• In Linux, every user has the following:
    • Username
    • User ID
    • Primary Group
      • For use with discretionary access control.
        • Meaning access is granted based on whom the user is and what groups they belong to.
  • Users and Passwords
    • User Account Information
      • Stored in /etc/passwd
    • Passwords and Account Aging
      • Stored in /etc/shadow
    • User account commands and defaults for the shadow utilities, are stored in:
      • /etc/login.defs
  • Account Defaults
    • Commands that use the login.defs file include:
    • /etc/login.defs
      • useradd
      • userdel
      • usermod
      • groupadd
      • groupdel
      • groupmod
      • change
    • /etc/login.defs contains:
      • Password ageing settings.
      • Minimum and Maximum numbers for user ids.
      • Minimum and Maxium numbers for system account ids.
      • Minimum and Maximum numbers for user group ids.
      • Minimum and Maximum numbers for system group ids. Screenshot_20230712_205604.png
    • Notice in most cases that system id numbers are less than 1000.
    • Also configured are whether to create initial home directories.
      • and the hash encoding type, such as MD5 or SHA512.
    • /etc/default/useradd
      • HOME=/home
        • Details where home directories are created.
      • INACTIVE=-1
      • EXPIRE=
        • Whether the account should expire.
      • SHELL=/etc/skel
        • Which shell to use - if not specified in the command line.
      • SKEL=/etc/skel
        • Where the skeleton directory resides.
        • Contains the files that are copied to each new users’ home directory.
        • An example of this is here: myuser@myhost:~$ ls -la /etc/skel total 28 drwxr-xr-x 1 root root 84 Jun 20 11:07 . drwxr-xr-x 1 root root 4040 Jul 13 21:46 .. -rw-r--r-- 1 root root 220 Apr 24 06:23 .bash_logout -rw-r--r-- 1 root root 3526 Apr 24 06:23 .bashrc -rw-r--r-- 1 root root 5290 Apr 14 04:58 .face lrwxrwxrwx 1 root root 5 Apr 14 04:58 .face.icon -> .face -rw-r--r-- 1 root root 807 Apr 24 06:23 .profile
      • Includes shell initialisation files such as:
        • .bash_logout
        • .bash_profile
        • .bashrc
      • Can add any other files you want here to add to a new user’s home directory.
        • Mozilla configuration files and so on.
      • CREATE_MAIL_SPOOL=yes
        • Whether to create a mail spool or not.
    • Pluggable Authentication Modules
      • Inside the /etc/pam.d/ directory.
        • Configuration files for the pluggable authentication modules are stored.
          • atd
          • crond
          • cups
          • login
          • passwd
          • polkit-1
          • remote
          • runuser
          • sshd
      • useradd command
        • Any options left out are taken from the system defaults.
        • sudo useradd test
          • Can check for passwords by going to viewing /etc/passwd
  • Breaking down the passwd file. test:x:1001:1001::/home/test:/bin/bash * test = username * x = placeholder for the password. * In previous versions of RHEL, the password was stored here. * x means we are using the shadow suite and the password is stored in the /etc/shadow file. * 1001 * User’s numeric ID number. * 1001 * Primary group ID number. * Every user has to belong to one group, which is their primary group. * When a user creates any files, they will be owned by this particular group. * Can cross-reference the group ID in the \etc\group file. * Can see the group listed here. * In Red Hat-based distributions, primary group is the group with the same name as the user. Created automatically. * Not all distributions act this way. * Fifth column bob is the GECOS field. * Used to store general records. * Essentially a comment field to store information about the user account. * This field is world-readable. * /home/test user’s home directory. * Also configured in the /etc/login.defs file. * /bin/bash * The shell to execute upon login. * Need to view the password and ageing information in the /etc/shadow file. * If there is a second column with 2 exclamation points, it means a password has not been set yet. * The user would not be able to login. * After entering in passwd USER and adding their user, you will see an encoded password for them in the /etc/shadow file. * Not all Linux distros create the home directory for a new user automatically (based on a setting by login.defs. * ls -a * Show all files. * If you do the above command in the home directory, you see all of the hidden files * .bash_logout * .bash_profile * .bashrc * .mozilla * The above are copied over from the /etc/skel directory. * Also listed in the login.defs configuration file. If you want to delete a user, but keep their home directory and files: * sudo userdel USER If want to delete a user and their home directory: * sudo userdel -r USER * View the /etc/passwd to verify deletion. * To verify the home directory is gone ls /home and then check the user is gone.

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