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

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.

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.

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.

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

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

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.

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

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

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)

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.

25.7 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.

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

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

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.

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?

Thank You!