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Logging In and Out of a Linux System
The figure below depicts a typical Linux system's login screen. This is the place where administrators and users alike begin their work. Linux is considered a secure network operating system and, as such, it requires users to present credentials for most, if not all, system access. As an administrator, it is vital for you to understand the various ways that users can access the system and how to best secure those ways.
The Linux login screen.
Before we get into a deep discussion of logging in and out of Linux and the security involved with such, we first must learn some basics. First of all, there are two primary ways to access a Linux system-locally or remotely. When one accesses the machine locally, he or she is said to be attempting console access. Console access implies sitting at the machine interactively for usage. Remote access deals with accessing the machine from a network location. Telnet is a great illustration of accessing a Linux machine remotely.
You probably know by now that not everyone should simply be allowed to log in to a server. In the Linux OS, there are many ways to control both who can log in interactively, as well as many ways to control who can log in remotely. The graphic displayed earlier in this topic represents a login prompt that a user would see if he or she accessed the Linux machine from the console. When the user types in his user name, it is displayed, or echoed, to the screen; however, when the user gets to the point of typing the password in, the screen turns off echoing so the password is not displayed in the clear. Take a look at the figure below and note that the user name is displayed, but the password is not, when root attempts a login.
Masked password at login.
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