Are Linux boxes vulnerable to be used by botnets?

[Matt Graham]

After a long battle with technology, Erich Newell wrote:
> I recommend Single Packet Authentication or Port Knocking for use in
> conjunction with your SSH service.

Single packet authentication: http://www.linuxjournal.com/article/9621
It looks interesting.  However, the program described in that article doesn't 
have an ebuild yet for some reason )-: .

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Matt G|There is no Darkness in Eternity/But only Light too dim for us to see