User Management & File Permissions

User & Group Management
User/Group/Password Files
Admin Privileges
File Permissions & Ownership

User & Group Management

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Adding Users

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Adding Users

To add a user, the useradd command can be used.

useradd username

If you want to create a user with it's own home directory, this can be done using the -m flag:

useradd -m username

The default contents of a users home directory are defined within the /etc/skel directory, please see HERE for more info on this.

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Adding a system user

useradd -r username

Once created, you'll want to restrict the account by disabling the ability for login, as mentioned here

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Default options

There are lots of different options that can be set when creating users and groups, the default options can be viewed using the below command:

useradd -D

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Additional options:

-e -expires

-e 2023/12/31

-c - comment

-c "full name"

-s -shell

-s /bin/sh

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Groups

When creating a user, you can also specify groups to add the user to, this is done using the -G flag:

useradd -G groupname username

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Comments

When creating a user, you can also opt to add a comment using the -c flag, for example this could be a name:

useradd -c "comment" username

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Passwords

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Set Password

Once a user has been created, you can add a password using the passwd command:

passwd username

Once run, you'll be prompted to enter a new password

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Changing password

Changing a users password can be done using the passwd command when signed in as that user.

You can either SSH to the server directly using the required user, or access as root and use su- username to access the user. Once accessed, the passwd command can be run alone to change the password:

passwd

You can also change a users password using root:

passwd username

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Additional Options:

chage

Checking user password metrics (password expiration, last time password changed)

chage -l username

Forcing password change at logon

chage -d 0 username

Temporary Password

When setting a password, you're able to set a temporary placeholder password that can be used to log in by the user, upon logging in the user will be prompted to change to a password of their choice. This can be achieved by using the -e flag after setting a password

passwd username #set as temp password

passwd -e #sets password as expired

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Deleting Users

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Deleting Users

Users can be deleted using the userdel command:

userdel username

The above command only removes the user from the system, without removing their home directory.

Remove user and home directory

userdel -r username

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Modifying Users

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Post Creation, users can be modified using the usermod command.

Add User To Group

usermod -a -G groupname username

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Lock/Unlock Users

usermod -L username

usermod -U username

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usermod -s /sbin/nologin username

or we can use the change shell (chsh) command:

chsh -s /bin/nologin username

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Change User Home Directory

usermod -d /pathtonewhome username
chown username:usergroup /pathtonewhome

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Groups

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Viewing Groups

groups username

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Creating Groups

New groups can be created using the groupadd command:

groupadd groupname

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Managing group users

Users can be added/removed from a group with 2 main methods,

They can be added when initially created, as mentioned above
They can be added after creation using the usermod command, as mentioned above
They can be added using the gpasswd command.
Add a user to a group
```
gpasswd -a username groupname
```
Removing a user from a group
```
gpasswd -d username groupname
```
Add user to a group as an admin
```
gpasswd -A username groupname
```

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Deleting groups

Groups can be removed using the groupdel command, note that this doesn't delete the users that are part of this group.

groupdel groupname

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Modifying Groups

There are various group properties that can be modified using the groupmod command.

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Change Group Name

groupmod -n newname oldname

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Change Group ID

groupmod -g NEWID groupname

User/Group/Password Files

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/etc/passwd

used to store user and system accounts

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/etc/shadow

Used to store user passwords (encrypted)

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/etc/group

Used to store group information, and which users are included in a group.

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/etc/skel

This directory is used to define the default items that will get added to a new users home directory

For example, this could contain a README file that contains rules for using the server.

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/etc/default/useradd

This directory is used to define the default options used when the useradd command is used.

For example, this could specify a location that user directories are created, default group to be added to, and more.

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/etc/sudoers

This file is used to define which users have sudo access/

Admin Privileges

User & group privileges

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For users to have escalated privilege on a server (root access), they need to be granted this permission.

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sudo

Users with sudo access have full administrator permissions, this means that they can essentially perform any task on the system.

There are 2 methods we can use to grant users sudo access:

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1. usermod

sudo usermod -aG sudo username

You can then validate that this has worked by checking the groups that the specified user is included in:

groups username

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2 Editing the sudoers file directly

Users with sudo access are defined within the /etc/sudoers file. This file should ONLY ever be edited using the visudo text editor - as this will check the syntax for any errors.

visudo

To add a new user to the sudoers group, we need to append a line to the /etc/sudoers file.

1. Edit the /etc/sudoers file using visudo:

visudo

2. Find the line that reads # User privilege specification and add the following line below it:

username ALL=(ALL:ALL) ALL

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Adding user groups to sudoers

In addition to adding specific users to the sudoers group, we can also add user groups. Once again, this is best achieved by editing the sudoers file directly using visudo.

1. Edit the sudoers file

visudo

2. Find the line that reads # User privilege specification and add the following line below it:

%groupname ALL=(ALL:ALL) ALL

Groups are defined by placing a % symbol in front of the group name.

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Giving users sudo privilege for specific tasks/commands

In addition to giving a user full sudo permission on a system, we can also implement a more limited set of sudo-enabled privileges.

For example, I want to add a user who has sudo permission to run updates on a system, but I don't want them to have all privileges. To do this, we would need to edit the sudoers file as above using the visudo command, once there you can add a line like the following:

username ALL=(ALL:ALL) /usr/bin/apt update,/user/bin/apt upgrade

Note: As best practice, you're best off specifying the full binary path of the commands you wish to grant access to. This prevents a user from renaming a binary to apt (in this example) and being able to run the command with sudo privilege.

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Wheel

The alternative to adding users to the sudoers file, is to add users to the Wheel group. The Wheel group is essentially an exclusion that can be added for users to allow access to certain roles.

By default, any users in the wheel group have full privileges on the server.

An example of how this could be utilised, would be to add a rule into the /etc/wheel file that specifies a group that can be used to perform a specific task. Users that need this privilege could then be added to this file.

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PolicyKit

PolicyKit (also known as polkit) is a toolkit for defining and handling authorizations in Linux systems. It is used to manage privileges for unprivileged processes to perform tasks that normally require higher privileges, such as those of the root user. This allows for more fine-grained control over what users and processes are allowed to do without requiring full administrative access.

File Permissions & Ownership

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Linux File Permissions

Every file in Linux has permissions, these define which actions can be undertaken by the user,group, and other.

As seen on the file below, permissions are set at the start of the line using 10 characters.

-r--r-xrw- 1 root root       27 May 26 10:56 test.txt

These 10 characters are the permission classes, and are used as follows:

(For file type: - is a file, d is a directory).

Permissions can also be represented in number format.

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Changing file or directory permissions

(Numerical representation)

chmod 777 filename

(Letter representation)

chmod u=rwx,g=rwx,o=r

You can also use -R to chmod recursively:

chmod -R 777 directoryname

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Linux File Ownership

Files in Linux are owned by a user and group.

-rw-r--r-- 1 root root       27 May 26 10:56 test.txt

Changing user/group

chown newuser:newgroup

For instances where you want to chown a directory, and all of the subdirectories & files within, we can use the -R (recursive) flag:

chown -R newuser:newgroup directoryname

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FACL - File Access Control List

File Access Control Lists (FACLs) provide a robust mechanism for managing file permissions in Linux, offering greater flexibility and control than traditional Unix permissions. By using commands like setfacl and getfacl, administrators can easily set and view ACLs to fine-tune access to files and directories for multiple users and groups.

View file/directory ACL

getfacl filename

Grant an additional user permissions on a file

setfacl -m u:username:rwx filename

Remove a user's permissions on a file

setfacl -x u:username filename

Define default ownership/permissions for directories

setfacl -m d:u:username:rwx filename

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Sticky bits

In Linux, the sticky bit is a special permission that can be set on directories to control user access to the files within those directories. When the sticky bit is set on a directory, it restricts the deletion or renaming of files within that directory. Specifically, only the file's owner, the directory's owner, or the root user can delete or rename files.

Enable sticky bits

chmod o+t directoryname

Disable sticky bits

chmod o-t directoryname

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User Management & File Permissions

User & Group Management

Adding Users

Adding Users

Adding a system user

Default options

Additional options:

-e -expires

-c - comment

-s -shell

Groups

Comments

Passwords

Set Password

Changing password

Additional Options:

Checking user password metrics (password expiration, last time password changed)

Forcing password change at logon

Temporary Password

Deleting Users

Deleting Users

Modifying Users

Add User To Group

Lock/Unlock Users

Disable login access

Change User Home Directory

Groups

Viewing Groups

Creating Groups

Managing group users

Add a user to a group

Removing a user from a group

Add user to a group as an admin

Deleting groups

Modifying Groups

Change Group Name

Change Group ID

User/Group/Password Files

/etc/passwd

/etc/shadow

/etc/group

/etc/skel

/etc/default/useradd

/etc/sudoers

Admin Privileges

User & group privileges

sudo

1. usermod

2 Editing the sudoers file directly

Adding user groups to sudoers

Giving users sudo privilege for specific tasks/commands

Wheel

PolicyKit

File Permissions & Ownership

Linux File Permissions

Changing file or directory permissions

Linux File Ownership

Changing user/group

FACL - File Access Control List

View file/directory ACL

Grant an additional user permissions on a file

Remove a user's permissions on a file

Define default ownership/permissions for directories

Sticky bits