TELNET Protocol Overview

The TELNET protocol, designed for terminal-oriented remote login, is documented in RFC 854.

TELNET operates using the TCP Protocol, and depends heavily on option negotiation. TELNET options are documented in their own (usually short) RFCs. The organization of these RFCs, and instructions for registering new TELNET options, is found in RFC 855. Many TELNET options exist; a complete, current list can be found in the Internet Official Standards RFC, currently RFC 2400. A copy of this list appears below.

TELNET Options

Protocol   Name                           Number  State Status  RFC STD
========   =====================================  ===== ====== ==== ===
TOPT-BIN   Binary Transmission                 0  Std   Rec     856  27
TOPT-ECHO  Echo                                1  Std   Rec     857  28
TOPT-RECN  Reconnection                        2  Prop  Ele     ...
TOPT-SUPP  Suppress Go Ahead                   3  Std   Rec     858  29
TOPT-APRX  Approx Message Size Negotiation     4  Prop  Ele     ...
TOPT-STAT  Status                              5  Std   Rec     859  30
TOPT-TIM   Timing Mark                         6  Std   Rec     860  31
TOPT-REM   Remote Controlled Trans and Echo    7  Prop  Ele     726
TOPT-OLW   Output Line Width                   8  Prop  Ele     ...
TOPT-OPS   Output Page Size                    9  Prop  Ele     ...
TOPT-OCRD  Output Carriage-Return Disposition 10  Prop  Ele     652
TOPT-OHT   Output Horizontal Tabstops         11  Prop  Ele     653
TOPT-OHTD  Output Horizontal Tab Disposition  12  Prop  Ele     654
TOPT-OFD   Output Formfeed Disposition        13  Prop  Ele     655
TOPT-OVT   Output Vertical Tabstops           14  Prop  Ele     656
TOPT-OVTD  Output Vertical Tab Disposition    15  Prop  Ele     657
TOPT-OLD   Output Linefeed Disposition        16  Prop  Ele     658
TOPT-EXT   Extended ASCII                     17  Prop  Ele     698
TOPT-LOGO  Logout                             18  Prop  Ele     727
TOPT-BYTE  Byte Macro                         19  Prop  Ele     735
TOPT-DATA  Data Entry Terminal                20  Prop  Ele    1043
TOPT-SUP   SUPDUP                             21  Prop  Ele     736
TOPT-SUPO  SUPDUP Output                      22  Prop  Ele     749
TOPT-SNDL  Send Location                      23  Prop  Ele     779
TOPT-TERM  Terminal Type                      24  Prop  Ele    1091
TOPT-EOR   End of Record                      25  Prop  Ele     885
TOPT-TACACS  TACACS User Identification       26  Prop  Ele     927
TOPT-OM    Output Marking                     27  Prop  Ele     933
TOPT-TLN   Terminal Location Number           28  Prop  Ele     946
TOPT-3270  Telnet 3270 Regime                 29  Prop  Ele    1041
TOPT-X.3   X.3 PAD                            30  Prop  Ele    1053
TOPT-NAWS  Negotiate About Window Size        31  Prop  Ele    1073
TOPT-TS    Terminal Speed                     32  Prop  Ele    1079
TOPT-RFC   Remote Flow Control                33  Prop  Ele    1372
TOPT-LINE  Linemode                           34  Draft Ele    1184
TOPT-XDL   X Display Location                 35  Prop  Ele    1096
TOPT-ENVIR Telnet Environment Option          36  Hist  Not    1408
TOPT-AUTH  Telnet Authentication Option       37  Exp   Ele    1416
TOPT-ENVIR Telnet Environment Option          39  Prop  Ele    1572
TOPT-EXTOP Extended-Options-List             255  Std   Rec     861  32