Intro
Telnet is a computer protocol that was built for interacting with remote computers.
The word “Telnet” also refers to the command-line utility “telnet”, available under Windows OS and Unix-like systems, including Mac, Linux, and others. We will use the term “Telnet” mostly in the context of the telnet client software.
Telnet utility allows users to test connectivity to remote machines and issue commands through the use of a keyboard. Though most users opt to work with graphical interfaces, Telnet is one of the simplest ways to check connectivity on certain ports.
Enable telnet client in Windows
Telnet is disabled by default in Window’s settings, so you need to turn it on before you can do anything. Turning it on will help you to run the required diagnostics and check if a port is open. If you try to use telnet without turning it on first, you’ll receive a message like:
‘telnet’ is not recognized as an internal or external command, operable program or C:\>
Enabling telnet client through Command Prompt:
Install-WindowsFeature -name Telnet-Client
Enabling telnet client via GUI Win10:
Open Windows Start menu > Type “Control Panel” > Press Enter > “Programs” > “Programs and Features” > Turn Windows features on or off > Select “Telnet Client” > Press “OK”
Enabling telnet client via GUI WS2012+:
Open “Server Manager” > “Add roles and features” > click “Next” until reaching the “Features” step > tick “Telnet Client” > click “Install” > when the feature installation finishes, click “Close”.
Telnet Commands List
| Type this | To do this |
| open or o | Establish a Telnet connection with a host computer or remote server. You can use the full command, open, or abbreviate it to just o. For example, o domain.com 25 will connect your computer to a computer named domain.com port 25. |
| display | View the current settings for Telnet Client. Type display for a list of the current operating parameters. If you are in a Telnet session (connected to a Telnet server), to modify the parameters, press Ctrl+] to leave the Telnet session. To return to the Telnet session, press Enter. The following operating parameters are available: WILL AUTH (NTLM Authentication) WONT AUTH WILL TERM TYPE WONT TERM TYPE LOCALECHO off LOCALECHO on |
| quit or q | Exit from Telnet. |
| set | Set the terminal type for the connection, turn on local echo, set authentication to NTLM, set the escape character, and set up logging. >SET NTLM turns on NTLM. While you are using NTLM authentication, if you’re connecting from a remote computer, you will not be prompted to type a logon name and password. > SET LOCALECHO turns on local echoing. > SET TERM {ANSI|VT100|VT52|VTNT} sets the terminal type to the appropriate terminal type. You should use the VT100 terminal type if you are running normal command-line applications. Use the VTNT terminal type if you are running advanced command-line applications, such as . > ESCAPE + Character sets the key sequence to use for switching from session to command mode. For example, to set Ctrl+P as your escape character, typeset escape, press Ctrl+P, and then press Enter. > LOGFILE FileName sets the file to be used for logging Telnet activity. The log file must be on your local computer. Logging begins automatically when you set this option. > LOGGING turns on logging. If no log file is set, an error message is displayed. |
| unset | Turn off local echo or set authentication for the logon or password prompt. UNSET NLM turns off NLM. UNSET LOCALECHO turns off local echoing. |
| status | Move to the Telnet command prompt from a connected session |
| enter | Go to the connected session (if it exists) |
| ?/help | View Help information |
Parameter List
| Parameter | Description |
| /a | attempt automatic logon. Same as /l option except uses the currently logged on user s name. |
| /e <EscapeChar> | Escape character used to enter the telnet client prompt. |
| /f <FileName> | File name used for client side logging. |
| /l <UserName> | Specifies the user name to log on with on the remote computer. |
| /t {vt100 | vt52 | ansi | vtnt} | Specifies the terminal type. Supported terminal types are vt100, vt52, ansi, and vtnt. |
| <Host> [<Port>] | Specifies the hostname or IP address of the remote computer to connect to, and optionally the TCP port to use (default is TCP port 23). |
| /? | Displays help at the command prompt. Alternatively, you can type /h. |
Check the listen port
You can use this command for check the connection of a application.
telnet IPADDRESS PORT telnet 192.168.0.10 80 <- http telnet 192.168.0.10 25 <- smtp telnet 192.168.0.10 25
Check Connection and Close Connection
# telnet localhost 25 Trying 127.0.0.1... Connected to localhost.localdomain (127.0.0.1). Escape character is '^]'. 220 office-serv ESMTP Sendmail 8.13 ; Mon, 28 Mar 2019 08:00:00 +0900 ^] <- Ctrl ] telnet> quit Connection closed.
Telnet to Web Server (port 8o or 443
# telnet localhost 80 Trying 127.0.0.1... Connected to localhost.domain.com (127.0.0.1). Escape character is '^]'. GET / HTTP/1.0 <- Enter 2 times (http://localhost/index.html) HTTP/1.1 200 OK Date: Wed, 29 Jul 2009 15:16:50 GMT Server: Apache/1.3.37 (Unix) mod_fastcgi/2.4.6 mod_auth_passthrough/1.8 mod_log_bytes/1.2 mod_bwlimited/1.4 PHP/4.4.7 abbr. Last-Modified: Mon, 05 Feb 2007 11:04:57 GMT ETag: "1b200-b2c-45c70f59" Accept-Ranges: bytes Content-Length: 2860 Connection: close Content-Type: text/html <html> <html> <title>TEST</title> abbr. </body> </html> Connection closed by foreign host. #
Telnet to Mail Server
# telnet localhost 25 Trying 127.0.0.1... Connected to test-server.test-server (127.0.0.1). Escape character is '^]'. 220 test-server.example-sec.local ESMTP helo localhost 250 test-server.example-sec.local mail from:user01@example-sec.jp 250 ok rcpt to:user02@example-sec.jp 250 ok 502 unimplemented (#5.5.1) data 354 go ahead From: user01@example-sec.jp To: user02@example-sec.jp Subject: test this is test. . 250 ok 1184072108 qp 20747 502 unimplemented (#5.5.1) quit 221 test-server.example-sec.local Connection closed by foreign host.
C:\>telnet hostname 25 220 TEST.local Microsoft ESMTP MAIL Service, Version: 7.5. ready at Thu, 01 Jul 2018 11:11:11 -0500 helo test 250 ESXi-DEV-WEB01.local Hello [127.0.0.1] mail from:none@none.com 250 2.1.0 none@none.com….Sender OK rcpt to:youremail@domain.com 250 2.1.5 youremail@domain.com data 354 Start mail input; end with . subject:This is a test email This is the body of the test email sent via Telnet. . 250 2.6.0 Queued mail for delivery quit 221 2.0.0 TEST.local Service closing transmission channel Connection to host lost.