Re: DoS/DDoS Attack

[Nazareno Vicente Feito <nvfeito () advancedsl com ar>]

On Friday 14 January 2005 06:06 am, Faisal Khan wrote:
> Folks,
>
> Two quick questions.
>
> When IP (Source) addresses are spoofed, is there no way of determining (a)
> that the IP Source Addresses is spoofed and not the genuine one (b) to be
> able to determine the actual IP address that is sending DoS packets?
>
> Somehow I get the feeling I'm SOL when trying to find out the
> "genuine/actual" source IP address.
>
> If this is the case, then pretty much we all are helpless with DoS/DDoS
> attacks - considering one can write a script/program to keep incrementing
> or randomly assigning spoofed source addresses in the DoS packets being
> sent out.
>
> Faisal

I can't think of a way of reversing the process, the experiments I've done 
with spoofed ip's have been done in C using raw sockets, some folks tried 
with python, the language is indiferent, but what you do is alter the header 
of the packet, and tell the kernel of the OS that there's no need to add a 
header to the packet you're sending, then the kernel just place the packet on 
the net with the data you filled in. 
The main thing of a spoofed ip packet it's that you can fill the fields with 
any info you want (of course it's important the checksum matches, this is one 
way you could know if the packet is spoofed, and if it's not and the checksum 
does not match, there's an error, so one way or another you should get rid of 
the packet), check this with ethereal or another protocol analyzer.
In theory it should be no way of knowing what's the real source address (It's 
not like an smtp 'spoof' that you play with some rcpt to/mail from commands 
and you have the email headers added by the MTA), if you think about it a 
little bit, we're indeed helpless with DoS/DDoS attacks, if by that you mean 
syn floods and that kind of stuff, and if you dig deeper, you'll find out 
that if the operating system is in charge of stamping the ip address to a 
packet and the OS itself it's sufficiently flexible to let you do that from 
userspace, this is not considered a flaw, but a gift, the main problem is 
that not all people is this gift the way they should.
 

-- 

Saludos.
Nazareno Vicente Feito