The command needs to be executed in root mode.
1. Add blacklist command
ufw insert 1 deny from 192.168.3.99
Among them, insert 1 is to insert the rule into the first one, otherwise it is just added to the end, and ufw matches the rules in order, and if there is a rule matching, it will stop matching; so if there is a rule that is allowed to pass before, then add the rejected one rules would be invalid.
2. Remove the blacklist command
ufw delete deny from 192.168.3.99
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Original link: https://blog.csdn.net/ypp240124016/article/details/112650322