Editing commands in telnet. $ telnet options host port. To disconnect the Telnet client from the remote computer

Telnet is a network utility that allows you to connect to a remote port on any computer and establish an interactive communication channel, for example, to send commands or receive information. We can say that this is a universal browser in the terminal that can work with many network protocols.

This utility has been used very often in the past to remote control computer with Linux, but then it was replaced by the secure SSH protocol. But telnet is still used, for example, for network testing, port checking, and also for communicating with various IoT devices and routers. In this article we will look at what telnet is, as well as how to use telnet to solve your problems.

As I said, this utility is designed to create an interactive connection between remote computers. It works using the TELNET protocol, but this protocol is supported by many services, so it can be used to manage them. The protocol is based on TCP, and allows you to send regular string commands to another device. It can be used not only for manual control but also for interaction between processes.

To work with this protocol we will use the telnet utility, it is very easy to use. Let's look at the telnet syntax:

$ telnet options host port

Host is a domain remote computer to connect to, and port is the port on this computer. Now let's look at the main options:

-4 - force the use of ipv4 addresses;
-6 - force the use of ipv6 addresses;
-8 - use 8-bit encoding, for example, Unicode;
-E- disable support for Escape sequences;
-a- automatic login, takes username from environment variable USER;
-b- use a local socket;
-d- enable debugging mode;
-R- rlogin emulation mode;
-e- set the beginning symbol of the Escape sequence;
-l- user for authorization on a remote machine.

That's it for the telnet command to establish a connection. But connecting to a remote host is only half the battle. After establishing a connection, telnet can operate in two modes:

Line by line- this is the preferred mode, here a line of text is edited to local computer and is sent only when it is completely ready. Not all services always have this opportunity;
Character-by-character- all characters you type are sent to a remote server. It will be difficult to correct anything here if you make a mistake, because Backspace will also be sent as a symbol and a movement arrow too.

The use of telnet is to send special teams. Each service has its own commands, but the protocol has its own telnet commands that can be used in the telnet console.

CLOSE- close the connection to the server;
ENCRYPT- encrypt all transmitted data;
LOGOUT- exit and close the connection;
MODE- switch mode, from lowercase to character or from character to lowercase;
STATUS- view connection status;
SEND- send one of the telnet special characters;
SET- set the parameter value;
OPEN- establish a connection via telnet with a remote host;
DISPLAY- display the special characters used;
SLC- change the special characters used.

We will not consider all the commands, since you are unlikely to need them, and if you do, you can easily find them in the official documentation.

How to use telnet?

Next we will look at how to use telnet to solve your problems. Usually, the utility is already installed on most systems, but if this is not the case, then you can install telnet from the official repositories, for example, in Ubuntu:

sudo apt install telnet

Now let's move on to using the utility. Initially, it was used to remotely control a computer, but since the more secure SSH protocol was later developed, it was no longer used.

1. Server availability

The utility can still be useful when checking the availability of a node; to do this, simply pass it the IP address or host name:

telnet 192.168.1.243

You don't have to use telnet for this; ping is available.

2. Port check

Using telnet we can check the availability of a port on a host, and this can already be very useful. To check the telnet port run:

telnet localhost 123
$ telnet localhost 22

In the first case, we see that no one accepts the connection, but in the second, a message about a successful connection and a greeting from the SSH server are displayed.

3. Debugging

To enable debug mode and output more detailed information while running, use the -d option while connecting:

sudo telnet -d localhost 22

4. Telnet console

Using the telnet console too important point in the analysis of how to use telnet. In the main mode, you can execute commands on a remote server, but if you want to address the command specifically to telnet, for example, to configure its operation, you need to use a special character to open the console, usually the utility immediately tells you what this character is, for example, it is used by default "^[":

To activate it, you need to press the key combination Ctrl+[, then you will enter the telnet input prompt.

To see everything available commands, can you dial?. For example, you can view the connection status:

telnet>status

There are others here too interesting opportunities. Such things can be done on any connection using the telnet utility.

5. View the telnet site

One common way to use telnet is to test a website from the console. Yes, you won’t get a beautiful web page, but you can manually collect requests and see all the data sent by the server.

telnet opennet.ru 80

Then issue the command to the web server:

The web server will return the entire page, as well as the headers that are necessary for the browser to display it.

6. Telnet remote control

It is strongly recommended not to use insecure telnet for remote control because all commands and passwords can be eavesdropped by a third party. But sometimes, for example, for routers, telnet is still used for remote control. Everything works exactly the same as for other connections, only you need to use port 23, and telnet-server must be installed on the remote computer:

telnet localhost 23

Here you don’t even need to specify the port, because 23 will be used by default. Next, you need to enter your login and password, and then you will be able to execute commands on the remote system.

conclusions

In this article, we looked at examples of using telnet, as well as what this utility is, although it is no longer used for its main purpose, it can still be useful to many users and system administrators. If you have any questions, ask in the comments!

Application layer protocol TELNET (from English. T.E. rmina LNET work) - network protocol to implement a text interface over the network. Name telnet We also received client programs for implementing this protocol for almost all existing operating systems. The Telnet protocol is one of the oldest network protocols, developed as a means of communication between remote terminals in test mode. Therefore, it does not provide data encryption or modern authentication tools. The protocol is vulnerable to many network attacks, and cannot be used as a tool to manage network operating systems. Currently, the SSH (Secure SHell) network protocol is used for remote access to the system, the creation of which focused specifically on security issues. The relative security of Telnet sessions occurs only in a fully controlled network environment or with security applied network level(various implementations of VPN - virtual private networks). However, TELNET is still used to manage specialized network devices (Switches, routers, etc.), as well as for network diagnostics, debugging and studying other text-oriented (telnet-like) protocols based on the TCP transport. The current Telnet protocol standard is described in RFC 854.

In modern operating systems of the Windows family, the utility telnet.exe by default, not installed. To install it you need to go to Control Panel - Programs and Features - Turn it on or off Windows components and check the box for Telnet client. Or, in a command prompt running as an administrator, run the command:

pkgmgr /iu:"TelnetClient"

Command line format:

telnet [-a][-e Symbol][-f File][-l Name][-t Type][Host [Port]]

Command line options:

-l The user name to log into the remote system, assuming the TELNET ENVIRON option is supported.

-a Attempt automatic login into the system. Like the -l switch, but uses the current username under which you are logged in.

-e Service character for switching input mode in the telnet client window.

-f Client-side log file name. In the Russian help, this parameter is incorrectly interpreted as Login_File - “File name on the client side for logging into the system.”

-t Telnet terminal type. 4 types of terminals are supported: vt100, vt52, ansi and vtnt.

Knot The host name or IP address of the remote computer to which you are connecting. Port Port number or service name. If the number is not specified, then the standard Telnet port 23\TCP is used

When launched without parameters, the utility goes into command input mode:

Welcome to the Microsoft Telnet Client

Microsoft Telnet>

When entering a character ? or help Help information is displayed:

Teams may be shortened. Supported commands are:

c-close- close the current connection

d-display- display operation parameters

o - open hostname [Port]- connect to the site (by default, Port = 23)

q - quit- exit telnet

set-set- set parameters ("set?" to display a list of them)

sen - send- send strings to the server

st - status- display information about the current state

u - unset- reset parameters ("unset ?" to display a list of them)

? /h - help- issue a certificate

Some of the commands allow you to get hints for use when entered with a question character:

Telnet> set ?- get a hint on using the mode setting command. Example of information displayed:

bsasdel- symbol BackSpace Delete

crlf- carriage return mode; causes CR & LF characters to be sent

delasbs- symbol Delete will be sent as a character BackSpace

escape x- where x is the symbol for switching to telnet terminal mode and back

localecho- enable local echo.

logfile x- where x is the log file. In Russian translation it is incorrectly interpreted as "Login file current client into the system"

logging- recording the current session in the log. In Russian translation it is incorrectly interpreted as "executing a login"

mode x- where x=console is the console mode used to work with windowed applications (vi editor) and x=stream is the streaming mode used to work on the command line.

ntlm- enable NTLM authentication.

term x- type of emulated terminal. Where x is ansi, vt100, vt52, or vtnt.

To get a hint on how to cancel the installed parameters, use the command

Microsoft Telnet> unset?

bsasdel- symbol BackSpace will be sent as a character Delete

crlf- line feed mode; causes the CR character to be sent

delasbs- symbol Delete will be sent as a character Backspace

escape- the symbol for switching to telnet terminal mode and back is not set

localecho- disable local echo

logging- disable log recording. In the Russian-language version it is incorrectly interpreted as "disabling login"

ntlm- disable NTLM authentication.

Examples of commands in interactive mode:

open 192.168.0.1- connect to the Telnet server with an IP address 192.168.0.1

o zte-f660- connect to a Telnet server named zte-f660. Command abbreviation used o pen

set logfile C:\telnet.log- use as log file C:\telnet.log

set logging- record the current session to a log file.

display- display the parameters of the current session. Example of information displayed:

Mode switch symbol: "CTRL+]"

NTLM Authentication - Enabled

Local echo output - disabled

Mode new line- Symbol ENTER will be shipped as CR&LF

Current mode: Streaming

TERMINAL MODE

Preferred ANSI Terminal Type

In practice, the utility telnet.exe used as a diagnostic and debugging tool to connect not only to the Telnet server on TCP port 23, but also to any other TCP port, thereby allowing interaction with any command line-controlled application. So, for example, using the utility telnet you can connect to servers that support text (telnet-like) input of commands and data - SMTP, POP3, IMAP, etc. In addition, the utility can be used as a means of roughly checking the connectivity to any TCP port (checking whether a specific TCP port is listening).

telnet 192.168.1.1 8080- connect to host 192.168.1.1 on port 8080. In cases where the port is closed, the utility will report that connection is impossible. Moreover, to check the availability of a certain port, it is not even necessary that it be listened to by a service that supports text input, such as a VNC server. To disconnect from the remote server, you must enter the mode switching symbol (by default - CTRL+]).

The telnet.exe utility can be used, for example, to exchange with a mail server using the protocol POP3(Post Office Protocol ver. 3). This protocol is used by email client programs (Outlook, Outlook Express, The Bat, etc.) to receive Email, stored in mailbox user. This is the simplest protocol based on the exchange text messages. In order to study the interaction mail client with a mail server, you can implement a connection session using TELNET.

By default, the POP3 server listens for incoming connections via TCP on port 110 ("listens" on port tcp/110). Telnet command to connect to a server, for example pop.mail.ru

telnet pop.mail.ru 110

If the server is healthy, its invitation will appear in the telnet window

+OK mPOP POP3 v1.1

user [email protected]

After which, the server will prompt you to enter a password:

OK Password required for user [email protected]

You need to enter a password

pass VasinPass

The server will report the result of the password check:

You can request a list of letters with a directive list:

list

In response to this, the server will display a list and sizes of letters in the mailbox:

OK 10 messages (152527 octets)
1 48628 1 is the serial number, 48628 is the size
2 1829
3 2070
:

If necessary, you can view the headers of the letters. To do this, use the command top sequence number, space, number of lines from the message body

top 2 0

In response to this, you will see the header of the letter containing service information about the sender, date of sending, return address and some other data:

Received: from (HELO mx1.ks.pochta.ru) by node7-1.ks.pochta.ru with QIP.RU LMTP
for [email protected];
Fri, 08 Apr 2011 15:18:33 +0400
Received: from mx3.softkey.ru ()
: :

The directive is used to receive letters retr sequence number

retr 2 - receive a letter with sequence number 2

To delete a letter, use the directive dele sequence number For example, to remove the 2nd letter from the list received by the directive list:

If the deletion was successful, the server will display the following message:

OK message 2 deleted

Sometimes, the TELNET command can also be used to identify the service listening specified port, since many of them display either their banner or specific service information when connected. For example, the FTP server greeting: 220-FileZilla Server version 0.9.43 beta

220-written by Tim Kosse ( [email protected])

220 Please visit http://sourceforge.net/projects/filezilla/

And this is what the screen looks like when connecting to the RealVNC server:

RFB 003.008

Telnet Commands

Teams telnet allow you to communicate with a remote computer using Telnet protocol. You can run the command telnet without parameters to enter the telnet context indicated on the Telnet command line ( telnet). From the Telnet command line, use the following commands to control the computer running the Telnet client.

Teams tlntadmn allow you to remotely control a computer running a Telnet server. These commands are executed from the command line. Team tlntadmn without parameters displays the local server setting.

To use commands telnet from the Telnet command line

To launch the Telnet client and enter the Telnet command line

Syntax

telnet [\\ remote server]

Options

\\ remote server Displays the name of the connected server /? Displays help on the command line.

Notes

When using the command telnet The Telnet client starts without parameters.
At the Telnet command line, you must use Telnet commands.

To stop the Telnet client

Syntax

Options

none

Notes

To connect a Telnet client to a remote computer

Syntax

open [\\ remote server] [port]

Options

\\ remote server Specifies the name of the managed server. If the server is not specified, it is used local server. port Indicates the port to use. If no port is specified, the default port is used.

Notes

This command can be shortened to o.

Examples

To connect to the remote Redmond server on port 44, enter at the command prompt:
redmond 44

To disconnect the Telnet client from the remote computer

Syntax

close [\\ remote server]

Options

\\ remote server Specifies the name of the managed server. If no server is specified, the local server is used.

Notes

This command can be shortened to c.

Examples

To disconnect from the remote Redmond server, enter the command:
redmond 44

To configure Telnet client settings

none

Notes

This command displays the current operating parameters for the Telnet client. When operating in Telnet session mode (in other words, while connected to a Telnet server), you can exit the session to change settings by pressing CTRL+]. To return to the Telnet session, press ENTER.

Using commands tlntadmn You can remotely administer a computer running Telnet Server if both computers are running Windows XP. Teams tlntadmn cannot be used to remotely administer a Telnet server running Windows 2000 from a computer running Windows XP.
If you specify where to send event information without specifying the information type or types to be audited, only administrator log event information will be audited and sent to the specified location.

Examples

To send event information to the Event Viewer, enter:

tlntadmn config auditlocation=eventlog

To audit administrator logon events and failed logon attempts, enter:

tlntadmn config audit=+admin +fail

To set the primary domain for the computer running Telnet Server

Syntax

tlntadmn [\\ remote server] config [dom=domain name]

Options

\\ remote server Specifies the name of the server that you want to take control of. If no server is specified, the local server is used. dom=domain name Specifies the domain that you want to make primary. /? Displays help on the command line.

Notes

Using commands tlntadmn You can remotely administer a computer running Telnet Server if both computers are running Windows XP. Teams tlntadmn cannot be used to remotely administer a Telnet server running Windows 2000 from a computer running Windows XP.

Examples

To make the Redmond domain the primary domain on your local server, enter:

Using commands tlntadmn You can remotely administer a computer running Telnet Server if both computers are running Windows XP. Teams tlntadmn cannot be used to remotely administer a Telnet server running Windows 2000 from a computer running Windows XP.

Notes

To switch from the Telnet client to command mode, at the Telnet command prompt, press CTRL+]. To return to the Telnet client, press ENTER.

Telnet can be used as a command in operating system Windows. In fact, this is not only an executable service, but also a network protocol or even firmware, which, using a text field, allows the user to “communicate” with a remote computer, sending the system a command to perform a particular action, as well as receiving a log of its execution.

The name of the protocol stands for Terminal network, which literally means terminal network.

What is Terminal network

Using the telnet command, despite its low functionality, you can perform quite a lot of tasks, such as:

checking the connection to the server using an open port;
connection to remote devices (modem, router or switch) to send a command (reboot, shutdown or receive logs);
firmware update network equipment or mobile phone;
file transfer.

Many people believe that this connection works, as do most network services, based on the client-server principle. However, it is not. telnet is a completely two-way service with the same functionality and access levels.

Interestingly, the program itself includes minimum set functions, which contains only the connection option and the authentication process (verifying the authenticity of the username and password that is allowed access). All other options, both command and system, are called by the connected applications.

How to work with the shell?

To use the telnet command's functionality, you must use the command line. In the seventh Windows versions and above, the service is disabled by default and does not start automatically when the command is called. Therefore, you need to follow these steps to enable telnet:

How to manage the service?

Once you enter the Windows Terminal Management service, you need to know a few commands to be able to manage it. There are the following options for obtaining a list of control keys:

If you are familiar with the options, you can immediately connect to the desired resource with the required data. In this case, the server for the connection is “smatp.ya.ru” and the port is “25”.

Result:

The telnet command is very easy to use and quite convenient. However, the technology of its operation does not provide for safety at all. The shell runs under Windows in a completely unprotected mode and without encryption. Therefore, instead of this function, ssh has been used for quite some time.

According to the principle of organizing the connection, they are almost the same. But ssh protocol was developed taking into account possible protection technologies. In some cases more simple way connection is quite sufficient, for example, when you need to connect to equipment in order to download new firmware or restore the old one, as well as to change the system configuration.

If you use several protocols to connect at once, then it is faster, more convenient and safer to do this through special program, for example "Putty".

In this article I will tell you how to use telnet in the Windows 7 operating system. But first, I will briefly talk about what telnet is and what it is used for. Telnet is one of the network communication protocols. It appeared in the last century and was often used for remote connection To various devices online.

And so, let's move directly to our question. In a standard installation in the Windows 7 operating system this protocol disabled. If there is a need to use this protocol on workstations running Windows 7, you need to perform some manipulations to enable it. The need to use it may arise not only for system administrators, which with its help configure network devices and often use it at work, but also among ordinary users who are far from system administration.

There are at least two ways to enable the telnet protocol in Windows 7.

The first and easiest way is as follows:

In the box, put a checkmark - “Telnet Client”.

That's all, you have successfully enabled the Telnet protocol on your computer.

Another nuance is possible when enabling the protocol using this method; the following situation may occur: when you select “Turn Windows components on or off,” the list of components may be empty. We will be required to make a small adjustment to the registry. I remind you how to open the registry by clicking “Start” - just above Start in the search line we type regedit,

After this, you need to reboot the workstation and repeat the above steps.

The second method uses the command line; ordinary users rarely work with it and often do not know that such functionality is available. Therefore, for those who do not consider themselves an advanced user, it is better to use the first method of enabling the protocol.

So, click “Start” - just above Start in the search bar, type cmd and press Enter.

A black window opens in which the cursor blinks, this is what we need - command line. Next, in this line, type the command “dism/online/Enable-Feature/FeatureName:TelnetClient” and press Enter.

After a short pause, an information window appears indicating that the operation was completed successfully. The Telneta installation is complete.

I hope that my article will be useful. Remember that Telnet is far from a secure protocol, so when working with it you need to remember security measures.

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