1. TELNET
BY , S.AISHWARYA III-IT

2. APPLICATIONS

3. INTRODUCTION
TELNET is a general protocol, meant to support logging in from almost any type of terminal to almost any type of computer.
It allows a user at one site to establish a TCP connection to a login server or terminal server at another site.
A TELNET server generally listens on TCP Port 23.
The TELNET protocol provides a standardized interface, through which a program on one host (the TELNET client) may access the resources of another host (the TELNET server) as though the client were a local terminal connected to the server.

4. REMOTE LOGIN

5. HOW IT WORKS????? telnet xxx .xxx .xxx
That client establishes a TCP connection with the TELNET server on the destination system.
Once the connection has been established, the client program accepts generally  one character at a time, to the TELNET server.
The server on the destination machine accepts the characters sent to it by the client, and passes them to a terminal server.
The terminal server treats the remote user as it would any other user logged in to the system, including relaying commands to other applications.
The terminal server passes outputs back to the TELNET server, which relays them to the client, which displays them on the user's screen.

6. The master server listens for service requests from clients
The master server listens for service requests from clients. When it hears one, it spawns a slave server to handle that specific request, while the master goes back to listening for more requests.
The only thing that makes TELNET hard to implement is the heterogeneity of the terminals and operating systems that must be supported. Not all of them use the same control characters for the same purposes.
To accommodate this heterogeneity, TELNET defines a Network Virtual Terminal (NVT). Any user TELNETting in to a remote site is deemed to be on an NVT, regardless of the actual terminal type being used.

7. NVT

8. NVT
NVT uses two sets of characters, one for data and one for control. Both are 8-bit bytes
An NVT is an imaginary device having a basic structure common to a wide range of real terminals. Each host maps its own terminal characteristics to those of an NVT, and assumes that every other host will do the same.
The negotiated options is used by the TELNET protocol, because many hosts wish to provide additional services, beyond those available with the NVT.
Various options may be negotiated. Server and client use a set of conventions to establish the operational characteristics of their TELNET connection via the ``DO, DON'T, WILL, WON'T'' .

9. NVT CONTROL CHARACTERS

10. NEGOTIATIONS
When in echo mode, each character sent and echoed requires three TCP segments: the character, ACK+character echo, ACK of character echo.

11. Offer to enable an option

12. Request to enable an option

13. SUBOPTION NEGOTIATION

14. NVT character set for sub-option negotiation

15. CONTROLLING THE SERVER

16. Example of interrupting an application

17. OUT-OF BAND SIGNALLING
To make control characters effective in special situations, TELNET uses out-of-band signaling, a technique in which the control characters are preceded by IAC and are sent out of order to the remote process

18. ESCAPE CHARACTER
When a user wants characters interpreted by the client instead of the server, he can use an escape character, normally Ctrl+] (shown as ^]). This means that the command is meant for the client

20. MODES TELNET operates in one of the following modes:
Default mode-Echoing done by the client. Characters sent after the whole line is completed. Client waits for GA from the server before accepts the new line from the user.
Character mode-Line editing (echoing, character
erasing, line erasing) is done by the client. Completed line
is then sent to the server
Line mode-Characters echoed by the server. Delays possible (satellite). Increase traffic(3 segments sent for each character).

21. DEFAULT MODE

22. CHARACTER MODE

23. USER-INTERFACE

24. ADVANTAGES
Accessing Remote Computers: One of the biggest advantages of Telnet software is that it allows remote access to someone else's computer.
Saves Time
Router configuration: Since it uses plain text it is very easy to fix problems. This allows for more access and less problems with data transmission.
Universal: Telnet can be used on any computer. Even older systems can connect to newer machines with different operating system versions.

25. DISADVANTAGES
Telnet is considered insecure because it transfers all data in clear text. Which means if a user was sniffing a network, it's very possible they could grab your username and password as they were being transmitted.
 It is old
It may difficult to use
Users who are concerned about the data being transmitted should consider SSH as opposed to telnet.