This document discusses several protocols for transferring files and accessing remote files, including FTP, TFTP, and Telnet. FTP allows users to transfer files between computers over the Internet and supports both binary and text file transfers. TFTP is a simpler protocol than FTP that can only be used to send and receive files. Telnet enables users to establish remote command console sessions on servers to run programs and scripts remotely. Secure Shell (SSH) provides encrypted connections for secure remote access.

1. Lecture 7:
File Transfer and
Remote File Access
BITS 2513
INTERNET TECHNOLOGY
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2. Using FTP to Transfer Files
 Web servers (using HTTP) and e-mail software
(using SMTP) must encode data so it appears
as text when it travels over the Internet.
 FTP (File Transfer Protocol) offers an
alternative.
 An FTP site is a computer running an FTP
server application.
 On FTP server a program called daemon allows
to download and upload files
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3. Using FTP to Transfer Files (cont.)
 FTP can transfer binary files over the Internet
without the encoding and decoding overhead,
making it a popular protocol for moving files over
the Internet.
 FTP also use to upload files from your computer
to another computer on the internet
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4. Using FTP to Transfer Files
(cont.)
 Large organizations might maintain several FTP sites in
different parts of the world to speed up download time
across the globe.
 These are called mirror sites.
 A mirror site is a server that contains the same set of
files as a heavily used server to off-load some of the
burden of providing the files to the community using
them.
 Mirror sites also serve as a backup for the main server in
case the main server fails.
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5. How FTP Works
 An FTP server identifies users on an FTP site by their
user IDs.
 FTP client and server software create a session after
you are logged on.
 The FTP client has access to the file system on the
server.
 The local computer (the client) issues character-like
commands, and the remote computer (the server) replies
with numbers that are interpreted by the local computer.
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6. How FTP Works (cont.)
 FTP client and server software can access their
individual file system and transfer files in either
direction
Local host Remote host
File FTP
FTP File
System Client
Server System
6

7. How FTP Works (cont.)
 FTP server usually listen at port 21 for client
activity
 Most Web servers have FTP service running to
receive changes to Web pages files from web
developers
 FTP server normally run on the same computer
that is running a Web server
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8. Requirements for using FTP
 An FTP client installed on the computer.
 Login details of the FTP server to which you want to
connect:
 The FTP server address. This looks a lot like the address you
type in Internet Explorer to browse websites. One such example
is “ftp://ftmk.kutkm.edu.my”.
 The user name and password required for the FTP connection.
Some FTP servers let you connect to them anonymously. For
anonymous connections, you do not need a user name and
password.
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9. FTP Via a Web Browser
 Have you ever attempted to download software
from a Web site and clicked a hyperlink that
says “Click here to download now” or a similar
message?
 If you carefully note the URL after you click to
download the software, you will see that the
protocol changes from http:// to ftp:// in the Web
browser’s Address box.
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10. How to use FTP
 Logging on to an FTP site with a Web browser
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11. How to use FTP
 When you log on to an FTP site, you can browse through the
available files by changing directories, seeing listing of all files and
etc.
Using Windows
Explorer to
access FTP site
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12. How to use FTP
 An error occur when you attempt to upload a file to an
FTP site where you don’t have write permission
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13. FTP from a Command Prompt
 Most operating systems, including Windows 9x,
Windows NT, Windows 2000, and Windows XP,
offer FTP client software that runs from a
command prompt.
 A batch file is a file with a .bat file extension that
contains a list of DOS-like commands that can
be executed as a group.
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14. FTP from a Command Prompt
cont.)
 Another protocol similar to FTP is TFTP (Trivial
FTP).
 TFTP has fewer commands than FTP and can
be used only to send and receive files.
 It can be used for multicasting in which a file is
sent to more than one client at the same time
using the UDP (User Datagram Protocol).
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15. FTP session using
Windows XP FTP
utility
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16. FTP Commands
Opening and closing connection
 ftp - starts an FTP session
 open hostname - connects to the specified host
 close - closes the connection (but not the FTP session!)
 quit - terminates the FTP session
Browsing on a remote machine
 dir - gives a full directory listing on the remote machine
 dir test* - displays only files and directories whose name begins with
"test..."
 ls - same as dir, but provides a simplified listing of filenames
Directories in FTP
 pwd - prints the name of the current remote directory
 cd remote-directory - changes working directory on remote host
 cd .. - moves up one level in the directory structure on the remote host
 lcd directory - changes the default directory on local host
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17. Trivial File Transfer Protocol:
TFTP Usage and Design
 Transfer files between processes.
 Minimal overhead (no security).
 Designed for UDP, although could be used with
many transport protocols.
 Easy to implement
 Small - possible to include in firmware
 Used to bootstrap workstations and network
devices.
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18. Diskless Workstation Booting 1
The call for help
Help! I don't know who I am!
My Ethernet address is:
4C:23:17:77:A6:03
RARP
Diskless
Workstation
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19. The answer from the all-knowing
I know all! You are to be know as:
RARP
128.113.45.211
Server
Diskless
Workstation
RARP REPLY
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20. The request for instructions
I need the file named
boot-128.113.45.211
Diskless
Workstation
TFTP Request (Broadcast)
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21. The dialog
here is part 1
TFTP
I got part 1 Server
here is part 2
Diskless
Workstation
boot file
TFTP File Transfer
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22. TFTP Protocol
5 message types:
 Read request
 Write request
 Data
 ACK (acknowledgment)
 Error
22

23. Messages
 Each is an independent UDP Datagram
 Each has a 2 byte opcode (1st 2 bytes)
 The structure of the rest of the datagram
depends on the opcode.
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24. Message Formats
OPCODE FILENAME 0 MODE 0
OPCODE BLOCK# DATA
OPCODE BLOCK#
OPCODE BLOCK# ERROR MESSAGE 0
2 bytes 2 bytes
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25. Read Request
01 filename 0 mode 0
null terminated ascii string null terminated ascii string
containing name of file containing transfer mode
2 byte opcode
network byte order
variable length fields!
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26. Write Request
02 filename 0 mode 0
null terminated ascii string null terminated ascii string
containing name of file containing transfer mode
2 byte opcode
network byte order
variable length fields!
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27. TFTP Data Packet
03 block # data 0 to 512 bytes
2 byte block number
network byte order
2 byte opcode all data packets have 512 bytes
network byte order except the last one.
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28. TFTP Acknowledgment
04 block #
2 byte block number
2 byte opcode
network byte order
network byte order
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29. TFTP Error Packet
05 errcode errstring 0
null terminated ascii error string
2 byte opcode
network byte order
2 byte error code
network byte order
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30. TFTP transfer modes
 “netascii” : for transferring text files.
 alllines end with rn (CR,LF).
 provides standard format for transferring text files.
 both ends responsible for converting to/from
netascii format.
 “octet” : for transferring binary files.
 no translation done.
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31. NetAscii Transfer Mode
Unix - end of line marker is just 'n'
 receiving a file
 you need to remove 'r' before storing data.
 sending a file
 you need to replace every 'n' with "rn" before
sending
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32. TELNET
32

33. Initiating and Using Telnet
Sessions
 Telnet enable user to;
 create a remote command console session on a host.
 run command line programs, shell commands, and
scripts in a remote command console session just as
though you were locally logged on to the host and
using a local command prompt window.
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34. Telnet
 Telnet is a protocol used to pass commands
and replies between the client the UNIX
computer.
 All UNIX systems support some form of Telnet.
 Windows versions also include Telnet Client and
Telnet Server components.
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35. How Telnet Works
 To use Telnet you need to know IP address of
the host where the resource you want to use
 When you contact the host, the distant computer
and your computer negotiate how to
communicate with each other
 When client and server communicate they use
Telnet protocol
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36. How Telnet Works
 You can start the service manually every time
you want to connect to a host,
 Or you can configure the service so that it starts
every time your computer starts.
 Telnet clients cannot connect to a host unless a
Telnet server program (or service) is running
and listening for connection requests.
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37. How Telnet Works
 When run a Telnet client, it makes a connection request
to the host (Telnet server).
 If a Telnet server responds to the request, the Telnet
client and server negotiate the details of the connection,
such as flow control settings, window size, and terminal
type.
 After the connection details are successfully negotiated,
and logon credentials are validated, the Telnet server
program creates a Telnet command console session.
37

38. Telnet in Windows XP
 Use Run dialog box,
 type telnet mydomain.com
 Use web browser
 Enter telnet://mydomain.com in the address box
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39. Telnet in DOS prompt
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40. Telnet in HyperTerminal
40

41. Examples of TELNET client/server
 Tlntsvr.exe (Telnet Server) - preinstalled in
Windows OS
 Telnet.exe (Telnet Server) – preinstalled in
Windows OS
41

42. Telnet Commands
 open - to open hostname port number to
establish a Telnet connection to a host.
 close - to close an existing Telnet connection
 quit - to exit from Telnet
 status - to determine whether the Telnet client is
connected.
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43. Telnet Limitations
 Telnet is a character-based communication
protocol and not designed to transmit cursor
movements or GUI information.
 Can only run command line programs, shell
commands, scripts, and batch files
 Some editing programs, such as vi and Edit, can
be run over a Telnet connection
 However, these interactive programs are not
true GUI programs because cursor movement is
controlled by the keyboard, not the mouse.
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44. Telnet Limitations (cont.)
 By default, Telnet does not encrypt any data
sent over the connection (including passwords),
and so it is trivial to eavesdrop on the
communications and use the password later for
malicious purposes.
 Lacks an authentication scheme that makes it
possible to ensure that communication is carried
out between the two desired hosts
44

45. Using Secure Shell (SSH) Protocol
 SSH enables two computer to negotiate and
establish secure connection that use encryption
 May stop cracker who try to sniff password and
data
 Provide secure communication for doing email,
accessing Web, login to remote site and
publishing file in FTP (SFTP)
45

46. Using SSH Protocol (cont.)
 It uses public-key cryptography to authenticate the
remote computer and to allow the remote computer to
authenticate the user
 SSH provides confidentiality and integrity of data
exchanged between the two computers using
 encryption
 message authentication codes (MACs).
 PuTTY is a free SSH client and multiplatform - establish
a secure channel between a local and a remote
computer
46

47. Summary
 FTP sites can be accessed by client software,
such as Web browsers, operating system
command utilities, or GUI software dedicated to
FTP, such as WS_FTP Pro.
 FTP also can be accessed from a command
prompt.
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48. Summary (cont.)
 Telnet program runs on the computer and
connects PC to a server on the network.
 Enter commands through the Telnet program
and they will be executed as if you were entering
them directly on the server console.
 Enables you to control the server and
communicate with other servers on the network
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