Fail2ban is not blocking IPv6 on the current version Fail2ban on Debian Stretch.
You can upgrade to the Fail2ban from testing branch and enable IP6 version blocking.
See here Fail2ban not blocking IPv6
Fail2ban is not blocking IPv6 on the current version Fail2ban on Debian Stretch.
You can upgrade to the Fail2ban from testing branch and enable IP6 version blocking.
See here Fail2ban not blocking IPv6