FTP Client Application CSC 8560 Brian Jorgage 4/27/2004

Basic Approach Install FTP Server locally Observe interaction between command- line FTP and FTP server Read RFC 959 Iterative Development – “Build a little, test a little”

Major Design Issues Application built in Visual Basic 6.0 Winsock control used for socket connections Synchronization between client and server processes is critical Need to understand state machine for Winsock control – based on TCP

Login Login accomplished via USER and PASS commands sent over control connection Beginning of synchronization issues USER sent over control line generates 331 response code from server On receipt of 331 client sends PASS Initially these were implemented as distinct command buttons but later the synch logic was built in Present operation has Winsock.connect kick off whole process

FTP Sockets RFC specifies one control connection and one data connection for FTP – this requires 2 instances of Winsock control Commands sent over control connection (e.g. login, dir, get, put) Data sent over data connection (actual bits and bytes during file transfer) RFC specifies that PORT command sent from client will send connect information to server then Server will initiate the connection PORT command sends parameters h1, h2, h3, h4, p1, p2 H1 thru H4 are just IP address of client P1 and P2 are data connection port # converted to mod 256 (e.g. port 4440 = 17 * ) – thus P1=17, P2=88 Unable to get this working – TCP state is “listening” but connect request never arrives from server Alternate method is to send PASV command to server and wait for response Server sends an available port number and then client initiates the connection Second major synchronization issue – return code from server is 227 On receipt of 227 the data connection is established

Get and Put FTP Get command implemented as RETR command File transfer is not complete until server sends 226 code FTP Put command implemented as STOR 3 rd synch issue – handshake sequence occurs as follows: PASV sent and operation code is set on client (1=LIST, 2=RETR, 3=STOR) Server responds w/227 Client initiates data connection then branches on operation code to send LIST, RETR or STOR Server responds w/150 If operation is Put then parse file and send over data connection

Text mode vs Binary mode Text files and binary files handled differently in FTP Text files require “TYPE A” command Default mode in FTP is text mode Images and executables need to be sent in binary mode Binary files require “TYPE I” command

Navigation Server-side navigation accomplished via CWD command Client process responds to double-click event and parses out directory name then sends “CWD ” Move up to parent directory accomplished via CDUP command Client-side navigation accomplished via CommonDialog control accessed by “Browse” button DIR sent as “LIST” command Server response on data connection is displayed on ListBox control

Managing Connections Managing the state of the data and control lines turned out to be an issue Client needs to close the data connection after file is sent This is detected by the SendComplete event on Winsock for the data connection On SendComplete the data connection is closed by client process Data connection also needs to be closed on completion of LIST If Client process exits before both sockets are closed the application will hang Needed to implement logic to avoid this FTP command to disconnect is QUIT Client process checks state of both lines before exiting If either line is still open client sends a warning message

Raw FTP commands Raw FTP commands collected together under a menu selection This allows Get to be implemented as PASV + RETR menu selections Put can be called as PASV + STOR menu selections Also Dir can be called as PASV + LIST Text mode is “TYPE A” menu Binary mode is “TYPE I” menu PORT command is PORT menu – as stated before this connection method does not work Additional commands SYST, STAT, NOOP, HELP added to menu STOU (store unique) is a variation on STOR (i.e. PUT) that guarantees unique filenames on host – menu option is there but does not work

Remaining Issues Cerberus FTP server sends files in 8K chunks Each chunk generates a message box Message box was used during development but when it was taken out file transfer was incomplete It seems that the server is closing the connection before the last chunk of data is received When msg box is left in it forces the correct sequence to occur Other features to add are Abort, Delete, Mkdir, Rename Determine how to implement connection as PORT command and then retire the use of PASV – default behavior should use PORT