Slide 1: By: Idania Vargas & Ricardo Vasquez
Slide 2:  A login that allows a user terminal to connect to a host computer via a network or direct telecommunications link, and to interact with that host computer as if the user terminal were directly connected to that host computer.
Slide 3:  Rlogin provides an almost identical process of logging in over a network to the ordinary Linux login. However, there are some security problems with rlogin, and so it has been slowly phased out in favour of telnet.  Most Linux distributions still support rlogin although you usually have to install it because it’s not automatically installed. Like telnet, rlogin requires a ’daemon’ working on the server to control connections.  Rlogin is from Berkeley Unix and was developed to work between Unix systems only, but it has been ported to other operating systems also.
Slide 4: Summary of the data flow from the server to the client.
Slide 5:  Telnet has taken over from rlogin because it provides more secure control over connections. It is also able to handle more complex exchanges of information, such as ‘control characters’. Telnet also enforces conditions on what those making the connection are able to connect to.  Internet services like telnet require a ‘daemon’ program to work. A daemon is a program that stays in the server’s memory, automatically monitoring the system to respond to requests for services.  Telnet is a standard application that almost every TCP/IP implementation provides. It works between hosts that use different operating systems. Telnet uses option negotiation between the client and server to determine what features each end can provide.
Slide 6: How does TELNET work?  User sends keystrokes to the terminal driver (The operating system       accepts them but does not interpret them). Characters are sent to the TELNET client and transformed into a universal character set called the network virtual terminal (NVT) The NVT characters travel through the internet and arrive at the TCP/IP stack (port 23) at the remote machine. The characters are delivered to the operating system and passed to the TELNET server. The TELNET server changes the characters from NVT form to characters the remote machine understands. The characters are sent to a Pseudoterminal driver The Pseudoterminal driver sends the characters to the operating system. The operating system passes the characters to the appropiate application program.
Slide 8: TELNET options  TELNET lets client and server negotiate options before or during the use of the service.  Some options include:  Echo: echo the data received on one side to the other.  Terminal type, terminal speed, and status.  In order to use these options the client and server require “option negotiation”.  Four characters are used for this purpose:  WILL : offers to enable, accepts a request to enable  WONT: rejects request to enable, offers to disable, accepts a request to disable  DO: approves an offer to enable, requests to enable  DONT: disapproves an offer to enable, approves an offer to disable, requests to disable
Slide 9: How does RLOGIN work?  Rlogin uses a single TCP connection between the client and server.  Option negotiation is not required when the operating system on the      client and server are known in advance. The client writes four strings to the server ( a byte of 0, the login name of the user on the client host , the login name of the user on the server host , and the terminal speed. The server responds and may or may not ask for a password. The server sends a request to the client asking for the terminal's window size. The client sends 1 byte at a time to the server and all echoing is done by the server. The operation is simple: everything typed by the user is sent to the server, and everything sent by the server to the client is displayed on the terminal.
Slide 10: References  Data Communications & Networking, 4th Edition, Behrouz Forouzan  Telnet and Rlogin: Remote Login  http://www.uic.rsu.ru/doc/inet/tcp_stevens/telnet.h tm#26_0  Remote Login to Another Linux System Developed by The ’Free Range’ Community Linux Training Centre Project Version 1.0  August 2002. http://www.fraw.org.uk/cltc/