Different protocols to command linux

RAW

RAW TCP are network protocols that allows data to be exchanged using a secure channel between two networked devices. Used primarily on Linux and Unix based systems to access shell accounts

Telnet

The Telnet protocol offers a way to remotely access multi-user terminal servers across networks and the Internet. Telnet consists of a client-server connection where an application either is the Telnet client or communicates through a separate Telnet terminal client to the multi-user terminal server.

Rlogin

RLOGIN is a Unix software utility that allows users to log in on another host via a network, communicating via TCP.  However, neither RLOGIN nor Telnet is a secure protocol, and their traffic can be easily seen with any port sniffing application.

SSH

Secure Shell or SSH  network protocol  that allows data to be exchanged using a secure channel between two networked devices. Used primarily on Linux and Unix based systems to access shell accounts, SSH was designed as a replacement for Telnet and other insecure remote shells, which sent information, notably passwords, in plain text, leaving them open for interception.

Serial

Most embedded Linux / BSD systems such as routers, servers and nas devices comes with console interface (serial port with RS-232). BIOS can uses this, and after boot BIOS screen I/O is redirected so that you can use the device. RS-232 is also used for communicating to headless server, where no monitor or keyboard is installed, during boot when operating system is not running yet and therefore no network connection is possible. You need to use a serial cable between your computer and embedded system or server. In this post I will cover five conman utilities used for serial communication under Linux / Unix / *BSD and Mac OS X.