To limit ssh access to a linux box based on originating IP address, edit /etc/hosts.allow:
* You can allow or deny based on ip address, subnet, or hostname.
* List rules in order of most to least specific. The file only gets read until a matching line is found, so if you start with ssdh : ALL : deny, no ssh connections will be allowed.
* You can control access to other tcp wrapped services as well - see the hosts.allow man page for details: http://linux.die.net/man/5/hosts.allow
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