CSF or Config Server Firewall is a great tool/firewall for your Linux Server if you want to learn how to install CSF on your box please refer to How to install CSF Firewall on your Centos Linux Server article.
Let's see now how you can check whether an IP is blocked on your server by CSF Firewall or not and if it is...how to unblock remove it from the blocked IP's.
Check if the IP is blocked
Unblock an IP address
Finally, once you unblocked the IP in question you need to restart CSF in order for changes to take effect.
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