if you use proftpd
you can try my partial conf in /etc/proftpd.conf
#
ServerName "ProFTPd"
ServerType standalone
### Restrictions, set to your needs
ServerIdent on ""
MaxClientsPerHost 2 "Two clients by hostname max"
MaxClientsPerUser 1 "Only one connection per user allowed"
MaxClients 4 "Too many users, please try again later"
MaxHostsPerUser 1 "Only one host per user allowed"
MaxLoginAttempts 1 "You've reached the max. login attempts"
#TransferRate RETR 128
#
Port 21
PassivePorts 32555 32565
TimesGMT off
UseReverseDNS off
#TimeoutLogin 120
TimeoutLogin 20
#TimeoutIdle 600
#TimeoutNoTransfer 900
#TimeoutStalled 3600
TimeoutIdle 300
TimeoutNoTransfer 600
TimeoutStalled 900
#
Other settings with google and main proftpd website