Tips on FTP Implementation

FTP Model User Interface User Server PI User PI File System Server DTP Commands User Interface User FTP Commands Server PI User PI FTP Replies Port L File System Server DTP User DTP File System Data Connection Port L-1 FTP Server FTP Client PI: Protocol Interpreter DTP: Data Transfer Process Jin Pyo Hong, HUFS

Commands: Minimum Implementation User Cmd arguments FTP Cmd Meaning open <hostname> Connect to FTP server [user] <username> USER User name <password> PASS Password system SYST Show the type of OS running on the remote machine nlist [<pathname>] NLST <pathname> Name list (ls) ls, dir LIST Directory list (ls –l) pwd PWD Print working directory cd CWD Change working directory lcd Change local working directory [type] ascii [type] binary TYPE A I A – NVT-ASCII text I – binary data get RETR Retrieve: download the file put STOR Store: upload the file PASV Set server into passive mode. Server will reply 227 response with its IP and port PORT h1,h2,h3,h4,p1,p2 Data port for FTP client (Set server into active mode) h1,h1,h3,h4: IPv4 address, p1,p2: port number close QUIT Close FTP session bye, quit Close FTP session and exit passive Passive mode on or off (toggle switch) If passive mode on, send PASV for data transfer Otherwise, send PORT (active mode, default) type FTP data type ? debug Debug mode on or off (toggle switch Jin Pyo Hong, HUFS Jin Pyo Hong, HUFS

FTP Replies Code Description 1yz 2yz 3yz 4yz 5yz Positive preliminary reply The command is being acted upon; expect a final reply code before sending another command 2yz Positive completion reply The command was successfully executed; a new command may be sent 3yz Positive intermediate reply The command was accepted, but the final result is being delayed because other information needs to be supplied from the client 4yz Transient negative completion reply The command failed, but the condition is temporary; the user agent should try again 5yz Permanent negative completion reply The command failed and will always fail if given again; the command should not be attempted again Jin Pyo Hong, HUFS

FTP Response Codes 125 Data connection already open; transfer starting. 150 File status okay; about to open data connection. 200 Command okay. 212 Directory status. 213 File status. 220 Service ready for new user. 221 Service closing control connection. 225 Data connection open; no transfer in progress. 226 Closing data connection. 227 Entering Passive Mode (203,253,69,113,201,131) 230 User logged in, proceed. 250 Requested file action okay, completed. 331 User name okay, need password. 350 Requested file action pending further information. 421 Service not available, closing control connection. 425 Can't open data connection. 426 Connection closed; transfer aborted. 450 Requested file action not taken. File unavailable (e.g., file busy). 500 Syntax error, command unrecognized. 502 Command not implemented. 530 Not logged in. 550 Requested action not taken. File unavailable (e.g., file not found, no access). Jin Pyo Hong, HUFS

Open and Closing FTP Sessions User Interface User PI Server PI open <host> or from argv 설계: State transition diagram - Event: User command, Reply msg Action: send FTP command … * reply msg에 대한 state transition도 포함해야 Establish Control Connection 220 Service ready <user name> USER <user name> <password> 331 Need passwd State transition table을 만든다면, if-then-else를 쓰지 않고, table-driven programming할 수 있지 않을까? PASS <password> 230 User logged in Data Transfer Session close bye, quit QUIT Jin Pyo Hong, HUFS 221 Service closing

Data Transfer: get, out, ls - active mode (passive mode off) Server PI Server DTP User Interface User DTP User PI User logged in How to know the listening socket address? - IP addr: control connection socket에서 /* getsockname(controlfd, …); */ - port: 0 - bind(listenfd, …); /* kernel이 사용하지 않는 port를 선택 */ - listen(listenfd, …); - getsockname(listenfd, …); - send PORT command - datafd = accept(listenfd, … ); get/put/ls <file> PORT h1,h2,h3,h4,p1,p2 200 Command Successful RETR/STOR/LIST <file> Establish data connection 150 Opening data connection Data Transfer Close connection 226 Transfer Complete Jin Pyo Hong, HUFS

Data Transfer: get, put, ls - passive mode on Server PI Server DTP User Interface User DTP User PI get/put/ls <file> IP addr, port of server DTP (server’s listening socket addr) PASV 227 Entering Passive Mode (203,253,69,113,201,131) RETR/STOR/LIST <file> Establish data connection 150 Opening data connection Data Transfer Close connection 226 Transfer Complete Jin Pyo Hong, HUFS

FTP command and Response Jin Pyo Hong, HUFS

More Specific Diagram Login sequence Jin Pyo Hong, HUFS Wait for S reply Wait for I reply Wait for P reply Wait for S reply , 3 4, 5 F Login sequence Jin Pyo Hong, HUFS

State Transition Diagram open, user 3 PASS Wait for I reply Wait for S reply 2 NULL 2 NULL open Open control Connection user USER Wait for User Cmd Jin Pyo Hong, HUFS

State Transition Diagram pwd, cd, ascii, binary, system, close 2 Success 1, 3 Error pwd, cd, ascii, binary, system, close PWD, CWD, TYPE A, TYPE I, SYST, QUIT Wait For S reply Wait for User Cmd 4, 5 Failure Jin Pyo Hong, HUFS Jin Pyo Hong, HUFS

State Transition Diagram ls, dir, nlist, get, put ls, dir, nlist Type A 2 PASV/ PORT Wait for User Cmd Wait for S reply - 1 Wait for S reply - 2 get, put PASV / PORT 2 Connect/Listen (LIST/NLIST/RETR/STOR) 2 Close Data Connection 2 Close Data Connection Data Transfer Wait for P reply 1 Pre Reply Jin Pyo Hong, HUFS Jin Pyo Hong, HUFS

Event State Table States Events Wait for user cmd (U_CMD) S reply wait for S reply-3 (S_REPLY3) P reply (P_REPLY) Data transfer (DATA) I reply (I_REPLY) From User Interface open Open control connection USER I_REPLY user nlist, ls, dir TYPE A  S_REPLY1 get, put PASV/PORT  S_REPLY2 pwd,, cd, ascii, binary, system PWD, CWD, TYPE A, TYPE I SYST  S_REPLY close, bye, quit QUIT Close control connection  S_REPLY3 lcd, type, passive, debug Local action  U_CMD From FTP Server 1yz Error Pre. Reply DATA 2yz Ignore Success  S_REPLY2 Connect or listen LIST/NLIST/RETR/STOR  P_REPLY success U_CMD or Exit (bye, quit) Close data connection if not closed Password does not required 3yz PASS S_REPLY 4yz, 5yz failure