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The emFTP FTP server is an optional extension which adds the FTP protocol to the stack.

  • Low memory footprint.
  • Multiple connections supported.
  • Independent of the file system: Any file system can be used.
  • Independent of the TCP/IP stack: Any stack with sockets can be used.
  • Project for executable on PC for Microsoft Visual Studio included.




FTP stands for File Transfer Protocol. It is the basic mechanism for moving files between machines over TCP/IP based networks such as the Internet. FTP is a client/server protocol, meaning that one machine, the client, initiates a file transfer by contacting another machine, the server and making requests. The server must be operating before the client initiates his requests. Generally a client communicates with one server at a time, while most servers are designed to work with multiple simultaneous clients.


Free FTP Server for Windows, Mac, Linux
The SEGGER free FTP Server is a free and complete application. It makes it possible to easily evaluate the emFTPServer core.

The Host applications (available for Windows, Linux and MacOS) use the exact same "engine", so the exact same emFTPServer code available for embedded applications. This way they are guaranteed to behave the same when run on a host as when run on an embedded target.

You are more than welcome to use Wireshark to sniff the communication between the server and any FTP client. emFTPServer is compatible with any RFC compliant FPC client, from browsers such as Firefox or Chrome to dedicated FTP Clients such as FileZilla down to simple command line clients such as ftp clients that come with most host operating systems. The provided utilities are for both evaluation as well as "productive use", meaning you are free to use them if you find them useful.

FTP Backgroundsftp-background-server.svg
The File Transfer Protocol (FTP) is an application layer protocol. FTP is an unusual service in that it utilizes two ports, a 'Data' port and a 'CMD' (command) port. Traditionally these are port 21 for the command port and port 20 for the data port. FTP can be used in two modes, active and passive. Depending on the mode, the data port is not always on port 20.

When an FTP client contacts a server, a TCP connection is established between the two machines. The server does a passive open (a socket is listen) when it begins operation; thereafter clients can connect with the server via active opens. This TCP connection persists for as long as the client maintains a session with the server, (usually determined by a human user) and is used to convey commands from the client to the server, and the server replies back to the client. This connection is referred to as the FTP command connection.

The FTP commands from the client to the server consist of short sets of ASCII characters, followed by optional command parameters. For example, the FTP command to display the current working directory is PWD (Print Working Directory). All commands are terminated by a carriage return-linefeed sequence (CRLF) (ASCII 10,13; or Ctrl-J, Ctrl-M). The servers replies consist of a 3 digit code (in ASCII) followed by some explanatory text. Generally codes in the 200s are success and 500s are failures. See the RFC for a complete guide to reply codes. Most FTP clients support a verbose mode which will allow the user to see these codes as commands progress.

If the FTP command requires the server to move a large piece of data (like a file), a second TCP connection is required to do this. This is referred to as the FTP data connection (as opposed to the aforementioned command connection). In active mode the data connection is opened by the server back to a listening client. In passive mode the client opens also the data connection. The data connection persists only for transporting the required data. It is closed as soon as all the data has been sent.

Active Mode FTPembos_ip_ftp_active_mode.svg
In active mode FTP the client connects from a random unprivileged port P (P > 1023) to the FTP server's command port, port 21. Then, the client starts listening to port P+1 and sends the FTP command PORT P+1 to the FTP server. The server will then connect back to the client's specified data port from its local data port, which is port 20.






Passive Mode FTPembos_ip_ftp_passive_mode.svg
In passive mode FTP the client connects from a random unprivileged port P (P > 1023) to the FTP server's command port, port 21. In opposite to an active mode FTP connection where the client opens a passive port for data transmission and waits for the connection from server-side, the client sends in passive mode the ?PASV? command to the server and expects an answer with the information on which port the server is listening for the data connection.

After receiving this information, the client connects to the specified data port of the server from its local data port.


FTP Reply Codes
Every FTP command is answered by one or more reply codes defined in [RFC 959]. A reply is an acknowledgment (positive or negative) sent from server to user via the control connection in response to FTP commands. The general form of a reply is a 3-digit completion code (including error codes) followed by Space , followed by one line of text and terminated by carriage return line feed . The codes are for use by programs and the text is usually intended for human users.

The first digit of the reply code defines the class of response. There are 5 values for the first digit:

  • 1yz: Positive preliminary reply
  • 2yz: Positive completion reply
  • 3yz: Positive intermediate reply
  • 4yz: Transient negative Completion reply
  • 5yz: Permanent negative Completion reply5yz: Permanent negative Completion reply

The second digit of the reply code defines the group of the response.

  • x0z: Syntax - Syntax errors, syntactically correct commands that don't fit any functional category, unimplemented or superfluous commands.
  • x1z: Information - These are replies to requests for information, such as status or help.
  • x2z: Connections - Replies referring to the control and data connections.
  • x3z: Authentication and accounting - Replies for the login process and accounting procedures.x4z: Unspecified as yet.
  • x5z: File system - These replies indicate the status of the Server file system vis-a-vis the requested transfer or other file system action.

The third digit gives a finer gradation of meaning in each of the function categories, specified by the second digit.




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