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Re: hi there !!!
From: Jeff Nathan <jeff () wwti com>
Date: Sun, 13 Jan 2002 13:36:55 -0800

Josh Steele wrote:

There really is no way to make your server "invisible" persay.  You can
use firewalling to block certain ports, etc. but say if you cut off ping
reply's, there is a chance you might affect other service's ran on the
server.  As far as OS detection..there are some steps you can take so that
(never implemented this) so that say you run FreeBSD
4.4-RELEASE, but if you do an OS detection on the machine it reports back
Windows2000, etc.  If you are not really concerned with that, but just
don't want people scanning you, install a IDS system that will notify you,
log, add firewalling, etc. of any scan attempts.

Josh

If you're using ipf, (FreeBSD,NetBSD, Solaris, OpenBSD pre 3.0), you can
use something like this to make it *very* hard to find a system.

(first block IP options and fragments too short to extract a valid
network/transport header from)
block in quick on fxp0 from any to any with ipopts
block in quick on fxp0 from any to any with short

(next block TCP flag combinations used in scanning and OS
identification)
block in quick on fxp0 proto tcp all flags SF/SFRA
block in quick on fxp0 proto tcp all flags /SFRA
block in quick on fxp0 proto tcp all flags F/SFRA
block in quick on fxp0 proto tcp all flags U/SFRAU
block in quick on fxp0 proto tcp all flags P

From that point on in your ruleset, you'd probably pass in connections
to whichever ports you want to run services on.  Making sure you're
careful about when state is established is fairly important as well
(with TCP, ensure you're adding new potential state entries based upon
SYNs).  Beyond that, ensure your firewall passes out acceptable traffic
and keeps state. Explicitly passing traffic in and out if the loopback
interface without state should be done as well. Complete the ruleset
with:

block in log quick all
block out log quick all

Also, the reason an interface is specified within the rules above is to
make outbound nmap scanning possible.

To adapt the rules above to pf (OpenBSD 3.0 and -current) simply drop
the first two rules included above above and replace the explicit drops
with these:

block in quick on fxp0 inet proto tcp all flags SF/SFRA
block in quick on fxp0 inet proto tcp all flags /SFRA
block in quick on fxp0 inet proto tcp all flags F/SFRA
block in quick on fxp0 inet proto tcp all flags U/SFRAU
block in quick on fxp0 inet proto tcp all flags P

-Jeff


On Thu, 3 Jan 2002, arendashu ph wrote:

hi there,,


i am a new member...and i want to ask u if there is a
protection against nmap ...i mean like if someone is
scaning me with nmap ..is there a way to make my
server invisible to nmap ?


        thnak you !!!


PS: please excuse my english !!!

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"Common sense is the collection of prejudices acquired by age eighteen."
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