Create new jail file in /etc/fail2ban/jail.d/test.local

[bottest]
enabled  = true
maxretry = 4
findtime = 60s
bantime  = 60s
action   = iptables[name=bloKKKED, port=http]
logpath  = %(nginx_access_log)s

This will create a bottest jail and will use a filter defined in /etc/fail2ban/filter.d/bottest.local. Example of regex matching nginx log:

[Definition]
failregex = client=<HOST> .*

ignoreregex =

datepattern = {^LN-BEG}

<HOST> is mandatory and has to match the IP address. Adapt regex to your logging format and lines you need to match.

This will now ban http request for 60 seconds from any client if it tries to access the site on port 80 more than 4 times within 60s. It will create a new iptables chain named f2b-bloKKKED. Truncated iptables -nvL output:

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
   73 66036 f2b-bloKKKED  tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:80

...
Chain f2b-bloKKKED (1 references)
 pkts bytes target     prot opt in     out     source               destination         
    4  5864 REJECT     all  --  *      *       10.21.21.1           0.0.0.0/0            reject-with icmp-port-unreachable
   35 17374 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0           

• fail2ban 0.11.2
• Debian 11 Bullseye

• https://www.fail2ban.org/wiki/index.php/MANUAL_0_8