File Transfer & Access (FTP, TFTP, NFS) Chapter 25

File Access & Transfer Many network systems provide file access on remote machines Lots of designs; each optimized for some goals Lower overall cost; use centralized file server Data archive Data sharing by multiple users, programs or sites

On-line Shared Access Two distinct forms of file sharing On-line access Multiple programs can access single file concurrently Changes take affect immediately; available to all Whole-file copying Program obtains local copy to access a file Often used for read-only data If must be modified, changed copy is transferred back

File sharing is usually easy to use Advantage: no change to application programs Disadvantages may be less obvious Network/Remote machine down: application may not work Remote could be overloaded or network congested Application appears unreliable Implementing integrated, transparent access may be difficult File names may be hard to map Mechanism must handle notions of ownership, authorization, and access protection Different file representations may make it difficult or impossible to implement all operations on all files

Sharing by File Transfer Alternative to transparent, integrated, on-line access User obtains local copy; operates on the copy Transfer mechanism not integrated w/file system Special client program used to transfer files Specify remote machine & any login information Client contacts server and asks for copy of file After transfer, user terminates client Have efficient usage of file once have copy

Whole-file transfer can be difficult Client & server must agree on: Ownership, protection, authorization, data format Exact detail of differences in representation techniques depends on the systems involved Information can be lost during translations

FTP – File Transfer Protocol The major TCP/IP file transfer software File transfer one of most used TCP/IP applications Accounts for much network traffic Early ARPANET transfer protocols evolved into current standard Other functions offered beyond file transfer: Interactive interface with remote server File format specification Authentication control

FTP Process Model FTP servers allow concurrent access by multiple clients Clients use TCP to connect Have usual master/slave functions Except, slave not perform all necessary computation Slave handles control connection from the client Carries commands telling which file to transfer Separate processes handle data transfer connection Also uses TCP as the transport protocol

Figure 25.1

TCP Port Number Assignment Multiple connections require multiple ports Control connection: Client: random, locally assigned port Server: uses well-known port (21) For data transfer: Client: unused port; server process: FTP data transfer port (20) Server cannot accept connection from any process Control connection used to communicate port number Client control process gets port for data transfer; creates transfer process to listen; sends number to server over control connection; waits for server to connect to the port Use NVT protocol for data on the control connection

Anonymous FTP FTP contains access authorization facilities Strict enforcement keeps arbitrary users away Anonymous FTP used to access public files Client does not need an account or password Uses login name of anonymous; password guest Have access, but only to public files

ftp> open ftp.cs.purdue.edu Connected to lucan.cs.purdue.edu. 220 lucan.cs.purdue.edu FTP server (Version wu-2.6.2(1) Mon Feb 28 15:45:56 EST 2005) ready. User (lucan.cs.purdue.edu:(none)): anonymous 331 Guest login ok, send your complete e-mail address as password. Password: 230- 230- Purdue University 230- Department of Computer Sciences 230- Access is allowed all day. Local time is Thu Aug 3 12:46:43 2006. 230- All transfers are logged with your host name and email address. 230- If you don't like this policy, disconnect now! 230- If your FTP client crashes or hangs shortly after login, try using a 230- dash (-) as the first character of your password. This will turn off 230- the informational messages which may be confusing your ftp client. 230- Report any problems to postmaster@cs.purdue.edu 230 User ftp logged in. Access restrictions apply.

Remote system type is UNIX. Using binary mode to transfer files. ftp> ls 200 PORT command successful. 150 Opening ASCII mode data connection for /bin/ls. total 14 drwx------ 2 0 0 512 Jan 17 2001 OLD d--x--s--x 2 0 0 512 Jun 15 2000 bin d--x--s--x 2 0 0 512 Jun 15 2000 dev d--x--s--x 3 0 0 512 Apr 3 1997 d--x--s--x 2 0 0 512 Jun 15 2000 lib drwxr-sr-x 57 0 0 1024 May 2 14:00 pub d--x--s--x 2 0 0 512 Apr 3 1997 usr 226 Transfer complete.

ftp> help Commands may be abbreviated. Commands are: ! debug mdir put size $ dir mget pwd status account direct mkdir quit struct append disconnect mls quote system ascii form mode recv sunique bell get modtime reget tenex binary glob mput rstatus trace bye hash newer rhelp type case help nmap rename user cd idle nlist reset umask cdup image ntrans restart verbose chmod lcd open rmdir win close ls prompt runique ? cr macdef proxy send delete mdelete sendport site ftp> quit 221-You have transferred 0 bytes in 0 files. 221-Total traffic for this session was 1476 bytes in 1 transfers. 221-Thank you for using the FTP service on lucan.cs.purdue.edu. 221 Goodbye.

TFTP FTP is the most general file transfer protocol in the TCP/IP suite Also most complex & difficult to program Many applications do not need full functionality Has second file transfer protocol Provides inexpensive, unsophisticated service Trivial File Transfer Protocol (TFTP) Intended for applications with less complex interactions between the client and the server

Secure File Transfer Original FTP has password mechanism Password and data are unencrypted Other protocols have added security Secure Sockets Layer FTP (SSL-FTP) Uses secure socket mechanism Secure File Transfer Program (sftp) Alternative to FTP; uses SSH tunnel Secure Copy (scp) Uses same syntax as Unix copy command Uses SSH

Summary Two forms of remote data access FTP uses whole-file copying Shared on-line access FTP uses whole-file copying Provides transfer and user interface facilities TFTP provides simple alternative for FTP Are secure alternatives to FTP