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How to Use Sudo on Debian, CentOS, and FreeBSD

Using a sudo user to access a server and execute commands at root level is a very common practice among Linux and Unix Systems Administrator. The use of a sudo user is often coupled by disabling direct root access to one's server in an effort to prevent unauthorized access.
In this tutorial of www.vcclhosting.com blog, we will be covering the basic steps for disabling direct root access, creating a sudo user, and setting up the sudo group on CentOS, Debian, and FreeBSD.

Prerequisites

Step 1: Installing sudo

Debian

apt-get install sudo -y

CentOS

yum install sudo -y

FreeBSD

cd /usr/ports/security/sudo/ && make install clean
or
pkg install sudo

Step 2: Adding the sudo user

A sudo user is a normal user account on a Linux or Unix machine.

Debian

adduser mynewusername

CentOS

adduser mynewusername

FreeBSD

adduser mynewusername

Step 3: Adding the new user to the wheel group (optional)

The wheel group is a user group which limits the number of people who are able to su to root. Adding your sudo user to the wheel group is entirely optional, but it is advisable.
Note: In Debian, the sudo group is often found instead of wheel. You can however manually add the wheel group using the groupadd command. For the purpose of this tutorial, we will use the sudo group for Debian.

The difference between wheel and sudo.

In CentOS and Debian, a user belonging to the wheel group can execute su and directly ascend to root. Meanwhile, a sudo user would have use the sudo sufirst. Essentially, there is no real difference except for the syntax used to become root, and users belonging to both groups can use the sudo command.

Debian

usermod -aG sudo mynewusername

CentOS

usermod -aG wheel mynewusername

FreeBSD

pw group mod wheel -m mynewusername

Step 4: Making sure your sudoers file is setup properly

It is important to ensure that sudoers file located in /etc/sudoers is setup properly in order to allow sudo users to effectively use the sudo command. In order to accomplish that, we will view the contents of /etc/sudoers and edit them where applicable.

Debian

vim /etc/sudoers
or
visudo

CentOS

vim /etc/sudoers
or
visudo

FreeBSD

vim /etc/sudoers
or
visudo
Note: The visudo command will open /etc/sudoers using the system's preferred text editor (usually vi or vim).
Start reviewing and editing below this line:
# Allow members of group sudo to execute any command
This section of /etc/sudoers often looks like this:
# Allow members of group sudo to execute any command %sudo ALL=(ALL:ALL) ALL
In some systems, you may not find %wheel instead of %sudo; in which case, this would be the line under which you would start modifying.
If the line starting with %sudo in Debian or %wheel in CentOS and FreeBSD is not commented out (prefixed by #), this means that sudo is already setup and is enabled. You can then move to the next step.

Step 5: Allowing a user that belongs to neither the wheel nor the sudo group to execute the sudo command

It is possible to allow a user that is in neither user groups to execute the sudo command by simply adding them to /etc/sudoers as follows:
anotherusername ALL=(ALL) ALL

Step 6: Restarting the SSHD Server

In order to apply the changes you made to /etc/sudoers, you need to restart the SSHD server as follows:

Debian

/etc/init.d/sshd restart

CentOS 6

/etc/init.d/sshd restart

CentOS 7

systemctl restart sshd.service

FreeBSD

/etc/rc.d/sshd start

Step 7: Testing

After you have restarted the SSH server, log out and then log back in as your sudo user, then attempt to execute some testing commands as follows:
sudo uptime sudo whoami
Any of the below commands will allow the sudo user to become root.
sudo su - sudo -i sudo -S
Notes:
  • The whoami command will return root when coupled with sudo.
  • You will be prompted to enter your user's password when executing the sudo command unless you explicitly instruct the system to not prompt sudo users for their passwords. Please note that is not a recommended practice.

Optional: allowing sudo without entering the user's password

As previously explained, this is not a recommended practice and is included in this tutorial for demonstration purposes only.
In order to allow your sudo user to execute the sudo command without being prompted for their password, suffix the access line in /etc/sudoers with NOPASSWD: ALL as follows:
%sudo ALL=(ALL:ALL) ALL NOPASSWD: ALL
Note: You need to restart your SSHD server in order to apply the changes.

Step 8: Disable direct root access

Now that you have confirmed that you can use your sudo user without issues, it is time for the eighth and final step, disabling direct root access.
First, open /etc/ssh/sshd_config using your favorite text editor and find the line containing the following string. It may be prefixed with a # character.
PermitRootLogin
Regardless of the prefix or the value of the option in /etc/ssh/sshd_config, you need to change that line to the following:
PermitRootLogin no
Finally, restart your SSHD server.
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