Managing Users and Groups

5.2. Managing Unix Users and Unix Groups

The database of Unix users and groups consists of the textual files /etc/passwd (list of users), /etc/shadow(encrypted passwords of users), /etc/group (list of groups), and /etc/gshadow (encrypted passwords of groups). Their formats are documented in passwd(5), shadow(5), group(5), and gshadow(5) respectively. While these files can be manually edited with tools like vipw and vigr, there are higher level tools to perform the most common operations.

Using getent to Consult the User Database

The getent (get entries) command checks the system databases (including those of users and groups) using the appropriate library functions, which in turn call the name service switch (NSS) modules configured in the /etc/nsswitch.conf file. The command takes one or two arguments: the name of the database to check, and a possible search key. Thus, the command getent passwd kaliuser1 will return the information from the user database regarding the user kaliuser1.

5.2.1. Creating User Accounts

Although Kali is most often run while authenticated as the root user, you may often need to create non-privileged user accounts for various reasons, particularly if you are using Kali as a primary operating system. The most typical way to add a user is with the adduser command, which takes a required argument: the username for the new user that you would like to create.

The adduser command asks a few questions before creating the account but its usage is fairly straightforward. Its configuration file, /etc/adduser.conf, includes many interesting settings. You can, for example, define the range of user identifiers (UIDs) that can be used, dictate whether or not users share a common group or not, define default shells, and more.

The creation of an account triggers the population of the user's home directory with the contents of the /etc/skel/ template. This provides the user with a set of standard directories and configuration files.

In some cases, it will be useful to add a user to a group (other than their default main group) in order to grant additional permissions. For example, a user who is included in the sudo group has full administrative privileges through the sudo command. This can be achieved with a command such as adduser user group.

5.2.2. Modifying an Existing Account or Password

The following commands allow modification of the information stored in specific fields of the user databases:

  • passwd—permits a regular user to change their password, which in turn, updates the /etc/shadow file.
  • chfn—(CHange Full Name), reserved for the super-user (root), modifies the GECOS, or "general information" field.
  • chsh—(CHange SHell) changes the user's login shell. However, available choices will be limited to those listed in /etc/shells; the administrator, on the other hand, is not bound by this restriction and can set the shell to any program chosen.
  • chage—(CHange AGE) allows the administrator to change the password expiration settings by passing the user name as an argument or list current settings using the -l user option. Alternatively, you can also force the expiration of a password using the passwd -e user command, which forces the user to change their password the next time they log in.

5.2.3. Disabling an Account

You may find yourself needing to disable an account (lock out a user) as a disciplinary measure, for the purposes of an investigation, or simply in the event of a prolonged or definitive absence of a user. A disabled account means the user cannot login or gain access to the machine. The account remains intact on the machine and no files or data are deleted; it is simply inaccessible. This is accomplished by using the command passwd -l user(lock). Re-enabling the account is done in similar fashion, with the -u option (unlock).

5.2.4. Managing Unix Groups

The addgroup and delgroup commands add or delete a group, respectively. The groupmod command modifies a group's information (its gid or identifier). The command gpasswd group changes the password for the group, while the
gpasswd -r group command deletes it.

Working with Several Groups

Each user may be a member of many groups. A user's main group is, by default, created during initial user configuration. By default, each file that a user creates belongs to the user, as well as to the user's main group. This is not always desirable; for example, when the user needs to work in a directory shared by a group other than their main group. In this case, the user needs to change groups using one of the following commands: newgrp, which starts a new shell, or sg, which simply executes a command using the supplied alternate group. These commands also allow the user to join a group to which they do not currently belong. If the group is password protected, they will need to supply the appropriate password before the command is executed.

Alternatively, the user can set the setgid bit on the directory, which causes files created in that directory to automatically belong to the correct group. For more details, see SECURITY setgid directory and sticky bit.

The id command displays the current state of a user, with their personal identifier (uid variable), current main group (gid variable), and the list of groups to which they belong (groups variable).