ProFTPD is a very powerful, well documented open source FTP server.
It includes advanced functionalities like chroot, virtual hosts and users, accountancy and quotas. ProFTPD uses access and configuration syntax that is similar to the syntax used by Apache HTTP Server. ProFTPD allows the use of a MySQL database to manage FTP accounts in lieu of management by the machine’s users. Its architecture is modular, which has made it possible for extensions to be written for SSL/TLS encryption support (FTPS protocol) and authentication via RADIUS, LDAP and SQL databases.
It is available under the GPL.
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