ACK packet).



> 
> So, what I am saying is that the syn flood will leave the box but it
> will leave differently then we all think. So, can someone confirm this?
> I have been seeing this in two different environments now.
> 
> 

Sure, I'll confirm:

Packets with src=127.0.0.1 will be droped by routers and firewalls. If you
screw with DNS and windowsupdate.com you will have a lot of RST packets
flying inside your LAN.
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