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Use Fail2Ban on GNU/Linux to block botnet's attacks
Synchronet now have a built-it support to block incomming connections (see Blocking "Hackers") but it's feature protect SBBS services only. If you running another services/daemos on same server, you can use Fail2Ban and the to block at firewall level the ip address from the attacker.
You must config an setup the sbbs log file that are explained here: UNIX
These examples where made on Debian GNU/Linux, but you can adjust and apply for another distribution like Ubuntu, Arch, Fedora, etc.
First step, install fail2ban package:
apt-get install fail2ban