Reverse telnet
Encyclopedia
Reverse telnet is a specialized application of telnet
, where the server side of the connection reads and writes data to a TTY
line (RS-232
serial port), rather than providing a command shell to the host device. Typically, reverse telnet is implemented on an embedded device (e.g. terminal/console server), which has an Ethernet network interface and serial port(s). Through the use of reverse telnet on such a device, IP-networked users can use telnet to access serially-connected devices.
In the past, reverse telnet was typically used to connect to modem
s or other external asynchronous devices. Today, reverse telnet is used mostly for connecting to the console port of a router, switch or other device.
telnet 172.16.1.254 2002
(The syntax in the above example would be valid for the command-line telnet client packaged with many operating systems, including most Unixes, or available as an option or add-on.)
In this example, 172.16.1.254 is the IP address
of the console device, and 2002 is the TCP port associated with a TTY
line on the server.
TELNET
Telnet is a network protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communications facility using a virtual terminal connection...
, where the server side of the connection reads and writes data to a TTY
TTY
TTY may stand for:* Teleprinter or Teletypewriter or Teletype Printer, a typewriter paired with an electronic communication channel, used for telecommunications or as a computer terminal....
line (RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
serial port), rather than providing a command shell to the host device. Typically, reverse telnet is implemented on an embedded device (e.g. terminal/console server), which has an Ethernet network interface and serial port(s). Through the use of reverse telnet on such a device, IP-networked users can use telnet to access serially-connected devices.
In the past, reverse telnet was typically used to connect to modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
s or other external asynchronous devices. Today, reverse telnet is used mostly for connecting to the console port of a router, switch or other device.
Example
On the client, the command line for initiating a "reverse telnet" connection might look like this:telnet 172.16.1.254 2002
(The syntax in the above example would be valid for the command-line telnet client packaged with many operating systems, including most Unixes, or available as an option or add-on.)
In this example, 172.16.1.254 is the IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
of the console device, and 2002 is the TCP port associated with a TTY
TTY
TTY may stand for:* Teleprinter or Teletypewriter or Teletype Printer, a typewriter paired with an electronic communication channel, used for telecommunications or as a computer terminal....
line on the server.