Telnet Protocol

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.

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.

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

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

Commands

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

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.

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.

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

255(IAC),251(WILL),3

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

and

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 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.

• Suppress Go Ahead

  The original telnet implementation defaulted to "half duplex" operation. This means that data traffic could only go in one direction at a time and specific action is required to indicate the end of traffic in one direction and that traffic may now start in the other direction. [This similar to the use of "roger" and "over" by amateur and CB radio operators.] The specific action is the inclusion of a GA character in the data stream.

  Modern links normally allow bi-directional operation and the "suppress go ahead" option is enabled.

• echo

  The echo option is enabled, usually by the server, to indicate that the server will echo every character it receives. A combination of "suppress go ahead" and "echo" is called character at a time mode meaning that each character is separately transmitted and echoed.

  There is an understanding known as kludge line mode which means that if either "suppress go ahead" or "echo" is enabled but not both then telnet operates in line at a time mode meaning that complete lines are assembled at each end and transmitted in one "go".

• linemode

  This option replaces and supersedes the line mode kludge.

• remote flow control

  This option controls where the special flow control effects of Ctrl-S/Ctrl-Q are implemented.

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.

• Interrupt Process

  This is used by the client to cause the suspension or termination of the server process. Typically the user types Ctrl-C on the keyboard. An IP (244) character is included in the data stream.

• Abort Output

  This is used to suppress the transmission of remote process output. An AO (238) character is included in the data stream.

• Are You There

  This is used to trigger a visible response from the other end to confirm the operation of the link and the remote process. An AYT (246) character is incorporated in the data stream.

• Erase character

  Sent to the display to tell it to delete the immediately preceding character from the display. An EC (247) character is incorporated in the data stream.

• Erase line

  Causes the deletion of the current line of input. An EL (248) character is incorporated in the data stream.

• Data Mark

  Some control functions such as AO and IP require immediate action and this may cause difficulties if data is held in buffers awaiting input requests from a (possibly misbehaving) remote process. To overcome this problem a DM (242) character is sent in a TCP Urgent segment, this tells the receiver to examine the data stream for "interesting" characters such as IP, AO and AYT. This is known as the telnet synch mechanism.

  A DM not in a TCP Urgent segment has no effect.

The telnet command

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

telnet <remote host>

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 134.220.1.20 ...
Connected to ccub.wlv.ac.uk.
Escape character is '^]'.
SENT do SUPPRESS GO AHEAD
SENT will TERMINAL TYPE (reply)
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

NEWS CONTENTS

• 20180114 : Using telnet to debug connection problems ( Jan 14, 2018 , bash-prompt.net )
• 20180114 : Using telnets command mode ( Using telnet's command mode, )
• 20070425 : ShellTelnet ( ShellTelnet, Apr 25, 2007 )

Old News ;-)

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

Jan 14, 2018 | bash-prompt.net

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 samba.example.com 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 samba.example.com 445

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

telnet samba.example.com 445
Trying 172.31.25.31...

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 samba.example.com 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 samba.example.com 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 samba.example.com 445
Server lookup failure:  samba.example.com:445, 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 smaba.example.com .

Always double check the spelling and the correct hostname (is it samba.example.com or samba1.example.com ) 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 samba.example.com 445
Trying 172.31.25.31...
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 samba.example.com 445
Trying 172.31.25.31...
Connected to samba.example.com.
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 samba.example.com 445
Trying 172.31.25.31...
Connected to samba.example.com.
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 samba.example.com 445
Trying 172.31.25.31...
Connected to samba.example.com.
Escape character is '^]'.
^]
telnet> quit
Connection closed.

Conclusion

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:

telnet>

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