Sv: function for whitelisting IPs

[dovecot at ptld.com]

> I have found that dynamic IP blocking programs such as sshguard or 
> fail2ban
> are a CPU burden since that table needs to be refreshed as new IPs are 
> added
> or removed so I have stopped using them.

Have you seen ipset?
https://ipset.netfilter.org/

It is built for dynamically adding/remove IP's from a firewall without 
changing a table or rules or reloading the firewall. It holds a hashmap 
in memory of what IP's to block and integrates into the kernel. However 
you have to build your own mouse trap to use it. I don't know of 
anything out of the box that would automatically add IP's to it, i wrote 
my own script that gets fed log lines from rsyslog to do it.