We have greatly improved support for FTP in our ETL software, fists of all support for SSL was introduced which makes FTP secure plus the log for FTP is more detailed so it is much easy now to resolve connection problems.
What is FTP
How FTP works
A client makes a connection to the server on TCP port 21. This connection, called the control connection, remains open for the duration of the session, with a second connection, called the data connection, on port 20 opened as required to transfer file data. The control connection is used to send administrative data (i.e. commands, identification, passwords). Commands are sent by the client over the control connection in ASCII and terminated by a carriage return and line feed. For example "RETR filename" would transfer the specified file from the server to the client. Due to this two port structure, FTP is considered out-of-band, as opposed to an in-band protocol such as HTTP.
The server responds on the control connection with three digit status codes in ASCII with an optional text message, for example "200" (or "200 OK.") means that the last command was successful. The numbers represent the code number and the optional text represent explanations (i.e. <OK>) or needed parameters (i.e. <Need account for storing file>). A file transfer in progress over the data connection can be aborted using an interrupt message sent over the control connection.
FTP can be run in active mode or passive mode, which control how the second connection is opened. In active mode the client sends the server the IP address port number that the client will use for the data connection, and the server opens the connection. Passive mode was devised for use where the client is behind a firewall and unable to accept incoming TCP connections. The server sends the client an IP address and port number and the client opens the connection to the server.
FTP has no encryption tools meaning all transmissions are in clear text; user names, passwords, FTP commands and transferred files can be read by anyone sniffing on the network. This is a problem common to many Internet protocol specifications written prior to the creation of SSL, such as HTTP, SMTP and Telnet. The common solution to this problem is to use either SFTP (SSH File Transfer Protocol), or FTPS (FTP over SSL), which adds SSL or TLS encryption to FTP.
A host that provides an FTP service may additionally provide anonymous FTP access. Users typically login to the service with an 'anonymous' account when prompted for user name. Although users are commonly asked to send their email address in lieu of a password, no verification is actually performed on the supplied data.