How To Edit the Sudoers File on Ubuntu and CentOS

Introduction

Privilege separation is one of the fundamental security paradigms implemented in Linux and Unix-like operating systems. Regular users operate with limited privileges in order to reduce the scope of their influence to their own environment, and not the wider operating system.
A special user, called root, has "super-user" privileges. This is an administrative account without the restrictions that are present on normal users. Users can execute commands with "super-user" or "root" privileges in a number of different ways.
In this article, we will discuss how to correctly and securely obtain root privileges, with a special focus on editing the /etc/sudoers file.
We will be completing these steps on an Ubuntu 16.04 server, but most modern Linux distributions should operate in a similar manner.
This guide assumes that you have already completed the initial server setup discussed here. Log into your server as regular, non-root user and continue below.

How To Obtain Root Privileges

There are three basic ways to obtain root privileges, which vary in their level of sophistication.

Log In As Root

The simplest and most straight forward method of obtaining root privileges is simply to log into your server as the root user from the onset.
If you are logging into a local machine (or using out-of-band Console Access if you are using DigitalOcean), simply enter "root" as your username at the login prompt and enter the root password when asked.
If you are logging in through SSH, specify the root user prior to the IP address or domain name in your SSH connection string:

ssh root@server_domain_or_IP

If you have not set up SSH keys for the root user, enter the root password when prompted.

Use "su" to Become Root

Logging in as root is usually not recommended, because it is easy to begin using the system for non-administrative tasks, which is dangerous.
The next way to gain super-user privileges allows you to become the root user at any time, as you need it.
We can do this by invoking the su command, which stands for "substitute user". To gain root privileges, simply type:

su

You will be prompted for the root user's password, after which, you will be dropped into a root shell session.
When you have finished the tasks which require root privileges, return to your normal shell by typing:

exit

Use "sudo" to Execute Commands as Root

The final, and most complex, way of obtaining root privileges that we will discuss is with the sudo command.
The sudo command allows you to execute one-off commands with root privileges, without the need to spawn a new shell. It is executed like this:

sudo command_to_execute

Unlike su, the sudo command will request the password of the user calling the command, not the root password.
Because of its security implications, sudo access is not granted to users by default, and must be set up before it functions correctly. If you followed the initial server setup guide, you already completed a bare-bones configuration.
In the following section, we will discuss how to modify the configuration in greater detail.

What is Visudo?

The sudo command is configured through a file located at /etc/sudoers.

Warning

Never edit this file with a normal text editor! Always use the visudo command instead! Because improper syntax in the /etc/sudoers file can leave you with a system where it is impossible to obtain elevated privileges, it is important to use the visudo command to edit the file.
The visudo command opens a text editor like normal, but it validates the syntax of the file upon saving. This prevents configuration errors from blocking sudo operations, which may be your only way of obtaining root privileges.
Traditionally, visudo opens the /etc/sudoers file with the vi text editor. Ubuntu, however, has configured visudo to use the nano text editor instead.
If you would like to change it back to vi, issue the following command:

sudo update-alternatives --config editor

Output
There are 4 choices for the alternative editor (providing /usr/bin/editor).

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /bin/nano            40        auto mode
  1            /bin/ed             -100       manual mode
  2            /bin/nano            40        manual mode
  3            /usr/bin/vim.basic   30        manual mode
  4            /usr/bin/vim.tiny    10        manual mode

Press <enter> to keep the current choice[*], or type selection number:

Select the number that corresponds with the choice you would like to make.
On CentOS, you can change this value by adding the following line to your ~/.bashrc:

export EDITOR=`which name_of_editor`

Source the file to implement the changes:

. ~/.bashrc

After you have configured visudo, execute the command to access the /etc/sudoers file:

sudo visudo

How To Modify the Sudoers File

You will be presented with the /etc/sudoers file in your selected text editor.
I have copied and pasted the file from Ubuntu 16.04, with comments removed. The CentOS /etc/sudoers file has many more lines, some of which we will not discuss in this guide.

/etc/sudoers

Defaults        env_reset
Defaults        mail_badpass
Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

root    ALL=(ALL:ALL) ALL

%admin ALL=(ALL) ALL
%sudo   ALL=(ALL:ALL) ALL

#includedir /etc/sudoers.d

Let's take a look at what these lines do.

Default Lines

The first line, "Defaults env_reset", resets the terminal environment to remove any user variables. This is a safety measure used to clear potentially harmful environmental variables from the sudo session.
The second line, Defaults mail_badpass, tells the system to mail notices of bad sudo password attempts to the configured mailto user. By default, this is the root account.
The third line, which begins with "Defaults secure_path=...", specifies the PATH (the places in the filesystem the operating system will look for applications) that will be used for sudo operations. This prevents using user paths which may be harmful.

User Privilege Lines

The fourth line, , which dictates the root user's sudo privileges, is different from the preceding lines. Let's take a look at what the different fields mean:

• root ALL=(ALL:ALL) ALL
  The first field indicates the username that the rule will apply to (root).
• demo ALL=(ALL:ALL) ALL
  The first "ALL" indicates that this rule applies to all hosts.
• demo ALL=(ALL:ALL) ALL
  This "ALL" indicates that the root user can run commands as all users.
• demo ALL=(ALL:ALL) ALL
  This "ALL" indicates that the root user can run commands as all groups.
• demo ALL=(ALL:ALL) ALL
  The last "ALL" indicates these rules apply to all commands.

This means that our root user can run any command using sudo, as long as they provide their password.

Group Privilege Lines

The next two lines are similar to the user privilege lines, but they specify sudo rules for groups.
Names beginning with a "%" indicate group names.
Here, we see the "admin" group can execute any command as any user on any host. Similarly, the sudo group can has the same privileges, but can execute as any group as well.

Included /etc/sudoers.d Line

The last line might look like a comment at first glance:

/etc/sudoers

. . .

#includedir /etc/sudoers.d

It does begin with a #, which usually indicates a comment. However, this line actually indicates that files within the /etc/sudoers.d directory will be sourced and applied as well.
Files within that directory follow the same rules as the /etc/sudoers file itself. Any file that does not end in ~ and that does not have a . in it will be read and applied to the sudo configuration.
This is mainly meant for applications to alter sudo privileges upon installation. Putting all of the associated rules within a single file in the /etc/sudoers.d directory can make it easy to see which privileges are associated with which accounts and to reverse credentials easily without having to try to manipulate the /etc/sudoers file directly.
As with the /etc/sudoers file itself, you should always edit files within the /etc/sudoers.d directory with visudo. The syntax for editing these files would be:

sudo visudo -f /etc/sudoers.d/file_to_edit

How To Give a User Sudo Privileges

The most common operation that users want to accomplish when managing sudo permissions is to grant a new user general sudo access. This is useful if you want to give an account full administrative access to the system.
The easiest way of doing this on a system set up with a general purpose administration group, like the Ubuntu system in this guide, is actually to just add the user in question to that group.
For example, on Ubuntu 16.04, the sudo group has full admin privileges. We can grant a user these same privileges by adding them to the group like this:

sudo usermod -aG sudo username

The gpasswd command can also be used:

sudo gpasswd -a username sudo

These will both accomplish the same thing.
On CentOS, this is usually the wheel group instead of the sudo group:

sudo usermod -aG wheel username

Or, using gpasswd:

sudo gpasswd -a username wheel

On CentOS, if adding the user to the group does not work immediately, you may have to edit the /etc/sudoers file to uncomment the group name:

sudo visudo

/etc/sudoers

. . .
%wheel ALL=(ALL) ALL
. . .

How To Set Up Custom Rules

Now that we have gotten familiar with the general syntax of the file, let's create some new rules.

How To Create Aliases

The sudoers file can be organized more easily by grouping things with various kinds of "aliases".
For instance, we can create three different groups of users, with overlapping membership:

/etc/sudoers

. . .
User_Alias      GROUPONE = abby, brent, carl
User_Alias      GROUPTWO = brent, doris, eric, 
User_Alias      GROUPTHREE = doris, felicia, grant
. . .

Group names must start with a capital letter. We can then allow members of GROUPTWO to update the apt database by creating a rule like this:

/etc/sudoers

. . .
GROUPTWO    ALL = /usr/bin/apt-get update
. . .

If we do not specify a user/group to run as, as above, sudo defaults to the root user.
We can allow members of GROUPTHREE to shutdown and reboot the machine by creating a "command alias" and using that in a rule for GROUPTHREE:

/etc/sudoers

. . .
Cmnd_Alias      POWER = /sbin/shutdown, /sbin/halt, /sbin/reboot, /sbin/restart
GROUPTHREE  ALL = POWER
. . .

We create a command alias called POWER that contains commands to power off and reboot the machine. We then allow the members of GROUPTHREE to execute these commands.
We can also create "Run as" aliases, which can replace the portion of the rule that specifies the user to execute the command as:

/etc/sudoers

. . .
Runas_Alias     WEB = www-data, apache
GROUPONE    ALL = (WEB) ALL
. . .

This will allow anyone who is a member of GROUPONE to execute commands as the www-data user or the apache user.
Just keep in mind that later rules will override earlier rules when there is a conflict between the two.

How To Lock Down Rules

There are a number of ways that you can achieve more control over how sudo reacts to a call.
The updatedb command associated with the mlocate package is relatively harmless on a single-user system. If we want to allow users to execute it with root privileges without having to type a password, we can make a rule like this:

/etc/sudoers

. . .
GROUPONE    ALL = NOPASSWD: /usr/bin/updatedb
. . .

NOPASSWD is a "tag" that means no password will be requested. It has a companion command called PASSWD, which is the default behavior. A tag is relevant for the rest of the rule unless overruled by its "twin" tag later down the line.
For instance, we can have a line like this:

/etc/sudoers

. . .
GROUPTWO    ALL = NOPASSWD: /usr/bin/updatedb, PASSWD: /bin/kill
. . .

Another helpful tag is NOEXEC, which can be used to prevent some dangerous behavior in certain programs.
For example, some programs, like "less", can spawn other commands by typing this from within their interface:

!command_to_run

This basically executes any command the user gives it with the same permissions that "less" is running under, which can be quite dangerous.
To restrict this, we could use a line like this:

/etc/sudoers

. . .
username  ALL = NOEXEC: /usr/bin/less
. . .

Miscellaneous Information

There are a few more pieces of information that may be useful when dealing with sudo.
If you specified a user or group to "run as" in the configuration file, you can execute commands as those users by using the "-u" and "-g" flags, respectively:

sudo -u run_as_user command

sudo -g run_as_group command

For convenience, by default, sudo will save your authentication details for a certain amount of time in one terminal. This means you won't have to type your password in again until that timer runs out.
For security purposes, if you wish to clear this timer when you are done running administrative commands, you can run:

sudo -k

If, on the other hand, you want to "prime" the sudo command so that you won't be prompted later, or to renew your sudo lease, you can always type:

sudo -v

You will be prompted for your password, which will be cached for later sudo uses until the sudo time frame expires.
If you are simply wondering what kind of privileges are defined for your username, you can type:

sudo -l

This will list all of the rules in the /etc/sudoers file that apply to your user. This gives you a good idea of what you will or will not be allowed to do with sudo as any user.
There are many times when you will execute a command and it will fail because you forgot to preface it with sudo. To avoid having to re-type the command, you can take advantage of a bash functionality that means "repeat last command":

sudo !!

The double exclamation point will repeat the last command. We preceded it with sudo to quickly change the unprivileged command to a privileged command.
For some fun, you can add the following line to your /etc/sudoers file with visudo:

sudo visudo

/etc/sudoers

. . .
Defaults    insults
. . .

This will cause sudo to return a silly insult when a user types in an incorrect password for sudo. We can use sudo -k to clear the previous sudo cached password to try it out:

sudo -k

sudo ls

Output
[sudo] password for demo:    # enter an incorrect password here to see the results
Your mind just hasn't been the same since the electro-shock, has it?
[sudo] password for demo: 
My mind is going. I can feel it.

Conclusion

You should now have a basic understanding of how to read and modify the sudoers file, and a grasp on the various methods that you can use to obtain root privileges.
Remember, super-user privileges are not given to regular users for a reason. It is essential that you understand what each command does that you execute with root privileges. Do not take the responsibility lightly. Learn the best way to use these tools for your use-case, and lock down any functionality that is not needed.