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[maze at cela.pl (Maciej Żenczykowski)]

I have the following in my firewall:

-A INPUT -p tcp --dport 12345 -m recent --set
-A INPUT -p tcp --dport ssh -m state --state NEW \
   -m recent --update --seconds 43200 -j ACCEPT

This will only allow SSH from a computer which has telneted to port 12345 
(obviously I use something else) in the past 12 hours (even the simple 
windows telnet will work, it's just important to try to connect). 
Obviously the above could be made more complicated with more than 1 port 
having to be 'telneted' but I doubt it's worth the trouble.

Cheers,
MaZe.