1. CH 25-Remote Login (TELNET,Rlogin)
Xiaoying Cheng
Yinghsuan Huang
Chunli Yu
Cheng Zhang
Yi Zhang

2. Introduction Remote login: Telnet:
Login to one host and then remote login across the network to any other host
Telnet:
A standard application. It works between hosts that use different operating systems.

3. TELNET Protocol
Allow a user to log into a computer across an internet.
Establish a TCP connection.
Pass keystrokes from the user’s keyboard directly to the remote computer.
Carry output from the remote machine back to the user’s screen.
Transparent: user’s keyboard and display attach directly to the remote machine.

4. TELNET Three Services
1 It defines a network virtual terminal that provides a standard interface to remote systems.
2 Telnet includes a mechanism that allows the client and server to negotiate options, and it provides a set of standard options.
3 Telnet treats both ends of the connection symmetrically.
Thus , telnet allows an arbitrary program to become a client, either end can negotiate options.

5. TELNET Client – Server Model
Telnet server
Login shell
Terminal
driver
TCP/IP
TCP/IP
Pseudo-terminal
driver
TCP connection
User at a
terminal

6. How application programs implement a TELNET client and server
Operating
system
Client sends
To server
Server receives
From client
Server sends to
pseudo terminal
TELNET
client
Operating
system
TELNET
server
Client reads
From terminal
User’s
keyboard
& display
TCP/IP
internet

7. Pseudo terminal
Pseudo terminal describe the OS 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.
Each slave server connects a TCP stream from one client to a particular pseudo terminal.

8. Network Virtual Terminal (NVT)
An imaginary device from which both ends of the connection, the client and the server, map their real terminal to and from.
TCP connection
across internet
Server’s
system
User’s
Keyboard
& display
Client
Server
Client system format
NVT format
Server system format

9. NVT Format 7 – bit USASCII:
Each 7 – bit character is sent as an 8-bit byte with the high-order bit set to 0.
An end-of-line is transmitted as 2-character sequence CR followed by a LF. Such as \r\n
A carriage return is transmitted as 2-character sequence CR followed by a NUL (byte of 0). Such as \r\0

10. Control Functions
NVT as accepting input from a keyboard that can generate more than 128 possible characters. (Figure 25.4)
To pass control functions across TCP connection, TELNET encodes them using escape sequence – IAC to indicate the next byte is the command byte. (Figure 25.5)

11. 25.6 Forcing the Server to Read a Control Function
Method: TELNET uses out of band signal
Problem ? misbehaving application causes server’s buffer to fill up, and block server
Solution? Control function, SYNCH command and data mark causes TCP to send a segment with URGENT DATA bit set to bypass flow control and reach server immediately. Server then will discard all data until data mark and resume normal processing.

12. TELNET options
In TELNET , options are negotiable, the client and server can reconfigure their connection.
i.e. 7-bit or 8-bit data
The range of TELNET options is wide: some extend the capabilities in major ways while others deal minor details.
P. 493, list of Telnet option.

13. 25.8 TELNET Option Negotiation
Symmetric: both of sides
Will X? Do/Don’t X.
Different version communication
If don’t understand request, decline!

14. Trusted Hosts rsh 25.9 Rlogin (BSD UNIX)
administrators may choose a set of “trusted” machines which are shared and establish equivalences among user logins.
One user, different login names in different machines without password.
rsh
One variant rlogin. rsh diamond ls

15. Advantage 25.9 Rlogin (BSD UNIX) cont.
rlogin understand both local and remote computing environments, they communicate better than TELNET.
Unix stdin, stdout.
Re-direction
exports user’s environment to remote machine.

16. Demo rlogin and rsh Telnet: is not Telnet only
Sun lab doesn’t fully support rsh.
Neither rlogin
Telnet: is not Telnet only
telnet different port/protocol
Knowledge of TCP/IP
Being a hacker?

17. Thank You!