File Transfer Protocol (FTP) is a standard network protocol used for transferring files between a client and a server over a computer network, typically the Internet. It was initially defined in the 1970s and has since undergone several revisions and enhancements. FTP operates on the client-server model, where the client initiates the connection and requests file transfers, while the server listens for incoming connections and handles file transfer requests.
FTP utilizes the Transmission Control Protocol (TCP) as its underlying transport protocol, providing reliable and ordered delivery of data. It uses two separate channels to transfer files: the control channel and the data channel. The control channel is used for issuing commands and receiving responses, while the data channel is used for transferring the actual file data.
While FTP is widely used for file transfer, it has certain security vulnerabilities, such as transmitting data in plain text, making it susceptible to eavesdropping and unauthorized access. To address these concerns, secure alternatives like FTPS (FTP over SSL/TLS) and SFTP (SSH File Transfer Protocol) have been developed, which provide encryption and authentication mechanisms to protect data during transit.
Breakdown of the FTP process
Connection establishment: The client establishes a TCP connection with the server using the well-known port number 21. This connection is used for the control channel.
Authentication: After the connection is established, the client needs to authenticate itself by providing a valid username and password to access the FTP server. Anonymous FTP allows users to log in with a default username and sometimes with an email address as the password.
Command exchange: Once authenticated, the client can send various FTP commands to the server over the control channel. These commands instruct the server on what actions to perform, such as listing directory contents, uploading files, or downloading files.
Some common FTP functions and commands
USER: Specifies the username for authentication.
PASS: Specifies the password for authentication.
LIST: Requests a list of files and directories in the current directory.
RETR: Requests the server to send a specific file to the client.
STOR: Sends a file from the client to the server.
DELE: Deletes a file on the server.
MKD: Creates a new directory on the server.
RMD: Removes an existing directory on the server.
Response exchange: After receiving a command, the server processes it and sends a response back to the client over the control channel. These responses include status codes and human-readable messages, indicating the success or failure of the requested action.
Data transfer: For file transfers, a separate data channel is established between the client and the server. The data channel can operate in two modes: active and passive.
Active mode: In active mode, the client specifies an IP address and port number on which it listens for incoming data connections. The server then initiates a connection to the client’s specified address to transfer data.
Passive mode: In passive mode, the server provides the client with an IP address and port number on which it will listen for incoming data connections. The client then initiates a connection to the server’s specified address to transfer data.
Once the data channel connection is established, the file transfer occurs over this channel. The data is divided into smaller units called “packets” and sent between the client and the server.
Termination: After completing the file transfer or executing other commands, the client can close the control channel connection to terminate the FTP session.
FTP supports various file operations, such as uploading (putting) files from the client to the server, downloading (getting) files from the server to the client, renaming files, deleting files, creating directories, and navigating through directory structures. It also allows for resuming interrupted file transfers, setting file permissions, and querying file information.
Common FTP response codes include
200: Command OK.
331: User name okay, need password.
530: Not logged in.
550: Requested action not taken, file unavailable.