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Telnet Cheat Sheet

A quick reference guide to using Telnet, covering basic commands, connection management, and security considerations. This cheat sheet provides a concise overview for network troubleshooting and remote access tasks.

Basic Telnet Commands

Connecting to a Host

telnet <hostname>

Connect to the specified hostname using the default Telnet port (23).

Example:
telnet example.com

telnet <hostname> <port>

Connect to the specified hostname on a specific port.

Example:
telnet example.com 80 (Connect to port 80)

open <hostname> <port>

Opens a connection to the specified hostname and port from within the Telnet client.

Example:
open example.com 25 (Connect to port 25)

Telnet Client Commands

close

Closes the current connection.

quit or exit

Closes the current connection and exits the Telnet client.

status

Displays the current status of the Telnet client, including connection information.

display

Displays the Telnet client settings.

? or help

Displays a list of available Telnet commands.

Sending Special Characters

send <command>

Sends special Telnet commands to the server.

Common commands include: ao (Abort Output), ayt (Are You There), esc (Escape), ip (Interrupt Process), nop (No Operation), synch (Synchronize).

send ao

Aborts the output on the remote system.

send ip

Interrupts the current process on the remote system.

Advanced Telnet Usage

Negotiation Options

set <option> <value>

Sets various Telnet options. Common options include echo (local echoing), crmod (carriage return mode), and termtype (terminal type).

Example:
set echo on (Enable local echoing)

unset <option>

Unsets a previously set Telnet option.

Example:
unset echo (Disable local echoing)

toggle <option>

Toggles a boolean Telnet option between on and off.

Example:
toggle crlf (Toggle carriage return/line feed mode)

Local Echo and Line Mode

set echo

Enables local echoing, so characters typed are displayed on your screen.

unset echo

Disables local echoing; useful if the server is already echoing characters.

set crlf

Sends carriage return and line feed characters. Some servers may require this.

unset crlf

Sends only carriage return characters.

Terminal Emulation

set termtype <type>

Sets the terminal type to a specific value (e.g., vt100, ansi). Some servers adapt their output based on terminal type.

Example:
set termtype vt100

display termtype

Displays the currently set terminal type.

Troubleshooting and Security

Common Issues

If you cannot connect, verify the hostname or IP address and port number are correct.

Check if a firewall is blocking the connection on port 23 (or the specified port).

If you see garbled output, try setting the terminal type (set termtype) to a common value like vt100 or ansi.

If characters are not displaying correctly, ensure local echoing is enabled or disabled as needed (set echo or unset echo).

Security Considerations

Warning: Telnet transmits data in plaintext, including usernames and passwords. This makes it vulnerable to eavesdropping and should generally be avoided on untrusted networks.

Consider using SSH (Secure Shell) instead, which encrypts all transmitted data.

If you must use Telnet, avoid transmitting sensitive information and only connect to trusted servers.

Never use Telnet over public Wi-Fi or other untrusted networks.

Alternatives to Telnet

SSH (Secure Shell) is the preferred alternative for secure remote access. It encrypts all transmitted data, protecting against eavesdropping.

Command: ssh <username>@<hostname>

For remote desktop access, consider using protocols like RDP (Remote Desktop Protocol) or VNC (Virtual Network Computing), which offer graphical interfaces and often support encryption.

Practical Telnet Examples

Connecting to an SMTP Server

telnet mail.example.com 25

Connects to the SMTP server on mail.example.com on port 25. You can then issue SMTP commands to test mail server functionality.

HELO example.com

Initiates an SMTP HELO handshake. Replace example.com with your domain.

QUIT

Closes the SMTP connection.

Connecting to a Web Server

telnet example.com 80

Connects to the web server on example.com on port 80. You can then send HTTP requests.

GET / HTTP/1.0

Sends a simple HTTP GET request for the root page. Press Enter twice after this line.

Press Enter Twice

After the GET command, you must press Enter twice to send the request.

Port Scanning with Telnet

Telnet can be used to quickly check if a port is open on a remote host.

Try connecting to various ports using telnet <hostname> <port>. If the connection succeeds, the port is likely open. If the connection fails or times out, the port is likely closed or filtered.

This is a rudimentary form of port scanning and should be used cautiously, respecting network policies and avoiding unauthorized scanning.
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