In Unix systems, the "chroot" command changes the apparent root operating system – so after logging in you appear to be working from drive "X:" rather than "C:" for example. Most Unix virgins will run into this most frequently when booting from a Windows system disk in order to rebuild a Windows installation.
You can also use it to set up a "chroot jail," that will let attackers log in, but keep them locked in a harmless and quarantined section of the site. Vancoillie puts his at /etc/ssh/sshd_config; instructions for setting up and useng it are under Modify SSHD_config on this blog page.
A more detailed explanation of how to set up a chroot jail is here.
These tips only apply to Unix servers, of course, and to relatively lightweight, generic automated attacks, not those modified for a particular site, or those for which more than a few zombie PCs have been arrayed to keep probing until they find a weakness.
Windows or other servers have similar functions for root access and the potential to set up a honeypot directory that quarantine attackers who do penetrate.
Vancoillie's example does give a better idea of how some botnets prob for potential targets, looking for easy openings and then moving on if they don't find them.
These precautions are very basic – like locking the door before you leave the house.
Cops will give you the same advice if your house is ever robbed; your security doesn't have to be so tight no thief can ever get in, it only has to be tough enough to give them a little more incentive to try the neighbor's place rather than yours.
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