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Posted 6 Jun 2007

Quick tool : A minimalistic Telnet library

, 6 Jun 2007
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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 Virtual Sales Lab
Belgium Belgium

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Comments and Discussions

QuestionProblem executing telnet commands Pin
sleepyman27-Jun-11 20:01
membersleepyman27-Jun-11 20:01 
AnswerRe: Problem executing telnet commands Pin
Tom Janssens27-Jun-11 20:51
memberTom Janssens27-Jun-11 20:51 
GeneralRe: Problem executing telnet commands [modified] Pin
sleepyman27-Jun-11 21:39
membersleepyman27-Jun-11 21:39 
GeneralProblem Pin
fares121-May-11 3:58
memberfares121-May-11 3:58 
GeneralRe: Problem Pin
Tom Janssens22-May-11 2:57
memberTom Janssens22-May-11 2:57 
GeneralMy vote of 5 Pin
medophin11-May-11 15:30
membermedophin11-May-11 15:30 
QuestionProblem with commands... Pin
raananv31-Dec-10 2:12
memberraananv31-Dec-10 2:12 
AnswerRe: Problem with commands... Pin
Tom Janssens31-Dec-10 4:50
memberTom Janssens31-Dec-10 4:50 
I'm sorry but this is very hard to do without the server, + I'm not usre whether you use telnet or simply a tcp-ip stream ?

GeneralRe: Problem with commands... Pin
raananv31-Dec-10 5:23
memberraananv31-Dec-10 5:23 
GeneralMy vote of 5 Pin
Kapil Singh Verma30-Dec-10 19:55
memberKapil Singh Verma30-Dec-10 19:55 
GeneralMy vote of 5 Pin
clueponic30-Nov-10 0:25
memberclueponic30-Nov-10 0:25 
GeneralHelpful article Pin
Member 469491012-Oct-10 16:11
memberMember 469491012-Oct-10 16:11 
Generalminilistic telnet Pin
john Saxby21-Sep-10 9:59
memberjohn Saxby21-Sep-10 9:59 
GeneralRe: minilistic telnet Pin
Tom Janssens23-Sep-10 7:18
memberTom Janssens23-Sep-10 7:18 
Generalwow perfect :-) :-) :-) Pin
Member 18269033-Sep-10 20:12
memberMember 18269033-Sep-10 20:12 
GeneralRe: wow perfect :-) :-) :-) Pin
Tom Janssens3-Sep-10 22:01
memberTom Janssens3-Sep-10 22:01 
Questionconsole output message Pin
apfelwein23-Aug-10 23:45
memberapfelwein23-Aug-10 23:45 
GeneralMy vote of 5 Pin
freemap9123-Aug-10 21:16
memberfreemap9123-Aug-10 21:16 
GeneralRe: My vote of 5 Pin
Tom Janssens23-Aug-10 23:27
memberTom Janssens23-Aug-10 23:27 
Generaltail -f Pin
sundalongqueso14-Aug-10 23:30
membersundalongqueso14-Aug-10 23:30 
GeneralRe: tail -f Pin
Tom Janssens15-Aug-10 14:58
memberTom Janssens15-Aug-10 14:58 
GeneralRe: tail -f Pin
sundalongqueso18-Aug-10 12:59
membersundalongqueso18-Aug-10 12:59 
GeneralRe: tail -f Pin
Tom Janssens19-Aug-10 0:59
memberTom Janssens19-Aug-10 0:59 
GeneralConnection failed Pin
jenny1826-Apr-10 16:00
memberjenny1826-Apr-10 16:00 
AnswerRe: Connection failed Pin
Tom Janssens26-Apr-10 17:17
memberTom Janssens26-Apr-10 17:17 

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