0blivion nº2:(TELNET.txt):15/03/2000 << Back To 0blivion nº 2
_____________________________________________________ / Oblivion Underground Magazine \ / Issue 2 15/04/2000 \ ▌ Telnet Protocol ▌ \ by Slider / \_____________________________________________________/ - SOF - TELNET TELNET has a status of : recommended. It is described in RFC 854 TELNET Protocol Specifications and RFC 855 TELNET Option Specifications. The TELNET protocol provides a standardized interface, through which a program on one host (the TELNET client) can access the resources of another host (the TELNET server) as though the client were a local terminal connected to the server. For example, a user on a workstation on a LAN may connect to a host attached to the LAN as though the workstation were a terminal attached directly to the host. Of course, TELNET can be used across WANs as well as LANs. TELNET Operation TELNET protocol is based on three ideas: 1. The Network Virtual Terminal (NVT) concept. 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. 2. A symmetric view of terminals and processes. 3. Negotiation of terminal options. The principle of 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* mechanism. The two hosts begin by verifying their mutual understanding. Once this initial negotiation is complete, they are capable of working on the minimum level implemented by the NVT. After this minimum understanding is achieved, they can negotiate additional options to extend the capabilities of the NVT to reflect more accurately the capabilities of the real hardware in use. Because of the symmetric model used by TELNET, both the host and the client may propose additional options to be used. Network Virtual Terminal The NVT has a printer (or display) and a keyboard. The keyboard produces outgoing data, which is sent over the TELNET connection. The printer receives the incoming data. The basic characteristics of an NVT, unless they are modified by mutually agreed options are: - The data representation is 7-bit ASCII transmitted in 8-bit bytes. - The NVT is a half-duplex device operating in a line-buffered mode. - The NVT provides a local echo function. All of these can be negotiated by the two hosts. For example, a local echo is preferred because of the lower network load and superior performance but there is an option for using a remote echo, although no host is required to use it. An NVT printer has an unspecified carriage width and page length. It can handle printable ASCII characters (ASCII code 32 to 126) and understands some ASCII control characters. TELNET Command Structure The communication between client and server is handled with internal commands, which are not accessible by users. All internal TELNET commands consist of 2 or 3-byte sequences, depending on the command type. The Interpret As Command (IAC) character is followed by a command code. If this command deals with option negotiation, the command will have a third byte to show the code for the referenced option. TELNET Basic Commands The primary goal of the TELNET protocol is the provision of a standard interface for hosts over a network. To allow the connection to start, the TELNET protocol defines a standard representation for some functions. To see more about Telnet and other protocols view Oblivion mag, or wait for it to appear on the site. Slider. - EOF -