TELNET (TELecommunication NETwork) describes a network protocol used on the Internet or local area network (LAN) to establish an interactive communication to remote devices for administration, configuration and handling. Beginning with RFC 15 the protocol was developed in 1969 and standardized as IETF STD 8 - hence it was one of the first standardised protocol purposed for networking over the internet.
Alternativeley the term telnet also refers to applications and libraries which implements the client part of the protocol and is therefore the interface between the remote device and the user.
Telnet is set up to work as a server-client application based on a reliable connection-oriented transport layer (OSI layer 4). Typically every telnet-client is prepared to use the TCP port 23 as the default port, however one user can chose to switch to an arbitrary port if claimed by the telnet-server.
Due to the fact the development initially started in early years and was dedicated for academic use, no hard security mechanisms had been implemented so far and bears some risks for the user to suffer from tamper attacks. The most important shortcomings are:
Nevertheless when working in an secure network the advantages
dominate and hence telnet is still in use.
Establish a telnet-session (both Linux and Microsoft Windows®) by starting telnet to the remote device with the IP-address 192.168.2.123:
telnet 192.168.2.123
Assumed there is a functioning network the remote-device should respond like shown below:
Festo-00-0E-F0-01-FF-FF login:
Now type the login-name (in our case root) and proceed with enter:
Festo-00-0E-F0-01-FF-FF login: root
After this you are requested to fill in your password and confirm it with enter:
Password:
In case the requested password fits you will get access to the system with a comment equal to that shown below:
BusyBox v1.5.1 (2007-10-12 10:31:26 CEST) Built-in shell (ash) Enter 'help' for a list of built-in commands. ~ $
From now on you can interact with the system until you close the session typing the command:
/$ ls bin ffxusr lost+found root usr dev include mnt sbin var etc lib proc target www ffx linuxrc ramdisk tmp /$ exit