CH 25 Remote Login TELNET Rlogin Xiaoying Cheng

Introduction l Remote login: – Login to one host and then remote login across

TELNET Protocol Allow a user to log into a computer across an internet. l

TELNET Three Services 1 It defines a network virtual terminal that provides a standard

TELNET Client – Server Model Telnet client Terminal driver User at a terminal TCP/IP

How application programs implement a TELNET client and server Client reads From terminal User’s

Pseudo terminal describe the OS entry point that allows a running program like the

Network Virtual Terminal (NVT) l An imaginary device from which both ends of the

NVT Format l 7 – bit USASCII: – Each 7 – bit character is

Control Functions l NVT as accepting input from a keyboard that can generate more

25. 6 Forcing the Server to Read a Control Function l Method: TELNET uses

25. 7 TELNET options l In TELNET , options are negotiable, the client and

25. 8 TELNET Option Negotiation l Symmetric: both of sides – Will X? Do/Don’t

25. 9 Rlogin (BSD UNIX) l Trusted Hosts – administrators may choose a set

25. 9 Rlogin (BSD UNIX) cont. l Advantage – rlogin understand both local and

Demo l rlogin and rsh – Sun lab doesn’t fully support rsh. – Neither

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

Introduction l Remote login: – Login to one host and then remote login across the network to any other host l 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. l – – – l 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 client Terminal driver User at a terminal TCP/IP Telnet server TCP/IP TCP connection Login shell Pseudo-terminal driver

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

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. l Each slave server connects a TCP stream from one client to a particular pseudo terminal. l

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

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

Control Functions l NVT as accepting input from a keyboard that can generate more than 128 possible characters. (Figure 25. 4) l 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 l 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 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 l In TELNET , options are negotiable, the client and server can reconfigure their connection. – i. e. 7 -bit or 8 -bit data l 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 l Symmetric: both of sides – Will X? Do/Don’t X. l Different version communication – If don’t understand request, decline!

25. 9 Rlogin (BSD UNIX) l Trusted Hosts – administrators may choose a set of “trusted” machines which are shared and establish equivalences among user logins. l One user, different login names in different machines without password. l rsh – One variant rlogin. rsh diamond ls

25. 9 Rlogin (BSD UNIX) cont. l Advantage – rlogin understand both local and remote computing environments, they communicate better than TELNET. l Unix stdin, stdout. l Re-direction l exports user’s environment to remote machine.

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