Getting started with FTP - The File Transfer Protocol

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FTP or File transfer protocol, is a standard and popular way of uploading and downloading files, between one host to another host, over a TCP based network, such as Internet.

FTP is probably older than Generation X. The first specification for the File Transfer Protocol was published as RFC 114, on 16th April 1971.

A lot of websites makes their files available through FTP. Examples of such websites includes, CentOS, Ubuntu, etc.

FTP may run in active or passive mode, these modes determines how the data connection is established between one host to another.

In both cases, the client creates a TCP control connection from a random unprivileged port to the FTP server command port 21.

In active modes, the client starts listening for incoming data connections from the server, the client sends the FTP command PORT N+1, to inform the server on which port it is listening.

Passive mode can be used, if client is behind a firewall and unable to accept connection from the server.

In this mode, the client uses the control connection to send a PASV command, to the server and then receives a server IP address and port number from the server, which the client, then uses to open a data connection from an arbitrary client port to the server IP address and port number received.

The server responds over 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 for the response from the server.

FTP server may need username and password to allow access to the files it hosts, this is only when server is configured that way, however if server is not configured to accept username and password, anonymous read only access is granted by the server, if allowed.
Anonymous access gives anyone read access to the files hosted on the server. Some companies use FTP to give access to a common storage to their customers so that they can upload files, which can be accessed by the company later or vice versa.

ftp://, represents FTP destinations, compare that to HTTP://, that represents website destinations.

This was introduction to FTP protocol in general. In next chapter, we will see how to setup FTP server on my Ubuntu Linux machine.