About FTP

Active FTP: With active FTP,  the client connects to the FTP server using Port 21. When the client requests data, the server initiates the the data transfer back to the client. Data is then transferred back using Port 20 on the server. The client receives the data through a random higher port (greater than 1024) that has been pre-negotiated during the initial Port 21 connection.  Active FTP does not always work if clients are using NAT or behind a firewall. The server doesn't understand which of the many computers under NAT is requesting the data.

Passive FTP: Under passive FTP, the client connects to the FTP server using Port 21.  When the client requests data, the client initiates the data transfer, not the server (unlike active FTP). The client is in charge of the FTP data transfer, not the server. The port of the data transfer on both the client and the server is a higher port which is pre-negotiated during the initial Port 21 connection on the client and on the server. Since the client is in control of the data requests, passive FTP works best when using NAT and when clients are behind a firewall. Windows has a default setting of active FTP.  Linux has a default setting of passive FTP.

Secure FTP (FTPS): When opening a normal FTP connection, the username and password are not encrypted during the FTP session.  Hackers eavesdropping on your network could obtain your FTP login information. Secure FTP (FTPS) provides encryption of the username, password and the entire session. This is accomplished by using the SSL/TLS protocol for channel encryption.

SSH FTP (SFTP): SFTP is an abbreviation of "SSH File Transfer Protocol."  It is not FTP over SSL. For the session, SFTP uses SSH keys for for encryption and access. It may be configured so a password and username aren't required. Instead, an encrypted SSH key is given to the client computer.  The advantage is that a hacker will never guess your username and password.  

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