1. Computer Networking From LANs to WANs: Hardware, Software, and Security Chapter 13 FTP and Telnet

2. Computer Networking From LANs to WANs: Hardware, Software, and Security2 Objectives Describe the purpose and operation of the File Transfer and Telnet protocols Show how File Transfer Protocol clients and servers are configured Discuss the various FTP and Telnet commands Identify important security considerations when using FTP and Telnet Show how Telnet clients and servers are configured

3. Computer Networking From LANs to WANs: Hardware, Software, and Security3 FTP Clients and Servers File Transfer Protocol (FTP) –Uses connection-oriented TCP for guaranteed reliability Text file –Contains ASCII text characters Binary file –Contains any number of binary digit combinations FTP transfers files efficiently and reliably among host computers –Allows convenience of remote file storage capabilities

4. Computer Networking From LANs to WANs: Hardware, Software, and Security4 FTP Clients and Servers (cont’d.) FTP client-server application –Client and server both use two ports –FTP server port 20: data transfers –FTP server port 21: control –FTP client: any port above 1024 Figure 13-1 FTP client-server interaction

5. Computer Networking From LANs to WANs: Hardware, Software, and Security5 FTP Commands FTP information exchange –Requests sent by client Command or action to be performed –Responses sent by server Greeting: initial response Last response: request accepted or rejected May consist of multiple lines Last line begins with three ASCII digits (form code)

6. Computer Networking From LANs to WANs: Hardware, Software, and Security6 FTP Commands (cont’d.) Table 13-2 Common FTP server response codes

7. Computer Networking From LANs to WANs: Hardware, Software, and Security7 FTP Clients Is provided for many hardware types and operating systems Anonymous FTP –Allows public access to files stored on the FTP server Authenticated FTP –Requires valid username and password to access private files on the FTP server Starting a Windows FTP client program –Command Prompt window –Run option on the Start menu

8. Computer Networking From LANs to WANs: Hardware, Software, and Security8 FTP Clients (cont’d.) Vendors provide their own FTP client application Figure 13-2(a) Windows Vista Client FTP Commands Figure 13-2(b) Fedora Linux Client FTP Commands

9. Computer Networking From LANs to WANs: Hardware, Software, and Security9 FTP Clients (cont’d.) Sample anonymous FTP session –Userid: the word “anonymous” –Password: e-mail address or the word “guest”

10. Computer Networking From LANs to WANs: Hardware, Software, and Security10 FTP Clients (cont’d.) FTP DIR command lists each file in the directory Table 13-3 UNIX-style file permission codes

11. Computer Networking From LANs to WANs: Hardware, Software, and Security11 FTP Client (cont’d.) Sample authenticated FTP session

12. Computer Networking From LANs to WANs: Hardware, Software, and Security12 FTP Clients (cont’d.) CoreFTP program –Windows application allowing mouse and keyboard for controls –Site Manager window allows FTP client configuration for each server –FTP session established by clicking the Connect button –Offers advantages over Command Prompt based client Kasablanca –Linux FTP client program

13. Computer Networking From LANs to WANs: Hardware, Software, and Security13 FTP Clients and Servers (cont’d.) Figure 13-7 CoreFTP Main window Figure 13-6 CoreFTP Default Site Manager window

14. Computer Networking From LANs to WANs: Hardware, Software, and Security14 FTP Clients and Servers (cont’d.) Figure 13-13 Entering user information into kasablanca, a Linux FTP client Figure 13-14 Linux FTP client kasablanca main file transfer window

15. Computer Networking From LANs to WANs: Hardware, Software, and Security15 Built-In FTP Clients Application programs using the Internet –Provide their own FTP client capability Example: HTML editing programs Figure 13-15 FTP client Site Properties setup

16. Computer Networking From LANs to WANs: Hardware, Software, and Security16 Secure FTP FTP information transmitted in plain text –Not secure Communication between client and server –Requires private and secure transmissions Secure shell (SSH) –Network protocol using encrypted communication channel between network devices Server fingerprint –Information about server SSH key –Enables encrypted communications channel

17. Computer Networking From LANs to WANs: Hardware, Software, and Security17 FTP Servers Operate on server class computer systems (typically) Installed as a service –Ports 20 and 21 by default –FTP server started automatically when system booted –Gracefully stopped when system shut down Responsibilities –Allows server access –Authenticates users (including anonymous FTP) –Provides server file structure access –Sets file transfer parameters

18. Computer Networking From LANs to WANs: Hardware, Software, and Security18 Windows FTP Server Figure 13-24 Windows Server 2008 Internet Information Services Microsoft Internet Information Services program (IIS) –Provides FTP service Record FTP server activity into a log file –Track FTP server use

19. Computer Networking From LANs to WANs: Hardware, Software, and Security19 Linux FTP Server FTP server package installed with Linux (typically) Services window will list FTP service Example: vsftpd Figure 13-29 vsftpd running Service Configuration window

20. Telnet Clients and Servers Connect to remote system easily –Perform user or administrator functions with little effort Telnet protocol (RFC 854) –Bidirectional, byte-oriented service using TCP transport (reliable message delivery) –Uses the client-server model Network Virtual Terminal (NVT) –Imaginary “reference terminal” Written to the set of published standards –Telnet translates NVT characteristics to a real terminal device (vice versa) Computer Networking From LANs to WANs: Hardware, Software, and Security20

21. Computer Networking From LANs to WANs: Hardware, Software, and Security21 Telnet Clients and Servers (cont’d.) Real physical terminals have features not in NVT –DEC VT320 terminal, IBM 3270 terminal Not compatible Figure 13-31 Telnet NVT implementation

22. Computer Networking From LANs to WANs: Hardware, Software, and Security22 Telnet Clients and Servers (cont’d.) Telnet protocol –Certain codes recognized and processed NVT –Defines standard control codes, nonprinting character functions Manufacturer specific codes may not be supported –Vendors provide extra functionality Negotiation process –Allows Telnet applications to accept or reject a specific option Commands: DO, DONT, WILL, WONT

23. Telnet Clients Built-in Telnet client –Provided when TCP/IP protocol suite is installed –Runs directly from Command Prompt or Start menu Run option –Linux: runs from Terminal session Authenticating Telnet session –Valid username and password required to gain remote system access –Cannot login to root or administrator account After authentication –Enter any valid command on the remote computer Computer Networking From LANs to WANs: Hardware, Software, and Security23

24. Computer Networking From LANs to WANs: Hardware, Software, and Security24 Telnet Clients (cont’d.) Telnet application programs available –PuTTY: free Telnet/SSH Telnet client offers option to log session activity to a file –Problems associated with Telnet often related to keyboard Figure 13-39 PuTTY Keyboard Preferences

25. Computer Networking From LANs to WANs: Hardware, Software, and Security25 Secure Telnet Communications Telnet communication between client and server –Not secure –Password displayed in plain text when using a packet sniffer SSH encryption –Circumvents unauthorized capture or viewing of Telnet interactions If the server supports SSH encryption –Client-server communication becomes encrypted –Uses SSH server key

26. Computer Networking From LANs to WANs: Hardware, Software, and Security26 Telnet Servers Available for most hardware and operating system platforms Microsoft non-server versions of Windows computers –Require third party Telnet server Windows Server operating systems –Contain built-in Telnet server –Installed when TCP/IP installed Telnet server service uses port 23 (default) –Service disabled when Telnet communication not allowed

27. Computer Networking From LANs to WANs: Hardware, Software, and Security27 Telnet 3270 Required for use on most IBM mainframe computers Telnet 3270 protocol (TN3270) –Described by RFC 1576 IBM terminal environment architecture –Standard NVT does work (easily) IBM provides 3270 server, client applications Telnet 3270 not very common nowadays –Should still be aware of TN3270 details

28. Computer Networking From LANs to WANs: Hardware, Software, and Security28 Troubleshooting Techniques FTP file types –Differences between text, binary files cause problems Only ASCII file type can be transferred without corruption in ASCII mode Directory navigation –Using a text based FTP product Necessary to learn directory tree navigation Product HELP –‘?’ command –Windows Help and Support options –Linux man (manual pages)

29. Computer Networking From LANs to WANs: Hardware, Software, and Security29 Troubleshooting Techniques (cont’d.) Telnet keyboard mapping –Understand Telnet supported keys Locate supported keys on the keyboard –Many different keyboard mapping issues arise Try to locate solution or compromise

30. Computer Networking From LANs to WANs: Hardware, Software, and Security30 Summary FTP and Telnet –Client-server applications FTP provides capability to copy files between computers –Text file contain ASCII text characters –Anonymous FTP and authenticated FTP Telnet provides bidirectional, byte-oriented service –Remote computer access Server fingerprint SSH key encrypts network communications