FTP File Transfer Protocol EE 122 Intro to Communication Networks Fall 2007 WF 4 5 30 in Cory 277 Vern Paxson TAs Lisa Fowler Daniel Killebrew Jorge Ortiz http inst eecs berkeley edu ee122 Materials with thanks to Jennifer Rexford Ion Stoica and colleagues at Princeton and UC Berkeley 1 File Transfer Protocol FTP Allows a user to copy files to from remote hosts Usage Client connects to the FTP server User provides a login id and password to become authenticated User can explore the directories User can download files from and upload files to the server A predecessor of the Web RFC 959 in 1985 No URL hypertext and helper applications to assist user Requirements for the user o o o o o Know the name of the server machine Have an account on the machine Can find the directory where the files are stored Know whether the file is text or binary Know what tool to run to render and edit the file 2 1 How Do You Transfer Files Today HTTP the usual Web transfer mechanism http FTP You may not have realized that you use it ftp links in web pages e g www kernel org sftp E g to upload your project files to EECS inst machines BitTorrent and other file sharing software scp Any others 3 Why Study FTP Helps cement familiarity with text status code based protocols similar to SMTP Illustrates use of multiple concurrent connections One for control commands replies Another connection for data depending on the command Illustrates reversal of roles For data connection FTP user s process plays the server role FTP server plays the client role We ll later use FTP as an example when looking at issues with Network Address Translation NAT 4 2 Example Commands Authentication USER specify the user name for login PASS specify the user s password Exploring the files LIST list the files for the given file specification CWD change to the given directory Downloading and uploading files TYPE set type to ASCII A or binary image I RETR retrieve the given file STOR upload the given file Closing the connection QUIT close the FTP connection 5 Server Response Codes 1xx positive preliminary reply The action is being started but expect another reply before sending the next command 2xx positive completion reply The action succeeded and a new command can be sent 3xx positive intermediate reply The command was accepted but another command is now required 4xx transient negative completion reply The command failed and should be retried later 5xx permanent negative completion reply The command failed and should not be retried 6 3 FTP Data Transfer Use separate data connection to Send lists of files LIST Retrieve a file RETR Upload a file STOR control data 7 Creating the Data Connection Client acts like a server Creates a socket o Assigned an ephemeral port number by the kernel Listens on socket Waits to hear from FTP server control socket 8 4 Creating Data Connection cont But the server doesn t know the port number After listening client tells it to the server Via the PORT command on the control connection PORT IP address port 9 Creating Data Connection cont Then the server initiates the data connection Connects to the socket on the client machine Client accepts to complete the connection Data now flows along second connection first connection remains open for more commands replies 10 5 FTP Communication Overview Client User Interface User Server File System Server PI tcp 21 Server DTP tcp 20 tcp random Command Connection tcp random Data Connection User PI User DTP File System Protocol Interpreter PI Data Transfer Protocol DTP 11 Why Out of Band Control Avoids need to mark end of data transfer Data transfer ends by closing data connection Control connection stays up Can abort a transfer without killing control connection Avoids requiring the user to log in again Done with an ABOR on the control connection Third party file transfer between two hosts Data connection could go to a different host o Send a different client IP address to the server o e g user coordinates transfer between two servers However Rarely needed and presents security issues 12 6 Example Traffic captured using tcpdump s 0 w ftp trace host ftp ee lbl gov Issue command ftp ftp ee lbl gov 13 What It Looks Like to the User Issue command ftp ftp ee lbl gov Connected ee lbl gov ftptoftp ee lbl gov 220NOTICE TO USERS This computer is a Federal computer system and is the property of the United States Government It is for authorized use only Users authorized or unauthorized have no explicit or implicit expectation of privacy LOG OFF IMMEDIATELY if you do not agree to the conditions stated in this warning Your ip address is 128 32 48 169 The local time is Wed Sep 27 15 04 44 2006 220 131 243 1 10 FTP server ready Name ftp ee lbl gov ee122 anonymous 331 Anonymous login ok send your complete email address as your password 14 7 Password 230 Anonymous access granted restrictions apply Remote system type is UNIX Using binary mode to transfer files ftp pwd 257 is current directory ftp 15 ftp ls bro 200 PORT command successful 150 Opening ASCII mode data connection for file list bro 0 9 current tar gz OLD bro 0 9a7 tar gz bro 0 9a8 tar gz bro 0 9a9 tar gz bro change log txt bro libidmef 0 7 2 modified tar gz bro pub 0 7 stable tar gz bro pub 0 8 stable tar gz bro pub 0 8a87 tar gz bro pub 0 8a88 tar gz bro pub 0 9a4 tar gz bro pub 0 9a4a tar gz 226 Transfer complete remote bro 283 bytes received in 0 0013 seconds 218 37 Kbytes s ftp 16 8 ftp get bro 0 9a9 tar gz TYPO 200 PORT command successful 550 bro 0 9a9 tar gz TYPO No such file or directory ftp get bro 0 9a9 tar gz 200 PORT command successful 150 Opening BINARY mode data connection for bro 0 9a9 tar gz 3440652 bytes 226 Transfer complete local bro 0 9a9 tar gz remote bro 0 9a9 tar gz 3440652 bytes received in 0 81 seconds 4161 86 Kbytes s ftp cd 250 CWD command successful ftp pwd 257 is current directory ftp cd secret files 550 secret files No such file or directory ftp quit 221 Goodbye 17 What It Looks Like On The Wire ftp ftp ee lbl gov Issue command Server sends this text ftp exactly ftp ee lbl gov 220 NOTICE TO USERS This computer is a Federal computer system and is the property LOG OFF IMMEDIATELY if you do not agree to the conditions stated in this warning Your ip address is 128 32 48 169 The local time is Wed Sep 27 15 04 44 2006 220 131 243 1 10 FTP server ready Name ftp ee lbl gov ee122 anonymous Client sends USER anonymous Server replies with exactly this text 331 Anonymous login ok send your complete email address
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