CH 25-Remote Login CH 25-Remote Login (TELNET,Rlogin)(TELNET,Rlogin)

Xiaoying Cheng

Yinghsuan Huang

Chunli Yu

Cheng Zhang

Yi Zhang

IntroductionIntroduction

Remote login:– 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 ProtocolTELNET 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 ServicesTELNET 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 ModelTELNET Client – Server Model

Telnet client

Terminaldriver

TCP/IP

User at a terminal

TCP/IPPseudo-terminal

driver

Telnet server Login shell

TCP connection

How application programs implement a How application programs implement a TELNET client and serverTELNET client and server

Operatingsystem

TCP/IPinternet

TELNETclient

User’s keyboard& display

Operatingsystem

TELNET server

Client sendsTo server

Server receivesFrom client

Server sends to pseudo terminal

Client reads From terminal

Pseudo terminalPseudo 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)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.

User’s Keyboard& display

Client ServerServer’ssystem

Client system format NVT format Server system format

TCP connectionacross internet

NVT FormatNVT 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\nA carriage return is transmitted as 2-

character sequence CR followed by a NUL (byte of 0). Such as \r\0

Control FunctionsControl 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 25.6 Forcing the Server to Read a Control FunctionRead 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 options25.7 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 Negotiation25.8 TELNET Option Negotiation

Symmetric: both of sides – Will X? Do/Don’t X.

Different version communication– If don’t understand request, decline!

25.9 Rlogin (BSD UNIX)25.9 Rlogin (BSD UNIX)

Trusted Hosts– 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

25.9 Rlogin (BSD UNIX) cont.25.9 Rlogin (BSD UNIX) cont.

Advantage– 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.

DemoDemo

rlogin and rsh– 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?