FTP

FTP, or File Transfer Protocol, is a standard network protocol used to transfer files between a client and a server over a TCP/IP network. It supports anonymous access and authenticated access, and it has a long history dating to the 1970s. The protocol was formalized in RFC 959 in 1985. FTP defines a control connection that carries commands and replies, and a separate data connection used for transferring file data. The control channel typically uses TCP port 21; the data channel can use port 20 in active mode or a dynamically allocated port in passive mode.

Users authenticate with a username and password, or may log in anonymously. Typical FTP operations include listing directories, uploading files, and downloading files. Common commands include USER, PASS, LIST, RETR (retrieve), and STOR (store). FTP supports various transfer modes and can handle binary and ASCII file types, with additional features such as appending data and resuming transfers.

FTP operates in two modes: active and passive. In active mode, the client opens a port and the server connects from its data port 20 to the client. In passive mode, the server opens a port and the client connects to it. Passive mode is generally more firewall- and NAT-friendly, but requires the server to manage a range of data ports and for those ports to be accessible.

Security considerations are significant. Standard FTP transmits credentials and file contents in plaintext, making eavesdropping and tampering possible. Secure alternatives include FTPS (FTP over TLS/SSL) and SFTP (SSH File Transfer Protocol). SFTP is not an extension of FTP but a different protocol secured by SSH. FTPS adds encryption to FTP, offering improved confidentiality and integrity.

Today, FTP remains supported by many clients and servers, but is often replaced by secure methods for modern file transfer needs.