http://vsftpd.beasts.org/ — The vsftpd project page is a great place to locate the latest documentation and to contact the author of the software.

http://slacksite.com/other/ftp.html — This website provides a concise explanation of the differences between active and passive mode FTP.

http://www.ietf.org/rfc/rfc0959.txt — The original Request for Comments (RFC) of the FTP protocol from the IETF.