FTPd

ftpd is an open-source implementation of the File Transfer Protocol (FTP) server. It is designed to be a secure and flexible FTP server that can be used in a variety of environments. The software is typically used to provide users with a way to upload and download files to and from a server using the FTP protocol.

ftpd supports a wide range of features, including support for anonymous logins, account management, and file permission control. It also includes a command-line interface and can be configured using a variety of methods, such as configuration files and script interfaces. The software is typically run in a server environment, but it can also be run as a result of the execution of a system call by a user.

One of the key features of ftpd is its ability to operate in Passive mode or Active mode. When running in passive mode, the client initiates the data transfer and the server opens a listen socket before beginning a data transfer. When running in active mode, the client initiates the data transfer and the server opens a connect socket to client's specified interface. Users can select the mode to be used based on whether a NAT is in place or not.

ftpd has been widely used in Unix-like operating systems. However, it is also available for use with other systems, such as Microsoft Windows. However, users should note that ftpd may have limited functionality on non-Unix systems, as many of its dependencies are written specifically with Unix in mind.