A fundamental part of system administration is configuring and managing users and groups. Part of this task involves monitoring the log in capabilities of all system entities.
In this guide, we will introduce the basic ideas behind user management and authentication logging.
We will be exploring these concepts on an Ubuntu 12.04 VPS, but you can follow along on any up-to-date Linux distribution.
Part one will cover how to view system users and find out who is logged into the system.
Every user on a Linux system, whether created as an account for a real human being or associated with a particular service or system function, is stored in a file called "/etc/passwd".
The "/etc/passwd" file contains information about the users on the system. Each line describes a distinct user.
Have a look by entering:
root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/bin/sh . . .
Each line is broken up into fields. These fields are delimited by the colon (:) character.
The only field that we are really interested in at the moment is the first one. Each is an independent username.
We can get this list without wading through the entire "/etc/passwd" by typing:
cut -d : -f 1 /etc/passwd
root daemon bin sys sync games . . .
You probably recognize "root" as the administrative user. Towards the end, you may see the user you are logged in as.
In between, you will probably see a number of other users whose usage seems at least somewhat clear. For instance, "www-data" is configured as the owner of web server processes.
This is done to separate functional privileges. That way, if an account is compromised or misused, the affect will be isolated.
You can learn more about the fields in "/etc/passwd" by clicking here.
The corresponding file for discovering system groups is "/etc/group".
We can see the whole file by typing:
root:x:0: daemon:x:1: bin:x:2: sys:x:3: adm:x:4: tty:x:5: disk:x:6: . . .
You can see that many of the group names mirror the users we discovered on our system. This is part of a configuration scheme called "user private groups", or UPG.
User private groups create a private group for each user and set that group as the primary group. The umask is then changed from 022 to 002.
This allows for more flexibility in shared directories by setting a flag called setgid, which gives files inside the directory the same group owner as the directory itself. This configuration is useful, but outside of the scope of this article.
Once again, we can pare down the information from the "/etc/group" file by using the cut command:
cut -d : -f 1 /etc/group
root daemon bin sys adm tty disk . . .
The output will be a list of each group on the system, one per line.
Many times, it will be more useful to find out which users are active on your system.
The "w" command is a simple way to list all of the currently logged in users, their log in time, and what the command they are currently using:
19:37:15 up 5:48, 2 users, load average: 0.33, 0.10, 0.07 USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT root pts/0 rrcs-72-43-115-1 19:15 38.00s 0.33s 0.33s -bash demoer pts/1 rrcs-72-43-115-1 19:37 0.00s 0.47s 0.00s w
The first line contains system uptime information. The following lines describe who is logged in.
An alternative that provides similar information is "who":
root pts/0 2013-09-05 19:15 (rrcs-72-43-115-186.nyc.biz.rr.com) demoer pts/1 2013-09-05 19:37 (rrcs-72-43-115-186.nyc.biz.rr.com)
User authentication on Linux is a relatively flexible area of system management. There are many ways of accomplishing the same objective with very simple tools.
You should now know how to find out where your server stores its user and group information. You can also see who is logged in at any given time.
In the next part, we will discuss how to restrict access to logins.
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