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Re: MacOS X 10.6.4 - pcap reception via NSE not working?
From: David Fifield <david () bamsoftware com>
Date: Thu, 19 Aug 2010 22:35:01 -0600

On Thu, Aug 19, 2010 at 09:03:42PM -0500, Kris Katterjohn wrote:
On 08/19/2010 08:02 PM, David Fifield wrote:
I spent most of today looking into this. It appears to be another change
in OS X BPF.


I can reproduce it with NSE and Nping. The common element is Nsock's
pcap capture. What's happening is that packets are being captured in
non-blocking mode, but they are not being flushed to user space until a
lot of them have been buffered in the kernel. I attached a test program
that illustrates this. If you run it as it is, it will not print out any
captures until you create a lot of traffic. If you comment out the call
to pcap_setnonblock, you will see captures right away.

The reason that port scans still work is that scan_engine.cc doesn't
call pcap_setnonblock, instead using a short timeout on platforms where
pcap descriptors are not selectable.

I found that removing pcap_set_nonblock and reducing the timeout in
Nsock makes NSE and Nping work. Another option is to set the
BIOCIMMEDIATE ioctl as described in the linked page. I'll look more into

Doug was suggesting using BIOCIMMEDIATE before (early 2007), but for a
different reason[1].  I had made some replies to his post, but I don't think
it was brought up again.

Perhaps using this is the way to go since not using it could cause problems in
the future: maybe other systems will behave differently as Guy Harris
describes they're supposed to be doing.  And with luck maybe we can still get
a performance boost like Doug described.

[1] http://seclists.org/nmap-dev/2007/q1/174

Thanks for finding that reference. I committed a change to use
BIOCIMMEDIATE whenever it is defined. This makes qscan.nse and Nping
work for me again.

This change only affects pcap in Nsock, so it won't have any performance
impact on the normal scanning engine. That will have to be handled
separately if someone wants to tackle it.

David Fifield
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