As title,
I would like to monitor the banned ip using cacti but I'm not able to monitor it,
I installed the fail2ban template on my cacti instance and I always got a NAN as a result
in graph.
all others graph works well.

What quantifiable attribute about the banned IP are you looking to graph?

_________________

/* TODO: need to add signature to posts */

the numbers of banned ip, what else?

The number of packets from banned IPs, maximum/average length of time IPs have been on the list, breakdown of banned IPs by offense, number of IPs that are about to be banned if they offend once more, etc.

Does the script thing that cacti uses to check fail2ban work if you run it manually? Is it throwing any errors? I've used neither fail2ban nor cacti, but if I had to guess, I'd say that cacti probably doesn't inherently have permission to access fail2ban's data...

_________________

/* TODO: need to add signature to posts */

cacti reads from a fail2ban scripts and cacti has the right persmission to access them.
if runned manually, yes it works.