1. Module 4
Remote Login

2. Remote Interactive Applications
Interactive remote computing applications
Users connect to and control remote machines as if they were actually there.
All operating system commands and functions can be utilized when remotely logging in.
Restrictions only exist when displaying video output to a remote terminal (video port redirection).
Users have access to all the commands available on the remote system without needing to provide specialized servers.
Interactive software tends to be more complex since things should be handled in real-time (control character sequences, echo input to the local screen, etc.)
Clients usually override the local interpretation of all keys except the “Escape” key.
This is to allow a user to truly control a remote machine as if they were sitting in front of it.
Problems do arise, however, when client applications fail to map all control commands to the appropriate remote commands.

3. Remote Login
Login to one host and then remote login across the network to any other host

4. TELNET A simple (standard) remote terminal protocol which:
Passes keystrokes.
Carries output to the client.
Provides a transparent service (appears as if we are directly attached to the destination machine).

5. TELNET Protocol TELNET provides three basic services:
Defines the Network Virtual Terminal (NVT) standard interface to remote systems (clients are built to use the standard interface).

6. TELNET provides three basic services:
TELNET Protocol
TELNET provides three basic services:
Provides a set of standard options (7-bit or 8-bit ASCII character sets) and option negotiation.

7. TELNET provides three basic services:
TELNET Protocol
TELNET provides three basic services:
Treats both ends of the connection symmetrically (either program can become a client).
In particular, TELNET does not force client input to come from a keyboard, nor does it force the client to display output on a screen. Thus, TELNET allows an arbitrary program to become a client. Furthermore, either end can negotiate options.

8. Illustration Of TELNET

9. TELNET: Pseudo Terminal
Is an entry point in the operating system (O.S.) that allows a running program (like the TELNET server) to transfer characters to the O.S. as if they came from a terminal.
The O.S. is required to support this, if TELNET is to be an application- level service. 8

10. TELNET:Network Virtual Terminal
Is defined by TELNET to accommodate heterogeneity among computer systems.
It handles different computers and operating systems by defining how data and command sequences are sent across the Internet. 10

11. Accommodating Heterogeneity
TELNET client and server map NVT into local computer’s representation.
We use the term pseudo terrninalt to describe the operating system entry point that allows a running program like the TELNET server to transfer characters to the operating system as if they came from a keyboard. It is impossible to build a TELNET server unless the operating system supplies such a facility. If the system supports a pseudo terminal abstraction, the TELNET server can be implemented with application programs. Each slave server connects a TCP stream from one client to a particular pseudo terminal.

12. TELNET NVT interpretation of USASCII control characters

13. Controlling the Remote Device
NVT keeps control commands separate from normal ASCII character sets
Reason
Greater flexibility
Avoids signal ambiguity
Greater flexibility - transfer all ASCII character sequences as well as all possible control functions.
Avoids signal ambiguity - client specifies the signals, so there is no confusion on how to treat input characters.

14. TELNET NVT Control Functions

15. Controlling the Remote Device
The IAC (Interpret as Command) octet:
Is a reserved byte of information which indicates a control code octet follows in the data stream.
It signifies the beginning of an “escape sequence”
It is denoted by decimal code 255.
When IAC is meant to appear as data, the sender doubles it (IAC-IAC).

16. TELNET Commands

17. TELNET Control Sequences And TCP
TELNET cannot rely on the conventional data stream alone to carry control sequences between client and server
misbehaving application that needs to be controlled might unintentionally block the data stream.

18. TELNET Control Sequences And TCP
Solution: use TCP’s urgent data to send control sequences

19. TELNET Option Negotiation
TELNET terminology, the request is WILL X, meaning will you agree to let me use option X;
The response is either DO X or DON'T X,

20. TELNET Option Negotiation
TELNET uses a symmetric option negotiation mechanism to allow clients and servers to reconfigure the parameters controlling their interaction.

21. TELNET Option Negotiation
Because all TELNET software understands a basic NVT protocol, clients and servers can interoperate even if one understands options another does not.

22. Remote Login
rlogin

23. rlogin Invented for BSD Unix Includes facilities specifically for Unix
Allows manager to configure a set of computers so that if two or more computers have same login id, X, the logins are owned by the same individual
Permits automatic authentication

24. Remote Commands in UNIX:
rsh (remote shell):
Invokes a command interpreter on the remote UNIX machine.
And passes any command line arguments to the command interpreter, skipping the login step completely.
Allows remote execution of a single command 27

25. Remote Commands in UNIX:
rsh does not ask for a password, so it can be used in programs as well as from the keyboard.
This may be a security issue at certain sites, so administrators may not allow executing programs to utilize this command.

26. Secure Remote Login (ssh)
Alternative to TELNET/rlogin
Transport layer protocol with service authentication
User authentication protocol
Connection protocol
Multiplexes multiple transfers
Uses encryption for privacy