Users and groups

Linux and localization.

Authentication is when you determine that a user is authentic - they are who they claim to be.

Managing user accounts

Adding accounts

A few files are used to create accounts (files on the same line are not linked):

Account creation files:                                 Built or modified after acct creation

/etc/default/useradd    -->                             --> /home/userid

/etc/login.defs         -->                             --> /etc/passwd

                                User account created    

/etc/skel               -->                             --> /etc/shadow

admin input             -->                             --> /etc/group

Account creation files

/etc/login.defs

Config file that contains directives for various shadow password suite commands. These settings are created after the OS installation.

shadow password suite: commands that deal with account credentials, including:

useradd
userdel
passwd

Directives include passwd length, how long before you have to change passwd, whether home dir is created by default, etc. Common directives include:

Name	Description
`PASS_MAX_DAYS`	Num of days before passwd change is required.
`PASS_MIN_DAYS`	Num of days after a passwd is changed that it can be changed again.
`PASS_MIN_LENGTH`	Min chars requried in password.
`PASS_WARN_AGE`	Num of days prior to passwd expiration that a warning is issued.
`CREATE_HOME`	Create user acct home directory. Default is ’no'.
`ENCRYPT_METHOD`	passwd hash method.

# lines that do not begin (-v) with $ or # (comment and blanks)
grep -v ^$ /etc/login.defs | grep -v ^\#
MAIL_DIR        /var/mail
...
UID_MIN			 1000   # lowest UID allowed for user accts (sometimes 500)
UID_MAX			60000
GID_MIN			 1000   # lowest GID allowed for group accts
GID_MAX			60000
LOGIN_RETRIES		5
LOGIN_TIMEOUT		60
...

# root is always 0
gawk -F: '{print $3, $1}' /etc/passwd | sort -n
0 root
1 daemon
2 bin

UID: User Identification Number (UID) is assigned to a user account or normal account.
- Humans use account names, Linux uses UIDs.
System accounts proide services (daemons) or perform special tasks.
root always has UID = 0.

/etc/default/useradd

Contains default values for useradd command:

# list all active directives
useradd -D
GROUP=100
HOME=/home              # Must set CREATE_HOME in /etc/login.defs to 'yes'
INACTIVE=-1             # Num of days after passwd expiration that the acct is deactivated
EXPIRE=
SHELL=/bin/sh           # default shell program
SKEL=/etc/skel          # skeleton directory
CREATE_MAIL_SPOOL=no

/etc/skel

Contains files. If you set up users to have a HOME directory, these files are copied to /user/home/:

ls -laog /etc/skel/
total 44
drwxr-xr-x   2  4096 Mar 20 20:51 .
drwxr-xr-x 168 12288 Mar 20 20:55 ..
-rw-r--r--   1   220 Apr  4  2018 .bash_logout
-rw-r--r--   1  3771 Apr  4  2018 .bashrc
-rw-r--r--   1  8980 Apr 16  2018 examples.desktop
-rw-r--r--   1  2078 Dec  6  2021 .kshrc
-rw-r--r--   1   807 Apr  4  2018 .profile

Files built or modified when acct created

These files are built or modified after a user account is created. Does not include group files.

/etc/passwd

Stores account information. When account is created, it is added to this file.

This file does not hold passwords due to security concerns. If it does, you can migrate passwords to the correct /etc/shadow file with the pwconv command.

cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin

Field num	Description
1	username
2	password field. `x` indicates there is a passwd in `/etc/shadow`
3	UID
4	GID
5	Comment field, traditionally contains user’s full name
6	user HOME dir
7	user default shell. `/sbin/nologin` and `/bin/false` prevents an acct from logging in to shell, usually for system service accounts. If you attempt to login, you get a message and the session terminates. Edit/create `/etc/nologin.txt` to customize the message. `/bin/false` just logs you out, no message.

/etc/shadow

Contains info about account’s password, even if one does not yet exist.

sudo cat /etc/shadow
root:$y$j9T$EDErVNwiCEqd1Nm5W26d..$aFG6ZaS4.iri2wuCLwuzcexksaNNYDpm1ntYDOGTb03:19804:0:99999:7:::
daemon:*:19773:0:99999:7:::
bin:*:19773:0:99999:7:::
sys:*:19773:0:99999:7:::
...
<username>:$y$j9T$qQNRlSwnLVRW/liifd7HW/$w22t73yRHnNvpqsSgDChfEucV825MollmauAvtbeYp9:19804:0:99999:7:::

Field num	Description
1	username. Only field shared with `/etc/passwd`
2	passwd, salted and hashed. `!` or `!!` indicates no password. `*` indicates account cannot log in with a password. `!<password-hash>` indicates account is locked.
3	Date of passwd change in Unix Epoch time
4	Num of days after a passwd is changed that it can be changed again.
5	Num of days before passwd change is required. Password expiration date
6	Num of days prior to passwd expiration that a warning is issued.
7	Num of days after a passwd expires that the account is deactivated
8	Date account expired in Unix Epoch time
9	special flag field for a special future use. It is blank.

Unix Epoch time is the number of seconds since Jan 1, 1970. etc/shadow expresses it in days, not seconds.

Create accounts

useradd

useradd <username>
-c # comment field, traditionally includes user's full name
-d # home dir specification
-D # display directives in /etc/default/useradd
-e # acct expirtaion date, set by EXPIRE directive
-f # num of days passwd expired before acct deactivated. -1 means never deactivate. INACTIVE directive
-g # group membership
-G # additional group membership
-m # create user acct /home. CREATE_HOME directive
-M # do NOT create /home
-s # acct shell. SHELL directive
-u # UID
-r # create system acct, not user acct

Rocky

# check directive settings
grep CREATE_HOME /etc/login.defs 
CREATE_HOME     yes
useradd -D | grep -i shell
SHELL=/bin/bash
# create user
sudo useradd <username>

Ubuntu

You have to add more options in Ubunutu bc /etc/login.defs does not have the same defaults as other distros:

# check directive settings
grep CREATE_HOME /etc/login.defs 
useradd -D | grep -i shell
SHELL=/bin/sh
# create user with home dir and bash shell
sudo useradd -md /home/linuxuser -s /bin/bash linuxuser
# check user files were created
grep ^linuxuser /etc/passwd
linuxuser:x:1001:1001::/home/linuxuser:/bin/bash
sudo grep ^linuxuser /etc/shadow
linuxuser:!:19813:0:99999:7:::  # no password yet
sudo ls -a /home/linuxuser/
.  ..  .bash_logout  .bashrc  .profile
sudo ls -a /etc/skel/
.  ..  .bash_logout  .bashrc  .profile

getent

View account records:

getent <filename> <username>

getent passwd linuxuser
linuxuser:x:1001:1001::/home/linuxuser:/bin/bash
# honors security settings on /etc/shadow
getent shadow linuxuser
sudo getent shadow linuxuser
linuxuser:!:19813:0:99999:7:::

Passwords

passwd

Set or modify the user password with passwd:

sudo password <username>
-d # rm acct passwd
-e # set passwd as expired, must change it at next login
-i # sets num of days for acct to become inactive after passwd expiration
-l # lock, places '!' in front of passwd in /etc/shadow
-n # num of days after passwd change that user can change passwd again
-S # display password status
-u # unlock, rm '!' in front of passwd in /etc/shadow
-w # num of days to issue warning before password expiration
-x # num of days until a passwd change is required

# get passwd status
# P usable
# NP no password
# L locked
sudo passwd -S linuxuser
linuxuser P 03/31/2024 0 99999 7 -1

chage

Same as passwd -S <username> but more human readable:

# view acct passwd status
sudo chage -l <username>
# interactively update passwd settings
sudo chage <username>

Modifying accounts

usermod

Utility to modify accounts, useful when you forget to check the distro’s account creation directives:

usermod [FLAG...] <username>
-c # modify comment field
-d # set new home dir. use -m option to move contents of current /home to new /home
-e # modify acct expiration date
-f # modify num of days for acct to become inactive after passwd expiration
-g # modify default group
-G # update additional group membership
-l # modify account username (login)
-L # lock acct, (!) in shadow file
-s # change acct shell
-u # change UID
-U # Unlock acct, rm (!) in shadow file

# lock account
sudo usermod -L linuxuser 
# display status
sudo passwd -S linuxuser 
linuxuser L 03/31/2024 0 99999 7 -1
# display shadow file entry- (!)
sudo getent shadow linuxuser
linuxuser:!$y$j9T$f4O7G12v7ecON6j8SAfiQ.$7kRt7qYpxngkDPaHATTNBlDGc6hHQc7sPuAW9iMKAJ.:19813:0:99999:7:::
# unlock and display status again
sudo usermod -U linuxuser 
sudo passwd -S linuxuser 
linuxuser P 03/31/2024 0 99999 7 -1

Deleting accounts

userdel

Deletes an account and all account files:

# -r deletes user /home dir and its contents
sudo userdel -r <username>

Troubleshooting

Check password

# check password
sudo getent passwd linuxuser 
linuxuser:x:1001:1001::/home/linuxuser:/bin/bash

# check shadow file, make sure no '!'
sudo getent shadow linuxuser
linuxuser:$y$j9T$f4O7G12v7ecON6j8SAfiQ.$7kRt7qYpxngkDPaHATTNBlDGc6hHQc7sPuAW9iMKAJ.:19813:0:99999:7:::

last, lastlog, and lastb

last and lastlog check when an account was last accessed:

last maintains more login in /var/log/wtmp and /var/log/wtmp.* files
lastlog maintains only most recent login in /var/log/lastlog
lastb searches for failed login attempts in /var/log/btmp

# all logins, specify additional files with -f option
sudo last -f /var/log/wtmp | grep linuxus
linuxuse pts/1        10.0.2.2         Mon Apr 22 08:31 - 08:34  (00:03)
linuxuse pts/0        10.0.2.2         Fri Apr 19 20:44    gone - no logout
linuxuse pts/0        10.0.2.2         Thu Apr 18 23:06 - 23:49  (00:42)
linuxuse pts/0        10.0.2.2         Thu Apr 18 22:13 - 23:06  (00:53)
linuxuse pts/0        10.0.2.2         Thu Apr 18 22:09 - 22:09  (00:00)
linuxuse tty2         tty2             Thu Apr 18 22:00 - down   (00:05)
linuxuse tty2         tty2             Thu Apr 18 21:55 - down   (00:03)
linuxuse tty2         tty2             Sun Apr 14 10:39 - crash (4+11:15)
linuxuse tty2         tty2             Sun Apr 14 10:34 - 10:35  (00:01)
linuxuse tty2         tty2             Thu Apr 11 22:02 - 10:31 (2+12:28)
linuxuse tty2         tty2             Thu Apr 11 21:53 - 22:01  (00:08)

# most recent login
lastlog
Username         Port     From             Latest
root                                       **Never logged in**
...
linuxuser        pts/1    10.0.2.2         Mon Apr 22 08:31:02 -0400 2024   # in last output
...

# failed login attempts
sudo lastb -f /var/log/btmp 
linuxuse ssh:notty    127.0.0.1        Thu Apr 18 23:04 - 23:04  (00:00)
linuxuse ssh:notty    127.0.0.1        Thu Apr 18 23:04 - 23:04  (00:00)
linuxuse ssh:notty    127.0.0.1        Thu Apr 18 23:04 - 23:04  (00:00)
linuxuse ssh:notty    127.0.0.1        Thu Apr 18 23:04 - 23:04  (00:00)

btmp begins Thu Apr 18 23:04:27 2024

Privilege issues

Check to make sure the users have the correct su privileges:

# members of admin can become su
# members of sudoers can execute any command as any user
sudo cat /etc/sudoers
...
# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo	ALL=(ALL:ALL) ALL

Check which groups the user belongs to with id:

id linuxuser
uid=1001(linuxuser) gid=1001(linuxuser) groups=1001(linuxuser),27(sudo)

GUI status

If a user can log into the terminal but cannot log into the GUI, check the GUI with systemctl:

# check GUI status
sudo systemctl status graphical.target
● graphical.target - Graphical Interface
     Loaded: loaded (/lib/systemd/system/graphical.target; static)
     Active: active since Thu 2024-04-18 22:12:54 EDT; 5 days ago
       Docs: man:systemd.special(7)

Apr 18 22:12:54 ubuntu22 systemd[1]: Reached target Graphical Interface.

Terminal status

You should check:

multiple users can log into the terminal
whether the terminal is corrupted
getty services are running. These services provide the login prompts for text-based terminals

# verify multiple users can log in
sudo systemctl status multi-user.target
● multi-user.target - Multi-User System
     Loaded: loaded (/lib/systemd/system/multi-user.target; static)
     Active: active since Thu 2024-04-18 22:12:54 EDT; 5 days ago
       Docs: man:systemd.special(7)

Apr 18 22:12:54 ubuntu22 systemd[1]: Reached target Multi-User System.

# check whether terminal is corrupted
c # means its a character file
- # means terminal is corrupted, rebuild with mknod command
ls -l /dev/tty?
crw--w---- 1 root tty 4, 0 Apr 21 09:44 /dev/tty0
crw--w---- 1 gdm  tty 4, 1 Apr 21 09:44 /dev/tty1
crw--w---- 1 root tty 4, 2 Apr 21 09:44 /dev/tty2
crw--w---- 1 root tty 4, 3 Apr 21 09:44 /dev/tty3
crw--w---- 1 root tty 4, 4 Apr 21 09:44 /dev/tty4
crw--w---- 1 root tty 4, 5 Apr 21 09:44 /dev/tty5
crw--w---- 1 root tty 4, 6 Apr 21 09:44 /dev/tty6
crw--w---- 1 root tty 4, 7 Apr 21 09:44 /dev/tty7
crw--w---- 1 root tty 4, 8 Apr 21 09:44 /dev/tty8
crw--w---- 1 root tty 4, 9 Apr 21 09:44 /dev/tty9

# check getty services
sudo systemctl status getty.target
● getty.target - Login Prompts
     Loaded: loaded (/lib/systemd/system/getty.target; static)
     Active: active since Thu 2024-04-18 22:12:29 EDT; 5 days ago
       Docs: man:systemd.special(7)
             man:systemd-getty-generator(8)
             http://0pointer.de/blog/projects/serial-console.html

Apr 18 22:12:29 ubuntu22 systemd[1]: Reached target Login Prompts.

Locked accounts

# check status of linuxuser. 'L' means 'locked'
sudo passwd -S linuxuser 
linuxuser L 03/31/2024 0 99999 7 -1

# verify locked w getent. '!' before passwd means its locked
sudo getent shadow linuxuser
linuxuser:!$y$j9T$f4O7G12v7ecON6j8SAfiQ.$7kRt7qYpxngkDPaHATTNBlDGc6hHQc7sPuAW9iMKAJ.:19813:0:99999:7:::

# unlock user
sudo usermod -U linuxuser 

# verify unlocked
sudo passwd -S linuxuser 
linuxuser P 03/31/2024 0 99999 7 -1

Expired accounts

Check account expiration with chage:

date
Wed Apr 24 02:51:34 PM EDT 2024

# view account status
sudo chage -l linuxuser
Last password change					: Mar 31, 2024
Password expires					: never
Password inactive					: never
Account expires						: never                           # never expires
Minimum number of days between password change		: 0
Maximum number of days between password change		: 99999
Number of days of warning before password expires	: 7

Authentication

Check or run the following:

PAM modules
pam_tally2
faillock
IdP config (LDAP, Kerberos)
SELinux or AppArmor

You can use sealert for SELinux machines:

# checks for policy violations
sealert -a /var/log/audit/audit.log
100% done
found 0 alerts in /var/log/audit/audit.log

Groups

Groups are part of Linux’s discretionary access control (DAC).
DAC: Traditional Linux security control where access to a file or any object is based on the user’s identity and current group membership.
Default group: When you create a user, it is given a default group.
- A process can have only one group at a time
- Users can be members of many groups but have only one default
- Default group is the acct’s current group when first logged in
- If default group is not designated when a user acct is created, a new group is created with same name as username and new GID
Have name and group identification number (GID). Names are for humans to read, GIDs are for linux to read
tracked in /etc/group
Group passwds stored in /etc/gshadow file

DO NOT allow access to groups with group passwords. Set user acct passwords, and grant access to groups via membership, not passwords.

View group membership

# GID is 4th field
grep linuxuser /etc/passwd | gawk -F : '{print $4}'
1001
# GID, but not group name
getent passwd linuxuser 
linuxuser:x:1001:1001::/home/linuxuser:/bin/bash
# display account name
sudo groups linuxuser 
linuxuser : linuxuser
# display GID
getent group linuxuser
linuxuser:x:1001:
# confirm GID
grep 1001 /etc/group
linuxuser:x:1001:

groupadd

Creates a group, tracked in /etc/group. Group must exist before you can add a user to it.

Debian prefers that you use addgroup

# -g specifies group number
sudo groupadd -g 1966 cream
# view group
getent group cream
cream:x:1966:
# <groupname>:<password-in-/etc/gshadow>:GID:<group-members>
grep cream /etc/group
cream:x:1966:
# check for passwd
sudo getent gshadow cream
cream:!::

Add user to group

Add users to groups with usermod:

# -a preserves existing group membership
# -G add the user acct to the group
sudo usermod -aG <groupname>
# view groups
sudo groups linuxuser 
linuxuser : linuxuser
# add user to group
sudo usermod -aG cream linuxuser 
# view user groups
sudo groups linuxuser 
linuxuser : linuxuser cream
# view user group membership
getent group cream
cream:x:1966:linuxuser

groupmod

Modifies a group:

groupmod [OPTIONS...] <groupname>
-g # modify GID
-n # modify name

# view group info
getent group cream
cream:x:1966:linuxuser
# change group name
sudo groupmod -n Cream cream
# confirm /etc/group was updated
getent group Cream
Cream:x:1966:linuxuser

groupdel

Deletes a group:

# delete group
sudo groupdel Cream
# check /etc/group cleanup
getent group Cream
# check user was removed from group
sudo groups linuxuser 
linuxuser : linuxuser
# check fs for files associated with deleted group
sudo find / -gid 1966 2>/dev/null

Environment setup

After user authenticates to the system and before the bash shell prompt displays, the user environment is configured.
When you start a bash shell, bash checks several files for configuration, called environment files (also called startup files)
Can start a shell in 3 ways:
- Default login (login to server w no GUI)
- Interactive shell spawned as subshell (such as from a GUI)
- non-interactive shell, such as when running a script

Environment variables

Store info about the current shell session

# user less pager
set
env
printenv
# view what is stored in env var
$ENVVARNAME

Environment files

Generally populated from the /etc/skel file. Users can edit these files after they are in their user account.
The first file found in the following order is ran, and the rest are ignored:
- .bash_profile
- .bash_login
- .profile
  .bashrc is run from a file in the preceding list. It is also always run when there is a non-interactive shell started

Global files

Global files:

/etc/profile
/etc/profile.d files
/etc/bash or /etc/bash.bashrc file (depends on distro)

Do not change global files. You can create a custom env file with an .sh extension and place it in /etc/profile.d. Files in this directory are run during bash login.

Querying users

You can audit user access history and other account information.

whoami

Displays current user name:

whoami

who

Displays info about every account on the system:

who
<username> tty2         2024-03-24 16:48 (tty2)

w

Like who, but more verbose:

# load average: 1m, 5m, 15m
# JCPU: total CPU time acct has used
# PCPU: CPU time current command has used
# WHAT: what the acct is currently running
w
 08:59:08 up 14:54,  1 user,  load average: 0.19, 0.13, 0.10
USER       TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
<username> tty2     tty2             24Mar24 23:24m  0.01s  0.01s /usr/libexec/gnome-session-binary --session=ubuntu

id

Useful in shell scripts.

Gather various data about the current user process or info about the provided user id:

id [username|UID]
-g # Display acct's current group's GID
-G # Display all grou membership GIDs
-n # Display acct name instead of UID
-u # Display acct UID

# current user
id
uid=1000(<current-user>) gid=1000(<current-user>) groups=1000(<current-user>),27(sudo),999(vboxsf)
# specified user
id linuxuser 
uid=1001(linuxuser) gid=1001(linuxuser) groups=1001(linuxuser)
# specified user
id -un 1001
linuxuser

# script example
grep USER /etc/profile
USER="`/usr/bin/id -un`"

last

Pulls info from the /var/log/wtmp file and shows a list of accounts and the last login/logout times:

# pull from /var/log/wtmp
last
<username> tty2         tty2             Sun Mar 24 16:48    gone - no logout
reboot     system boot  6.5.0-26-generic Sun Mar 24 16:47   still running
<username> tty2         tty2             Sat Mar 23 09:19 - crash (1+07:27)
reboot     system boot  6.5.0-26-generic Sat Mar 23 09:04   still running
<username> tty2         tty2             Thu Mar 21 23:09 - down   (00:05)
reboot     system boot  6.5.0-26-generic Thu Mar 21 23:07 - 23:14  (00:06)
reboot     system boot  6.5.0-26-generic Thu Mar 21 23:07 - 23:07  (00:00)
<username> tty2         tty2             Thu Mar 21 23:06 - down   (00:00)
reboot     system boot  6.5.0-26-generic Thu Mar 21 23:05 - 23:06  (00:01)
...

# pull from specific file (wtmp.1):
last -f /var/log/wtmp.1

Disk quotas (NEED TO FIGURE OUT)

https://www.linode.com/docs/guides/file-system-quotas/

You can limit the number of files a user can create and restrict the total fs space available to them to prevent users from filling up the hard drive with files.

Has four steps:

Modify /etc/fstab to enable fs quotas
Mount the fs. If it was already mounted, unmount it and mount it again.
Create the file quota.
Establish user or group quota limits and grace periods.