Most FTP servers use a common port number 21 by default. However using default port are subjected to many brute force attacks including many breakin attempts. It is a good idea to move FTP port to any other random number above 1024 because ports 1-1024 are reserved ports. It is very easy to change port number
To change Proftp port, open the config file /etc/proftpd.conf and change the port number this line
# Port 21 is the standard FTP port for Proftpd.
Port 3535
For Pure-FTP open the config file /etc/pure-ftpd/pure-ftpd.conf, change port 21 to our desired port
# IP address/port to listen to (default=all IP and port 21).
# Bind 127.0.0.1,3535
Be sure to permit your newly assigned port number through your firewall. Once this is done you can block port 21 using firewall. From now on you will need to mention our new port using ftp client like filezilla.
