Computer Networking From LANs to WANs: Hardware, Software, and Security

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

2. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

3. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

4. Figure 13-1 FTP client-server interaction 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

5. 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) Computer Networking From LANs to WANs: Hardware, Software, and Security

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

7. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

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

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

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

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

12. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

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

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

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

16. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

17. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

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

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

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 Security

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

22. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

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 Security

24. Figure 13-39 PuTTY Keyboard Preferences 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

25. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

26. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

27. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

28. 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) Computer Networking From LANs to WANs: Hardware, Software, and Security

29. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security

30. 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 Computer Networking From LANs to WANs: Hardware, Software, and Security