How to provide Sudo access using visudo in Linux

Lets discuss how to correctly and securely obtain root privileges, with a special focus on editing the /etc/sudoers file.

What is SUDO or Sudoers?

The sudo command offers a mechanism for providing trusted users with administrative access to a system without sharing the password of the root user.

When users given access via this mechanism precede an administrative command with sudo they are prompted to enter their own password. Once authenticated, and assuming the command is permitted, the administrative command is executed as if run by the root user.

It is executed like this:

sudo command_to_execute

Unlike su, sudo will request the password of the user calling the command, not the root password.

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.

Note: Never edit this file with a normal text editor! Always use the visudo command instead!

Because improper syntax in the 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 then 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.

How to modify Sudoers file?

sudoers file has many lines, some are commented and some of which we will not discuss in this article.

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

root        ALL=(ALL:ALL) ALL
user1        ALL=(ALL:ALL) ALL

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

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, 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 third and fourth lines, we are somewhat familiar with. The fourth line you added yourself, but you might not have investigated the details of what each portion was accomplishing.

user1 ALL=(ALL:ALL) ALL

The first field indicates the username that the rule will apply to (user1).
The first "ALL" indicates that this rule applies to all hosts.
The first "ALL" within the bracket indicates that the user1 user can run commands as all users.
The second "ALL" withing the bracket indicates that the user1 user can run commands as all groups.
The last "ALL" indicates these rules apply to all commands.

This means that our "root" and "user1" users can run any command using sudo, as long as they provide their password.

Group Privilege Lines

The last 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.

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:

You can also create aliases for: users -> User_Alias, run commands as other users -> Runas_Alias, host -> Host_Alias and command -> Cmnd_Alias

User_Alias OPERATORS = joe, mike, jude
Runas_Alias OP = root, operator
Host_Alias OFNET = 10.1.2.0/255.255.255.0
Cmnd_Alias PRINTING = /usr/sbin/lpc, /usr/bin/lprm

As you can see the alias OPERATORS includes the users joe, mike and jude, the alias OP includes the users root and operator, alias OFNET includes the network 10.1.2.0 (all the C class), and the command alias PRINTING includes the commands lpc and lprm.

So, a typical sudoers file may look like this:

User_Alias     OPERATORS = joe, mike, jude
Runas_Alias    OP = root, operator
Host_Alias     OFNET = 10.1.2.0/255.255.255.0
Cmnd_Alias     PRINTING = /usr/sbin/lpc, /usr/bin/lprm

OPERATORS ALL=ALL

#The users in the OPERATORS group can run any command from any terminal.

linux ALL=(OP) ALL

# The user linux can run any command from any terminal as any user in the OP group (root or operator).

user2 OFNET=(ALL) ALL

# user user2 may run any command from any machine in the OFNET network, as any user.

user3 ALL= PRINTING

# user user3 may run lpc and lprm from any machine.

greenlinux ALL=(ALL) ALL

# user greenlinux may run any command from any machine acting as any user.
If you want not to be asked for a password use this form:

greenlinux ALL=(ALL) NOPASSWD: ALL

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