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Date: Tue, 19 Apr 2005 14:53:53 -0400
From: Damian Gerow <dgerow@afflictions.org>
To: freebsd-stable@freebsd.org
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Subject: Re: FreeBSD and NMAP
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Thus spake Dominic Marks (dom@helenmarks.co.uk) [19/04/05 07:18]:
: On Tuesday 19 April 2005 12:11, pck wrote:
: > Hi,
: >
: > How can i hide from nmap that my OS is FreeBSD? Is this possible?
: 
: # sysctl -ad | grep random_id
: net.inet.ip.random_id: Assign random ip_id values
: # echo 'net.inet.ip.random_id=1' >> /etc/sysctl.conf

That doesn't hide the OS.  That just makes the IP ID field random.

One way to help:

    echo "net.inet.tcp.drop_synfin=1' >> /etc/sysctl.conf

(Note that you need the "options TCP_DROP SYNFIN" line in your kernel
config.)

Other than that... randomize the packet fingerprint data.  I know there's
been at least one daemon that did this on Linux, as well as a kernel patch
that did the same.  But I'd ask: why?  You're doing a significant amount of
work for very little in return.

  - Damian