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===== User Management on Linux CentOS / Redhat v7 =====

==== Base commands ====

=== List all users ===

List local and LDAP users
<cli prompt='>'>
[user1@linux1] /home/user1 > getent passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
...
</cli>

List parameters for a specific user
<cli prompt='>'>
[root@linux1] /root > lslogins shs
Username:                           shs
UID:                                1000
Gecos field:                        Sandra H-S
Home directory:                     /home/shs
Shell:                              /bin/bash
No login:                           no
Password is locked:                 no
Password not required:              no
Login by password disabled:         no
Password encryption method:         SHA-512
Primary group:                      shs
GID:                                1000
Supplementary groups:               secteam,sudo
Supplementary group IDs:            11,27
Last login:                         14:44
Last terminal:                      pts/1
Last hostname:                      192.168.0.13
Hushed:                             no
Password expiration warn interval:  7
Password changed:                   2020-Sep30
Maximum change time:                99999
Running processes:                  67
</cli>

List user logged in
<cli prompt='>'>
[root@linux1] /root > lslogins -u
  UID USER                PROC PWD-LOCK PWD-DENY LAST-LOGIN GECOS
    0 root                 151        0        0            root
 1000 shs                   68        0        0      12:35 Sandra H-S
</cli>

<cli prompt='>'>
[root@linux1] /root > passwd -S glpiagent
glpiagent PS 2024-02-13 0 99999 7 -1 (Password set, SHA512 crypt.)
</cli>


List all groups, local and LDAP
<cli prompt='>'>
[user1@linux1] /home/user1 > getent group
root:x:0:
bin:x:1:
daemon:x:2:
sys:x:3:
adm:x:4:
...
adgrp01:*:2222222:user1,user2,user3
</cli>

=== Change a user password ===

<cli prompt='#'>
# echo -e "MyOldPasswd\nMyNewPasswd\nMyNewPasswd" | passwd  
</cli>

<cli prompt='#'>
# echo "user01:MyNewPasswd" | chpasswd
</cli>

=== List all users LDAP ===

List local and LDAP users
<cli prompt='>'>
[user1@linux1] /home/user1 > strings /var/lib/sss/db/*
name=aix-users@test.lu,cn=groups,cn=test.lu,cn=sysdb
lu,cn=sysdb
T@test.lu,cn=users,cn=name=aix-users@test.lu,cn=groups,cn=test.lu,cn=sysdb
lu,cn=sysdb
T@test.lu,cn=users,cn=test.lu,cn=sysdb
&DN=@INDEX:UIDNUMBER:1234423298
@INDEX:UIDNUMBER:1234423298
@IDXVERSION
@IDX
name=user01@test.lu,cn=users,cn=test.lu,cn=sysdb
DN=@INDEX:GIDNUMBER:1234400513
@INDEX:GIDNUMBER:1234400513
@IDXVERSION
@IDX

...
</cli>

<cli prompt='>'>
[user1@linux1] /home/user1 > getent passwd -s sss user01
user01@test.lu:*:1234423298:1234400513:My test user:/home/user01@test.lu:/usr/bin/bash
</cli>

List users in an AD group
<cli prompt='>'>
[user1@linux1] /home/user1 > getent group grpadmin@test.lu
grpadmin@ad.lu:*:1234423233:user01@test.lu,user02@test.lu,user03@test.lu,
</cli>

=== Create user and group ===

Create group and user:
  groupadd -g 150 staff           (-r)
  useradd -u 200 -c 'Test User1' -d /home/user1 -g staff -G staff,apache -s /bin/bash  -p password user1 
To add a system user (add the options: -r -m), it will be ceated in the SYS_UID/SYS_GID id range specified in /etc/login.defs

Add a secondary group for the user1
  usermod -aG group2 user1
  
list member of a group
  groupmems -l group2
  
Lock a user (-U for unlock)
  usermod -L [LOGIN]

The **chage** utility.
<cli prompt='#'>
[root@linux01 ~]# chage -M 90 myadmin

[root@linux01 ~]# chage -l myadmin
Last password change                                    : May 23, 2025
Password expires                                        : May 23, 2026
Password inactive                                       : never
Account expires                                         : never
Minimum number of days between password change          : 0
Maximum number of days between password change          : 90
Number of days of warning before password expires       : 8

[root@linux01 ~]# lchage -l myadmin
Account is not locked.
Minimum:        0
Maximum:        90
Warning:        8
Inactive:       Never
Last Change:    05/23/2025
Password Expires:       05/23/2026
Password Inactive:      Never
Account Expires:        Never
</cli>

Usage: chage [options] [LOGIN]
<code>
Options:
  -d, --lastday LAST_DAY        set date of last password change to LAST_DAY
  -E, --expiredate EXPIRE_DATE  set account expiration date to EXPIRE_DATE
  -h, --help                    display this help message and exit
  -I, --inactive INACTIVE       set password inactive after expiration
                                to INACTIVE
  -l, --list                    show account aging information
  -m, --mindays MIN_DAYS        set minimum number of days before password
                                change to MIN_DAYS
  -M, --maxdays MAX_DAYS        set maximim number of days before password
                                change to MAX_DAYS
  -W, --warndays WARN_DAYS      set expiration warning days to WARN_DAYS
</code>

lock a user:
  passwd -l USER
  usermod -L USER
  
Unlock a user:
  passwd -u USER
  usermod -U USER

Verify the status of a user with passwd command
  P or PS: password is set (user is unlocked)
  L or LK: User is locked
  N or NP: No password is needed by the user
<cli prompt='#'>
[root@temp-rh8 ~]# passwd -S agent
agent PS 2023-10-16 0 99999 7 -1 (Password set, SHA512 crypt.)
</cli>

Change PAM config for pawword-history
  pam-config -a --pwhistory --pwhistory-remember=6

=== delete user and group ===

Removes the user entries from the /etc/passwd and /etc/shadow, files.
<cli prompt='$'>
$ userdel -r username
</cli>

Removes the group, -f: It used to delete a group even if it is the primary group of a user.
<cli prompt='$'>
$ groupdel -f groupname
</cli>

==== Defaults User parameters ====

Files for password management
  On Redhat 9 the file /etc/libuser.conf is used
  /etc/security/pwquality.conf
  /etc/security/pwhistory.conf


=== Password Algorithm ===

You can check the current password algorithm:
<cli prompt='#'>
[root@centos7 etc]# authconfig --test|grep hashing
 password hashing algorithm is sha512
[root@centos7 etc]# cat /etc/login.defs | grep ENCRYPT
ENCRYPT_METHOD SHA512
[root@centos7 ~]# cat /etc/sysconfig/authconfig | grep PASSWDALGORITHM
PASSWDALGORITHM=sha512
</cli>

Change the Algorithm:
<cli prompt='#'>
[root@centos7 etc]# authconfig --passalgo=sha512 --update
getsebool:  SELinux is disabled
</cli>

On Redhat 9 the file **/etc/libuser.conf** is used

Content of login.defs file:
<cli prompt='#'>
[root@centos7 etc]# cat /etc/login.defs
#
# Please note that the parameters in this configuration file control the
# behavior of the tools from the shadow-utils component. None of these
# tools uses the PAM mechanism, and the utilities that use PAM (such as the
# passwd command) should therefore be configured elsewhere. Refer to
# /etc/pam.d/system-auth for more information.
#

# *REQUIRED*
#   Directory where mailboxes reside, _or_ name of file, relative to the
#   home directory.  If you _do_ define both, MAIL_DIR takes precedence.
#   QMAIL_DIR is for Qmail
#
#QMAIL_DIR      Maildir
MAIL_DIR        /var/spool/mail
#MAIL_FILE      .mail

# Password aging controls:
#
#       PASS_MAX_DAYS   Maximum number of days a password may be used.
#       PASS_MIN_DAYS   Minimum number of days allowed between password changes.
#       PASS_MIN_LEN    Minimum acceptable password length.
#       PASS_WARN_AGE   Number of days warning given before a password expires.
#
PASS_MAX_DAYS   99999
PASS_MIN_DAYS   0
PASS_MIN_LEN    5
PASS_WARN_AGE   7

#
# Min/max values for automatic uid selection in useradd
#
UID_MIN                  1000
UID_MAX                 60000
# System accounts
SYS_UID_MIN               201
SYS_UID_MAX               999

#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN                  1000
GID_MAX                 60000
# System accounts
SYS_GID_MIN               201
SYS_GID_MAX               999

#
# If defined, this command is run when removing a user.
# It should remove any at/cron/print jobs etc. owned by
# the user to be removed (passed as the first argument).
#
#USERDEL_CMD    /usr/sbin/userdel_local

#
# If useradd should create home directories for users by default
# On RH systems, we do. This option is overridden with the -m flag on
# useradd command line.
#
CREATE_HOME     yes

# The permission mask is initialized to this value. If not specified,
# the permission mask will be initialized to 022.
UMASK           077

# This enables userdel to remove user groups if no members exist.
#
USERGROUPS_ENAB yes

# Use SHA512 to encrypt password.
ENCRYPT_METHOD SHA512
</cli>

==== User locked ====

Check locked users
<cli prompt='#'>
[root@Linux ~]# passwd -S user1 
user1 LK 2023-01-07 0 99999 7 -1 (Password locked.)
</cli>

To check if a system is configured to allow more or less than the usual three failed logins, we can check the value of deny in the **/etc/security/faillock.conf** file:
<cli prompt='#'>
# Deny access if the number of consecutive authentication failures
# for this user during the recent interval exceeds n tries.
# The default is 3.
 deny = 3
</cli>

<cli prompt='#'>
# faillock --user baeldung
baeldung:
When                Type  Source                                           Valid
2022-06-21 18:32:16 RHOST 192.168.0.22                                         V
2022-06-21 18:32:29 RHOST 192.168.0.22                                         V
2022-06-21 18:32:41 RHOST 192.168.0.22                                         V
</cli>

Unlock a user:
<cli prompt='#'>
# faillock --user baeldung  --reset
</cli>

Or 
<cli prompt='#'>
# rm /var/run/faillock/baeldung
</cli>


http://www.slashroot.in/how-are-passwords-stored-linux-understanding-hashing-shadow-utils

http://wiki.centos.org/HowTos/OS_Protection

http://onemoretech.wordpress.com/2014/02/23/sssd-for-ldap-auth-on-linux/
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