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Quick tool : A minimalistic Telnet library

, 6 Jun 2007 CPOL
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Send commands to your servers from your programs using the Telnet protocol


This article provides a minimalistic Telnet interface. Possible applications of this class include :

  • A Telnet console
  • A Windows GUI around some UNIX commands that need to be executed on a server
  • A program that executes scripts (scripted telnet)
This example offers both a minimal console, and a possibility to use a script, using file piping.


A little while ago one of my clients had the problem that a person who was managing a system using UNIX scripts (HP-UX) was leaving the company. This person was not to be replaced by a new employee, so his functions would have to be done by several other persons.

When estimating the cost of teaching the other people how to use UNIX, we made some calculations, and found a much cheaper solution: we built a simple Windows program that contained a GUI for every script used.

The user simply has to press some buttons, select some values and press the "Start" button. When the user presses the start button the UNIX commands are built using the parameters from the GUI, and sent to the server using Telnet.

Since I remember spending quite a few hours figuring out the minimal implementation of the Telnet protocol, I decided to do a simple rewrite for the users of codeproject.

Using the code

The class is actually quite simple to use. Please take a look at the example code :

//create a new telnet connection to hostname "gobelijn" on port "23"
TelnetConnection tc = new TelnetConnection("gobelijn", 23);

//login with user "root",password "rootpassword", using a timeout of 100ms, 
//and show server output
string s = tc.Login("root", "rootpassword",100);

// server output should end with "$" or ">", otherwise the connection failed
string prompt = s.TrimEnd();
prompt = s.Substring(prompt.Length -1,1);
if (prompt != "$" && prompt != ">" )
    throw new Exception("Connection failed");

prompt = "";

// while connected
while (tc.IsConnected && prompt.Trim() != "exit" )
    // display server output

    // send client input to server
    prompt = Console.ReadLine();

    // display server output


I suppose this is self-explanatory, isn't it ? Create the Telnet connection, login, send the login output to the screen, and while connected send serveroutput to the screen, read a command from the commandline, and again, send the serveroutput to the screen.
If the command is "exit" then the loop needs to finish.

Instead of reading the input from the console, the input can also be piped from a script: if the script was named telnetstuff.txt then execute the script as follows :

MinimalisticTelnet < telnetstuff.txt > output.txt

The result will be inside the file output.txt. Currently the servername, port, username and password is hardcoded, but it should be a piece of cake to change this into command line parameters.

How does it work

The TelnetConnection parses every byte received from the TcpClient, and if a byte series is a Telnet option request (DO, DONT, WILL, WONT), the client simply responds that the option is not available by sending (DONT, WONT) in return.

The only exception to these responses is the command SGA: suppress go ahead, since this command allows async traffic.

Points of Interest

The TelnetConnection.Read function assumes that if there is no data available for more then TimeOutMs milliseconds, the output is complete.

The login works by parsing the server output after the initial connection. It will look for a colon ":" in the screen output, and will send username and password after the colon.

To check if the connection succeeded, I look for whether the serveroutput ends with a "$" or a ">". If your Telnet server has another prompt, please replace this.

Votes & Comments

The votes & comments on my previous article gave me the motivation to publish a new article, so please keep your votes & comments coming !!


  • 2007-06-06: First version


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Tom Janssens
Founder Core bvba
Belgium Belgium
Tom Janssens, owner of Core, a software and consultancy company.
Father of two sons named Quinten & Matisse, and married to a beautiful woman named Liesbeth.

Comments and Discussions

QuestionThis throws an exception when trying to connect with the tcp socket PinmemberMember 1085520730-May-14 9:51 
QuestionSending commands to Cisco switch? [modified] PinmemberMember 1078158729-Apr-14 5:41 
QuestionTelnet over Telnet session PinmemberGabrielBarros26-Mar-14 10:15 
GeneralMy vote of 5 Pinmemberrcole1173-Jan-14 6:52 
QuestionA minimalistic Telnet library is not working for web application Pinmemberwskumar28-Nov-13 12:05 
QuestionMissing when Read output Multiline " ---more--------" PinmemberKieu Gol24-Nov-13 8:19 
GeneralSimple and Straight Forward PinmemberRichard Osafo24-Oct-13 4:48 
Questiontset: unknown terminal type network Terminal type? Pinmembersuperselector11-Aug-13 23:40 
QuestionThanks Tom, you deserve well the five star rating. PinmemberJane-Jane Doe30-Jul-13 0:35 
AnswerRe: Thanks Tom, you deserve well the five star rating. PinmemberTom Janssens30-Jul-13 0:38 
Questionhelp me . Pinmemberstudent_milworm14-Jul-13 22:52 
QuestionVery easy to use! Pinmemberalexdhu1-Jul-13 7:02 
GeneralMy vote of 5 PinmemberJon Sagara6-Jun-13 5:51 
GeneralRe: My vote of 5 PinmemberTom Janssens1-Jul-13 21:38 
QuestionIssue with login [modified] PinmemberTadejP8124-May-13 10:50 
QuestionCannot download Pinmemberitiwcsingkaww22-May-13 16:28 
Questionhow to i get a serial number of a device using the ip address through telnet using this code Pinmemberjinnah201328-Apr-13 9:24 
QuestionThanks and a little Question PinmemberMember 999687125-Apr-13 6:29 
QuestionThank You. PinmemberMember 999795118-Apr-13 18:19 
AnswerRe: Thank You. PinmemberTom Janssens18-Apr-13 21:53 
QuestionReading more than one line at a time Pinmemberslopesandsam9-Apr-13 20:45 
AnswerRe: Reading more than one line at a time PinmemberTom Janssens9-Apr-13 21:46 
GeneralRe: Reading more than one line at a time Pinmemberslopesandsam9-Apr-13 21:54 
GeneralMy vote of 5 Pinmembernieljake18-Jan-13 6:08 
BugBuggy PinmemberMember 96668847-Dec-12 15:32 
QuestionUnable to login Pinmemberrogerknit7-Nov-12 19:12 
AnswerRe: Unable to login PinmemberTom Janssens7-Nov-12 21:04 
QuestionLogin problem [modified] PinmemberLuis Velarde5-Sep-12 14:38 
AnswerRe: Login problem PinmemberLuis Velarde6-Sep-12 6:03 
GeneralRe: Login problem PinmemberTom Janssens6-Sep-12 7:38 
AnswerRe: Login problem PinmemberKieu Gol24-Nov-13 8:22 
QuestionTab Key Pinmemberprithumit28-Aug-12 4:40 
AnswerRe: Tab Key PinmemberTom Janssens28-Aug-12 5:11 
AnswerRe: Tab Key PinmemberEgyptianRobot28-Aug-12 5:47 
QuestionCannot retrieve all the result data PinmemberPhea Daravuth15-Jun-12 0:08 
AnswerRe: Cannot retrieve all the result data PinmemberTom Janssens15-Jun-12 1:46 
Questionusing dot notation for TelnetConnection parameter Pinmembergaraber16-May-12 17:09 
AnswerRe: using dot notation for TelnetConnection parameter Pinmembergaraber16-May-12 17:19 
GeneralRe: using dot notation for TelnetConnection parameter PinmemberTom Janssens17-May-12 21:21 
great Wink | ;)

QuestionLittle Change for write function with new lines - Login now Ok! Pinmemberrenatoosx8-Nov-11 3:13 
QuestionParse problem at login Pinmembersleepyman30-Jun-11 1:48 
AnswerRe: Parse problem at login PinmemberTom Janssens30-Jun-11 2:57 
GeneralRe: Parse problem at login Pinmembersleepyman30-Jun-11 7:35 
GeneralRe: Parse problem at login PinmemberTom Janssens1-Jul-11 0:44 
QuestionProblem executing telnet commands Pinmembersleepyman27-Jun-11 21:01 
AnswerRe: Problem executing telnet commands PinmemberTom Janssens27-Jun-11 21:51 
GeneralRe: Problem executing telnet commands [modified] Pinmembersleepyman27-Jun-11 22:39 
GeneralProblem Pinmemberfares121-May-11 4:58 
GeneralRe: Problem PinmemberTom Janssens22-May-11 3:57 
GeneralMy vote of 5 Pinmembermedophin11-May-11 16:30 

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