Users & Groups

User Management

Create User

useradd [username]

# options
-g   Main user group (gid)
-d   User Home directory, by default located at /home/<UserName>
-m  Create Home directory in case it doesn’t exist.
-s   Assign a specific shell to the user, by default it is /bin/bash.
-M dont create home directory

# example:
useradd -g sector1 -d /home/nicolas -m -s /bin/bash nicolas
sudo useradd -G wheel -m -d /home/frank -s /bin/bash frank


adduser [username]

Add Root User

useradd -ou 0 -g 0 [username]

add user to sudoers group (user will be created of doesnt exist)

//usermod -aG [group] [username]
usermod -aG [group1,group2,group3,...] [username]

Change User Home Directory

usermod -d [dir] [username]

Lock/Unlock User

usermod -L [username]
usermod -U [username]

Set New UID for User

usermod -u [username]

Deleting User

userdel [username]

Password

set password
passwd [username]

List commands you are allowed to run

sudo -l
sudo -ll

Change User Shell

chsh -s [shell ] [username]
chsh -s /bin/bash hesher

List commands another USER is allowed

sudo -l -U user

sudo Commands

Run as root:
sudo COMMAND

 Run as USER:
sudo -u USER COMMAND

Get a shell
sudo -s
sudo -s -u USER

Account Aging

sudo useradd -e 2020-12-31 charlie

you can set the number of days before an account with an expired password will get locked out:

sudo usermod -f 5 charlie

Account Expiration Date

  -I, --inactive INACTIVE       set password inactive after expiration
  -m, --mindays MIN_DAYS        set minimum number of days before password
  -M, --maxdays MAX_DAYS        set maximum number of days before password
  -W, --warndays WARN_DAYS      set expiration warning days to WARN_DAYS
  -d, --lastday LAST_DAY        set date of last password change to LAST_DAY

chage -M 20 john

chage -E 2020-12-31 charlie

Default Account Aging

for Red Hat or CentOS only

/etc/default/useradd

# useradd defaults file
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
CREATE_MAIL_SPOOL=yes

Ubuntu also has the useradd configuration file, but it doesn't work. No matter how you configure it, the Ubuntu version of useradd just won't read it. So, the write-up about this file only applies to Red Hat or CentOS.

Login/Logout Monitoring

monitor last logins

last
lastb
lastlog → last time a user has logged in

Authentication Log Files

/var/log/messages
/var/log/syslog
/var/log/secure
/var/log/auth.log

Configure Password complexity with pam

Debian-based

Ensuring that password meets a certain degree of complexity is equally crucial and further thwarts any attempts by hackers to infiltrate your system using brute force.

As a general rule, a strong password should have a combination of Uppercase, lowercase, numeric and special characters and should be at least 12-15 characters long.

To enforce password complexity in Debian / Ubuntu systems, you need to install the libpam-pwquality package as shown:

sudo apt install libpam-pwquality

Once installed, head out to the /etc/pam.d/common-password file from where you are going to set the password policies. Be default, the file appears as shown:

Locate the line shown below

password   requisite   pam_pwquality.so retry=3

Add the following attributes to the line:

minlen=12 maxrepeat=3 ucredit=-1 lcredit=-1 dcredit=-1 ocredit=-1 difok=4 reject_username enforce_for_root

The entire line should appear as shown:

Let’s flesh out what these directives stand for:

• retry=3: This option will prompt the user 3 times before exiting and returning an error.

• minlen=12: This specifies that the password cannot be less than 12 characters.

• maxrepeat=3: This allows implies that only a maximum of 3 repeated characters can be included in the password.

• ucredit=-1: The option requires at least one uppercase character in the password.

• lcredit=-1: The option requires at least one lowercase character in the password.

• dcredit=-1: This implies that the password should have at last a numeric character.

• ocredit=-1: The option requires at least one special character included in the password.

• difok=3: This implies that only a maximum of 3 character changes in the new password should be present in the old password.

• reject_username: The option rejects a password if it consists of the username either in its normal way or in reverse.

• enforce_for_root: This ensures that the password policies are adhered to even if it’s the root user configuring the passwords.

Redhat-based

For Debian and Ubuntu systems, we enforced the password policy by making changes to the /etc/pam.d/common-password configuration file.

For CentOS 7 and other derivatives, we are going to modify the /etc/pam.d/system-auth or /etc/security/pwquality.conf` ``` configuration file.

So, proceed and open the file:

sudo vim /etc/pam.d/system-auth

Locate the line shown below

password    requisite     pam_pwquality.so try_first_pass local_users_only retry=3 authtok_type=

Append the options in the line as shown.

minlen=12 lcredit=-1 ucredit=-1 dcredit=-1 ocredit=-1 enforce_for_root

You will end up having the line below:

Once done, save the password policies and exit the file.

Once again, when you try creating a user with a weak password that doesn’t adhere to the enforced policies, you will encounter the error shown in the terminal.

Configure Root Access

you can disable root logins entirely by setting root’s encrypted password to * or to some other fixed, arbitrary string. On Linux, passwd -l “locks” an account by prepending a ! to the encrypted password, with equivalent results. The * and the ! are just conventions; no software checks for them explicitly. Their effect derives from their not being valid password hashes. As a result, attempts to verify root’s password simply fail.

make sure only root has UID 0

awk -F: '($3 == "0")'  /etc/passwd

Converting Passwords

pwconv - convert to shadow passwords.
pwunconv - convert from shadow passwords.

/etc/shadow Format

Username
Hashed password
Days since epoch of last password change
Days until change allowed
Days before change required
Days warning for expiration
Days before account inactive
Days since epoch when account expires
Reserved

Lock/Unlock the Password

passwd -l account
passwd -u account 

set the account shell to no login

/sbin/nologin

Using chsh Command

chsh -s SHELL ACCOUNT
chsh -s /sbin/nologin jason

Manually Adding Users

Edit /etc/passwd with view and add a new line for the new account. Be careful with the syntax. Do not edit directly with an editor! vipw locks the file, so that other commands won't try to update it at the same time. You should make the password field be `*', so that it is impossible to log in.

Similarly, edit /etc/group if you need to create a new group as well.

Create the home directory of the user with mkdir.

Copy the files from /etc/skel to the new home directory.

Fix ownerships and permissions with chown and chmod. The -R option is most useful. The correct permissions vary a little from one site to another, but usually the following commands do the right thing:

cd /home/newusername
chown -R username.group .
chmod -R go=u,go-w .
chmod go= .

Set the password with passwd.

Group Management

Create a New Group

groupadd [name]

Change Group Name

groupmod -n [new name] [old name] 

Change the Group ID

groupmod -q 4000 [group]

chmod g+rwx [group]
chown [user]:[group] [file]

set GID

chmod g+s [group]

change primary group

usermod -g [group] [usename]
groupmod -g [gid] -n [new name] [oldname]

remove a user from a group

gpasswd -d [username] [groupname]

remove a group

groupdel [name]

list all system groups

cat /etc/groups

list current user groups

groups

list groups of another account

groups [username]
ip [username]