В третьем издании начало 31 главы про телнет выглядит так:
TCP/IP UNLEASHED
Third Edition
Karanjit S. Siyan, Ph.D.
Tim Parker, Ph.D.
Chapter 31
Using Telnet
Although terminal emulation isn’t used as widely today as it once was, it is still a necessary
tool for accessing shell accounts and other multiuser systems. Telnet is also used as the
basis of other protocols such as X-Windows that establish a session with an X-client.
Telnet is the ultimate thin client because all execution takes place on the Telnet server.
Telnet is often used as a troubleshooting tool as well. This chapter discusses the terminal
emulation service of the Internet: the TCP/IP Telnet protocol and its associated programs.
Understanding the Telnet Protocol
Telnet is one of the original TCP/IP protocols. In fact, RFC 15 (written in 1969!) provides
some interesting reading on the subject. Telnet is currently specified in RFC 854.
The Telnet protocol was created to simplify the connection to remote hosts. If you had
an IBM mainframe, you needed IBM terminals to connect to that mainframe. If you had
a DEC, you needed DEC terminals. The result was similar to the diagram in Figure 31.1.
IBM Terminal \
IBM Terminal > IBM Mainframe
IBM Terminal /
DEC Terminal \
DEC Terminal > DEC Mainframe
DEC Terminal /
XYZ Terminal \
XYZ Terminal > XYZ Mainframe
XYZ Terminal /
FIGURE 31.1
An unwieldy network prior to the creation of Telnet.
What was needed was a terminal emulation service that could speak all the proprietary
language sets. This would allow a user to sit at one terminal and work on many disparate
host systems. Enter Telnet.
Telnet is the standard Internet application protocol for logging in to remote hosts. It provides
the encoding rules and other services necessary to link a user’s system with a remote host.
Telnet uses the reliable TCP transport mechanism because it must maintain a reliable, stable
connection. The Telnet server listens, by default on well-known TCP port 23. The Telnet
client can be configured to connect to any other port on which another service is running.
This enables you to use the Telnet client to send commands to a specific application service.
This technique enables Telnet to be used for troubleshooting the application services.
Telnet can operate in any of the following modes:
• Half Duplex
• Character-at-a-time
• Line-at-a-time (a.k.a. Kludge Line Mode)
• Linemode
Half Duplex mode is obsolete.
In Character-at-a-time mode, each character typed is immediately sent to the remote host
for processing and then echoed back to the client. This can be painfully cumbersome
over slow networks, but is a common default with many implementations.
In Line-at-a-time mode, text is echoed locally, and completed lines are sent to the remote
host for processing.
In Linemode, character processing is done on the local system, under the control of the
remote system.
Additional details on Telnet can be found in the following RFCs:
15 Network subsystem for time sharing hosts. C.S. Carr. 1969.
854 Telnet Protocol Specification. J. Postel, J.K. Reynolds. 1983.
855 Telnet Option Specifications. J. Postel, J.K. Reynolds. 1983.
856 Telnet Binary Transmission. J. Postel, J.K. Reynolds. 1983.
857 Telnet Echo Option. J. Postel, J.K. Reynolds. 1983.
858 Telnet Suppress Go Ahead Option. J. Postel, J.K. Reynolds. 1983.
859 Telnet Status Option. J. Postel, J.K. Reynolds. 1983.
860 Telnet Timing Mark Option. J. Postel, J.K. Reynolds. 1983.
861 Telnet Extended Options: List Option. J. Postel, J.K. Reynolds. 1983.
1123 Requirements for Internet hosts—application and support. R.T. Braden. 1989.
1184 Telnet Linemode Option. D.A. Borman. 1990.
1205, 5250 Telnet interface. P. Chmielewski. 1991.
1372 Telnet Remote Flow Control Option. C. Hedrick, D. Borman. 1992.
1408 Telnet Environment Option. D. Borman, Editor. 1993.
1411 Telnet Authentication: Kerberos Version 4. D. Borman, Editor. 1993.
1412 Telnet Authentication: SPX. K. Alagappan. 1993.
1416 Telnet Authentication Option. D. Borman, Editor. 1993.
1571 Telnet Environment Option Interoperability Issues. D. Borman. 1994.
1572 Telnet Environment Option. S. Alexander. 1994.
2066 TELNET CHARSET Option. R. Gellens. 1997.
2217 Telnet Com Port Control Option. G. Clark. 1997.
For some slightly less-serious Telnet information, refer to the following RFCs:
748 Telnet randomly-lose option. M.R. Crispin. 1978.
1097 Telnet subliminal-message option. B. Miller. 1989.
Наиболее вероятным кандидатом выглядит абзац:
Telnet is the standard Internet application protocol for logging in to remote hosts. It provides
the encoding rules and other services necessary to link a user’s system with a remote host.
в котором речь идёт про encoding rules -- правила кодирования -- кодировки, но не про шифрование. Тем более, что предыдущие абзацы были про необходимость уникальных терминалов для каждого вида мейнфреймов, которым пришел на смену универсальный вариант -- Telnet.