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SMB parallelization
From: Ron <ron () skullsecurity net>
Date: Thu, 23 Sep 2010 23:02:30 -0500

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I'm finishing off a 15-hour day of SMB work, but it's been incredibly productive! This is my final submission for now, 
I hope I can still type coherently. :)

Anyway, I was asked to look into how to parallelize the SMB protocol today. I started by doing a quick and dirty 
prototype in Nmap for testing. Once I was off the clock, I went back and refined my code to make it work nicely. 

Basically, this removed all need for mutexes. Most (consumer) versions of Windows can handle roughly 11 SMB connections 
at once (globally). Once that's exceeded, an identifiable error message comes back. In my patch, if we see that error 
message, we back off for rand(1..10) seconds then try again. The 10th time we get it, we give up with the message, "Too 
many active connections" (the same error Windows itself uses). 

The second issue is that opening the pipe would fail at random. To solve that, I a) put a mutex around the create_file 
call (which seemed to have a race condition of some sort), and b) the rare occasions we DO get the error, we back off 
for 100ms (which is more than enough) and try again - up to 10 times again. I only got the error about 1/50 of the 
time, and never twice in 100ms. 

Me and Brandon did some largescale testing against a /16. Without the patch, it took about 4000s. After applying the 
patch, it's down to about 1000s. That's a 4x speedup! We compared the outputs, and the patched one was about 98% the 
size of the unpatched. The difference there could simply be explained by the number of hosts found - the unpatched scan 
found 28 more hosts. I suggested doing a Levenshtein distance calculation on the two files, but Brandon didn't want to 
spend the rest of the universe's lifespan on the calculation so we decided to say "the files were the same within 
experimental error". 

Anyway, I feel really good about the patch and it actually offers significant speedup over the original code. Let me 
know what you think! 

Ron
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