Reverse telnet - Citizendia

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. Telnet ( Tel ecommunication net work is a Network protocol used on the Internet or local area network (LAN connections In Telecommunications, RS-232 (Recommended Standard 232 is a standard for serial binary data signals connecting between a DTE ( Data Terminal Equipment 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 modems or other external asynchronous devices. Modem (from mo dulator- dem odulator is a device that modulates an analog carrier signal to encode Digital information 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 Unices, or available as an option or add-on. )

In this example, 172. 16. 1. 254 is the IP address of the server, and 2002 is the TCP port associated with a TTY line on the server. An Internet Protocol ( IP) address is a numerical identification ( Logical address) that is assigned to devices participating in a Computer network The Transmission Control Protocol (TCP is one of the core protocols of the Internet Protocol Suite.

A typical server configuration on a Cisco router would look like this:

version 12. 3
service timestamps debug uptim
service timestamps log uptime
no service password-encryption
!
hostname Terminal_Server
!
ip host Router1 2101 8. 8. 8. 8
ip host Router2 2102 8. 8. 8. 8
ip host Router3 2113 8. 8. 8. 8
!
!
interface Loopback0
 description Used for Terminal Service
 ip address 8. 8. 8. 8 255. 255. 255. 255
!
line con 0
 exec-timeout 0 0
 password MyPassword
 login
line 97 128
 transport input telnet
line vty 0 4
 exec-timeout 0 0
 password MyPassword
 login
 transport input none
!
end

Example

See also