Telnet is a protocol and command-line client that gives a remote text terminal over a TCP connection, traditionally to port 23. Everything you type, including the password, is sent as plain text with no encryption or server authentication.
Why it matters
Because it is trivially sniffed, Telnet was replaced by SSH for administration. It survives on old network equipment and embedded devices, where an exposed Telnet service with default credentials is a classic way for malware to recruit a botnet. Finding port 23 open on a public address is a red flag.
Telnet is also a handy debugging tool, since the client simply opens a raw TCP socket. You can talk to text-based services such as SMTP or HTTP by hand.
Example
$ telnet example.com 80
GET / HTTP/1.1
Host: example.com
If the TCP connection is accepted you see Connected to example.com, and the server replies with HTTP headers. A hang or Connection refused tells you the port is filtered or closed.
Common mistakes
- Using Telnet for real administration over untrusted networks.
- Treating a failed Telnet connection as proof the service is down; a firewall may simply block your source address.
- Leaving Telnet enabled on routers and cameras.
Related
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