Reverse telnet

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 modems 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 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.

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

Telnet

Telnet (Telecommunication network) is a network protocol used on the Internet or local area network (LAN) connections. It was developed in 1969 beginning with RFC 15 and standardized as IETF STD 8, one of the first Internet standards.

The term telnet also refers to software which implements the client part of the protocol. Telnet clients are available for virtually all platforms. Most network equipment and OSes with a TCP/IP stack support some kind of Telnet service server for their remote configuration (including ones based on Windows NT). Because of security issues with Telnet, its use has waned as it is replaced by the use of SSH for remote access.

"To telnet" is also used as a verb meaning to establish or use a Telnet or other interactive TCP connection, as in, "To change your password, telnet to the server and run the passwd command".

Most often, a user will be telnetting to a Unix-like server system or a simple network device such as a router. For example, a user might "telnet in from home to check his mail at school". In doing so, he would be using a telnet client to connect from his computer to one of his servers. Once the connection is established, he would then log in with his account information and execute operating system commands remotely on that computer, such as ls or cd.

On many systems, the client may also be used to make interactive raw-TCP sessions. It is commonly believed that a telnet session which does not include an IAC (character 255) is functionally identical. This is not the case however due to special NVT (Network Virtual Terminal) rules such as the requirement for a bare CR (ASCII 13) to be followed by a NULL (ASCII 0).

FTP over SSH

FTP over SSH refers to the practice of tunneling a normal FTP session over an SSH connection.

Because FTP uses multiple TCP connections (unusual for a TCP/IP protocol that is still in use), it is particularly difficult to tunnel over SSH. With many SSH clients, attempting to set up a tunnel for the control channel (the initial client-to-server connection on port 21) will protect only that channel; when data is transferred, the FTP software at either end will set up new TCP connections (data channels) which will bypass the SSH connection, and thus have no confidentiality, integrity protection, etc.

If the FTP client is configured to use passive mode and to connect to a SOCKS server interface that many SSH clients can present for tunneling, it is possible to run all the FTP channels over the SSH connection.

Otherwise, it is necessary for the SSH client software to have specific knowledge of the FTP protocol, and monitor and rewrite FTP control channel messages and autonomously open new forwardings for FTP data channels. Version 3 of SSH Communications Security's software suite, and the GPL licensed FONC are two software packages that support this mode.

FTP over SSH is sometimes referred to as secure FTP; this should not be confused with other methods of securing FTP, such as with SSL/TLS (FTPS). Other methods of transferring files using SSH that are not related to FTP include SFTP and SCP; in each of these, the entire conversation (credentials and data) is always protected by the SSH protocol.