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Telnet Protocol

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Teraterm HyperTerminal Exceed HostExplorer Humor Etc

Telnet is an old protocol that with ssh available probably outlived its usefulness. Still it is widely used.

Currently Teraterm is probably the best client for Windows environment. It  one of the few that understands middle mouse button and have some macro capabilities.  The product has non-intuitive configuration and there is no history of visited sites. but generally functionality is very good.  See special page devoted to it.

Please note that many poor soils reinvent the bicycle in a form of yet another macro language for terminal emulation. Expect is a more universal and often more powerful way of doing the same that frees you from the dependency of somebody design mistakes and the luck of understanding of compiler/interpreter technology .

The telnet command is occasionally used to automate connections to systems in situations in which the rlogin and rsh commands cannot be used. Automating a telnet connection requires the script to pass the login, password, and command to the telnet command to execute. The following is a Korn shell script fragment that automates the telnet session:

  sleep 2
  echo login^M
  sleep 2
  echo password^M
  sleep 2
  echo command^M
  sleep 2
 ) | telnet hostname


The Telnet Protocol by  Peter Burden

The Telnet protocol is often thought of as simply providing a facility for remote logins to computer via the Internet. This was its original purpose although it can be used for many other purposes.

It is best understood in the context of a user with a simple terminal using the local telnet program (known as the client program) to run a login session on a remote computer where his communications needs are handled by a telnet server program. It should be emphasized that the telnet server can pass on the data it has received from the client to many other types of process including a remote login server. It is described in RFC854 and was first published in 1983.

The Network Virtual Terminal

Communication is established using the TCP/IP protocols and communication is based on a set of facilities known as a Network Virtual Terminal (NVT). At the user or client end the telnet client program is responsible for mapping incoming NVT codes to the actual codes needed to operate the user's display device and is also responsible for mapping user generated keyboard sequences into NVT sequences.

The NVT uses 7 bit codes for characters, the display device, referred to as a printer in the RFC, is only required to display the "standard" printing ASCII characters represented by 7 bit codes and to recognise and process certain control codes. The 7 bit characters are transmitted as 8 bit bytes with most significant bit set to zero. An end-of-line is transmitted as the character sequence CR (carriage return) followed by LF (line feed). If it is desired to transmit an actual carriage return this is transmitted as a carriage return followed by a NUL (all bits zero) character.

NVT ASCII is used by many other Internet protocols.

The following control codes are required to be understood by the Network Virtual Terminal.

Name code Decimal Value Function
NULL NUL 0 No operation
Line Feed LF 10 Moves the printer to the next print line, keeping the same horizontal position.
Carriage Return CR 13 Moves the printer to the left margin of the current line.

The following further control codes are optional but should have the indicated defined effect on the display.

Name code Decimal Value Function
BELL BEL 7 Produces an audible or visible signal (which does NOT move the print head.
Back Space BS 8 Moves the print head one character position towards the left margin. [On a printing devices this mechanism was commonly used to form composite characters by printing two basic characters on top of each other.]
Horizontal Tab HT 9 Moves the printer to the next horizontal tab stop. It remains unspecified how either party determines or establishes where such tab stops are located.
Vertical Tab VT 11 Moves the printer to the next vertical tab stop. It remains unspecified how either party determines or establishes where such tab stops are located.
Form Feed FF 12 Moves the printer to the top of the next page, keeping the same horizontal position. [On visual displays this commonly clears the screen and moves the cursor to the top left corner.]

The NVT keyboard is specified as being capable of generating all 128 ASCII codes by using keys, key combinations or key sequences.


The telnet protocol also specifies various commands that control the method and various details of the interaction between the client and server. These commands are incorporated within the data stream. The commands are distinguished by the use of various characters with the most significant bit set. Commands are always introduced by a character with the decimal code 255 known as an Interpret as command (IAC) character. The complete set of special characters is

Name Decimal Code Meaning
SE 240 End of subnegotiation parameters.
NOP 241 No operation
DM 242 Data mark. Indicates the position of a Synch event within the data stream. This should always be accompanied by a TCP urgent notification.
BRK 243 Break. Indicates that the "break" or "attention" key was hit.
IP 244 Suspend, interrupt or abort the process to which the NVT is connected.
AO 245 Abort output. Allows the current process to run to completion but do not send its output to the user.
AYT 246 Are you there. Send back to the NVT some visible evidence that the AYT was received.
EC 247 Erase character. The receiver should delete the last preceding undeleted character from the data stream.
EL 248 Erase line. Delete characters from the data stream back to but not including the previous CRLF.
GA 249 Go ahead. Used, under certain circumstances, to tell the other end that it can transmit.
SB 250 Subnegotiation of the indicated option follows.
WILL 251 Indicates the desire to begin performing, or confirmation that you are now performing, the indicated option.
WONT 252 Indicates the refusal to perform, or continue performing, the indicated option.
DO 253 Indicates the request that the other party perform, or confirmation that you are expecting the other party to perform, the indicated option.
DONT 254 Indicates the demand that the other party stop performing, or confirmation that you are no longer expecting the other party to perform, the indicated option.
IAC 255 Interpret as command

There are a variety of options that can be negotiated between a telnet client and server using commands at any stage during the connection. They are described in detail in separate RFCs. The following are the most important.

Decimal code Name RFC
1 echo 857
3 suppress go ahead 858
5 status 859
6 timing mark 860
24 terminal type 1091
31 window size 1073
32 terminal speed 1079
33 remote flow control 1372
34 linemode 1184
36 environment variables 1408

Options are agreed by a process of negotiation which results in the client and server having a common view of various extra capabilities that affect the interchange and the operation of applications.

Either end of a telnet dialogue can enable or disable an option either locally or remotely. The initiator sends a 3 byte command of the form

	IAC,<type of operation>,<option>

The following patterns of option negotiation are defined.

Sender Sent Receiver Responds Implication
WILL DO The sender would like to use a certain option if the receiver can handle it. The receiver says it can support the option. Option is now in effect
WILL DONT The sender would like to use a certain option if the receiver can handle it. The receiver says it cannot support the option. Option is not in effect.
DO WILL The sender requests that the receiver use a certain option. The receiver says it can support the option. Option is now in effect.
DO WONT The sender requests that the receiver use a certain option. The receiver says it cannot support the option. Option is not in effect.
WONT DONT The sender will not use a certain option. Option disabled. DONT is only valid response.
DONT WONT The sender requests that the receiver does not use a certain option. Option disabled. WONT is only valid response.

For example if the sender wants the other end to suppress go-ahead it would send the byte sequence

The final byte of the three byte sequence identifies the required action.

For some of the negotiable options values need to be communicated once support of the option has been agreed. This is done using sub-option negotiation. Values are communicated via an exchange of value query commands and responses in the following form.

 IAC,SB,<option code number>,1,IAC,SE


IAC,SB,<option code>,0,<value>,IAC,SE

For example if the client wishes to identify the terminal type to the server the following exchange might take place

Client   255(IAC),251(WILL),24
Server   255(IAC),253(DO),24
Server   255(IAC),250(SB),24,1,255(IAC),240(SE)
Client   255(IAC),250(SB),24,0,'V','T','2','2','0',255(IAC),240(SE)
The first exchange establishes that terminal type (option number 24) will be handled, the server then enquires of the client what value it wishes to associate with the terminal type. The sequence SB,24,1 implies sub-option negotiation for option type 24, value required (1). The IAC,SE sequence indicates the end of this request. The repsonse IAC,SB,24,0,'V'... implies sub-option negotiation for option type 24, value supplied (0), the IAC,SE sequence indicates the end of the response (and the supplied value).

The encoding of the value is specific to the option but a sequence of characters, as shown above, is common.

Telnet Negotiable Options

Many of those listed are self-evident, but some call for more comments.

Telnet control functions

The telnet protocol includes a number of control functions. These are initiated in response to conditions detected by the client (usually certain special keys or key combinations) or server. The detected condition causes a special character to be incorporated in the data stream.

The telnet command

On most Unix systems a telnet session can be initiated using the telnet command. Most users simply type

telnet <remote host>
but if the user just types telnet then various options and subcommands are available which can be used to study the behaviour of the session.

Here's an exmaple of a telnet session from scitsc to ccub

bash$ telnet
telnet> toggle options
Will show option processing.
telnet> open ccub
Trying ...
Connected to
Escape character is '^]'.
RCVD do TERMINAL TYPE (don't reply)
RCVD will SUPPRESS GO AHEAD (don't reply)
RCVD will ECHO (reply)
SENT do ECHO (reply)
RCVD do ECHO (reply)
SENT wont ECHO (reply)

UNIX(r) System V Release 4.0 (ccub)

RCVD dont ECHO (don't reply)
login: Login timed out after 60 seconds
Connection closed by foreign host.

These pages were produced to support a communication systems module that is no longer taught. Further communication systems notes are available on-line.

Author : Peter Burden

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[Jan 14, 2018] Using telnet to debug connection problems

Jan 14, 2018 |

Telnet, the protocol and the command line tool, were how system administrators used to log into remote servers. However, due to the fact that there is no encryption all communication, including passwords, are sent in plaintext meant that Telnet was abandoned in favour of SSH almost as soon as SSH was created.

For the purposes of logging into a remote server, you should never, and probably have never considered it. This does not mean that the telnet command is not a very useful tool when used for debugging remote connection problems.

In this guide, we will explore using telnet to answer the all too common question, "Why can't I ###### connect‽".

This frustrated question is usually encountered after installing a application server like a web server, an email server, an ssh server, a Samba server etc, and for some reason, the client won't connect to the server.

telnet isn't going to solve your problem but it will, very quickly, narrow down where you need to start looking to fix your problem.

telnet is a very simple command to use for debugging network related issues and has the syntax:

telnet <hostname or IP> <port>

Because telnet will initially simply establish a connection to the port without sending any data it can be used with almost any protocol including encrypted protocols.

There are four main errors that you will encounter when trying to connect to a problem server. We will look at all four, explore what they mean and look at how you should fix them.

For this guide we will assume that we have just installed a Samba server at and we can't get a local client to connect to the server.

Error 1 - The connection that hangs forever

First, we need to attempt to connect to the Samba server with telnet . This is done with the following command (Samba listens on port 445):

telnet 445

Sometimes, the connection will get to this point stop and hang indefinitely:

telnet 445

This means that telnet has not received any response to its request to establish a connection. This can happen for two reasons:

  1. There is a router down between you and the server.
  2. There is a firewall dropping your request.

In order to rule out 1. run a quick mtr to the server. If the server is accessible then it's a firewall (note: it's almost always a firewall).

Firstly, check if there are any firewall rules on the server itself with the following command iptables -L -v -n , if there are none then you will get the following output:

iptables -L -v -n
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

If you see anything else then this is likely the problem. In order to check, stop iptables for a moment and run telnet 445 again and see if you can connect. If you still can't connect see if your provider and/or office has a firewall in place that is blocking you.

Error 2 - DNS problems

A DNS issue will occur if the hostname you are using does not resolve to an IP address. The error that you will see is as follows:

telnet 445
Server lookup failure:, Name or service not known

The first step here is to substitute the IP address of the server for the hostname. If you can connect to the IP but not the hostname then the problem is the hostname.

This can happen for many reasons (I have seen all of the following):

  1. Is the domain registered? Use whois to find out if it is.
  2. Is the domain expired? Use whois to find out if it is.
  3. Are you using the correct hostname? Use dig or host to ensure that the hostname you are using resolves to the correct IP.
  4. Is your A record correct? Check that you didn't accidentally create an A record for something like .

Always double check the spelling and the correct hostname (is it or ) as this will often trip you up especially with long, complicated or foreign hostnames.

Error 3 - The server isn't listening on that port

This error occurs when telnet is able to reach to the server but there is nothing listening on the port you specified. The error looks like this:

telnet 445
telnet: Unable to connect to remote host: Connection refused

This can happen for a couple of reasons:

  1. Are you sure you're connecting to the right server?
  2. Your application server is not listening on the port you think it is. Check exactly what it's doing by running netstat -plunt on the server and see what port it is, in fact, listening on.
  3. The application server isn't running. This can happen when the application server exits immediately and silently after you start it. Start the server and run ps auxf or systemctl status application.service to check it's running.
Error 4 - The connection was closed by the server

This error happens when the connection was successful but the application server has a built in security measure that killed the connection as soon as it was made. This error looks like:

telnet 445
Connected to
Escape character is '^]'.
Connection closed by foreign host.

The last line Connection closed by foreign host. indicates that the connection was actively terminated by the server. In order to fix this, you need to look at the security configuration of the application server to ensure your IP or user is allowed to connect to it.

A successful connection

This is what a successful telnet connection attempt looks like:

telnet 445
Connected to
Escape character is '^]'.

The connection will stay open for a while depending on the timeout of the application server you are connected to.

A telnet connection is closed by typing CTRL+] and then when you see the telnet> prompt, type "quit" and hit ENTER i.e.:

telnet 445
Connected to
Escape character is '^]'.
telnet> quit
Connection closed.

There are a lot of reasons that a client application can't connect to a server. The exact reason can be difficult to establish especially when the client is a GUI that offers little or no error information. Using telnet and observing the output will allow you to very rapidly narrow down where the problem lies and save you a whole lot of time.

Using telnet's command mode

When you use the Solaris telnet command to log in from one site to another, don't forget that you can enter telnet's command mode at any time during your session, usually by entering the following key sequence:

[CTRL] ] (the control key and right-bracket key at the same time)

When this is successful, you'll see the following prompt:


Then, from the "telnet>" prompt, enter "?" to learn more about the available telnet commands. A few commands that will give you more help at the prompt are shown below:

display displays many current Telnet settings
send ? displays commands you can "send" to the remote site
set ? displays variables that can be set

One of my favorites (mostly when fooling around) is:

send ayt

which means "send this message: are you there?".

[Apr 25, 2007] ShellTelnet Rudenko software.

Very interesting and in many ways more advanced then anything I saw, as it emulates OFM on the client side, but still pretty buggy... It's amazing what people can do with the old protocol. Really unfortunate that the author abandoned the development.

ShellTelnet is a Windows telnet client intended for use with a Unix/Linux shell account.

In addition to the classic telnet functionality, ShellTellnet introduces a new way of browsing Unix hosts. ShellTelnet allows the user to browse files and folders on a remote machine using the Windows Explorer interface.

ShellTelnet includes a hidden FTP client. It makes possible to transfer files without a use of slow and archaic utilities like Kermit and zmodem.

To use ShellTelnet you need a local Windows machine and a remote machine running Unix or Linux. It is assumed that the default shell on the remote machine is BASH.

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Last modified: January, 15, 2018