I have often noticed that while running ftp server with ProFTP there are strange f’s coming with ipadress like FFFF:11.33.44.99

I got this fixed by disabling IPv6 used by Proftp. Just put this one line in your proftp configuration file /etc/proftpd.conf

UseIPv6 off

You can see the changes while analysing the /var/log/secure log file for any login attempts.