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Linux superuser access, explained

Here’s how to configure Linux superuser access so that it's available to those who need it—yet well out of the way of people who don't need it.
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On Linux, everything starts at "root." The hard drive has a root partition (/), and the default initial user (at least traditionally) is root (often referred to as the "superuser"). Historically, you used the root account to log in, create secondary "normal" users, and then mostly forget about it.

These days, though, the root user is redundant to requirements. The Anaconda installer can create an initial user for you, and then you can perform "superuser" tasks with the sudo command. A computer's admin must understand how to configure superuser access so that it's well out of the way of people who don't need it and available to those who require it.

Lock root at installation

An attacker with enough knowledge to attempt a break-in also knows that every Unix and Linux system once had a user called root. That's one less guess an attacker must make when attempting unauthorized entry into a system. Locking down the root account is a significant foil to uninvited guests.

During installation, you have the option to lock the root account.

Locking the root account during installation
(Seth Kenlon, CC BY-SA 4.0)

Locking the root account prevents you from logging in as the root user. If you do that, grant the user you create administrative privileges during user creation.

Granting administrative privileges during user creation
(Seth Kenlon, CC BY-SA 4.0)

Making the user an admin adds that user to the special wheel group. By default, members of wheel can run any command with sudo with, essentially, root privilege. It seems nearly the same as using su to switch to the superuser account, but there are safeguards and advantages to sudo.

[ Free download: Advanced Linux commands cheat sheet. ]

Disable root

If you didn't lock the root account during installation, or if you're unsure, then you can disable the root account later, as long as you have administrative privileges. If you don't, then use the root account to add your user to the sudoers file and then use sudo to disable root.

To disable root, set the root shell to /sbin/nologin:

$ sudo sed -i 's_root:/bin/bash_root:/sbin/nologin_' /etc/passwd

Test your change by attempting to switch to the superuser account with su:

$ sudo su -
[sudo] password for tux: 
This account is currently not available.

Delegate administration

The su command uses an all-or-nothing model. If you have the password for root, you get all the power. If you don't have the password, you have no admin power. The problem with this approach is that a sysadmin must choose between handing over the master key to the system and withholding the key and all control of the system. That's not always what you want. Sometimes you want to delegate.

Suppose you want to grant a user permission to run a specific application, such as the groupadd command, that usually requires administrative permissions. You don't want to give this user permission to do any administrative task. You just want to allow them to create new groups.

To grant selective privileges to a single command or a group of commands, edit /etc/sudoers with the visudo command. The visudo command assumes you want to edit text with vi but also allows you to override that default by setting the variable EDITOR:

$ sudo EDITOR=nano visudo

Find the section of the sudoers file defining command permissions, and add the user and command you want to allow. For instance, to permit the user shadowman to run the groupadd command:

## Next comes the main part: which users can run what software on 
## which machines (the sudoers file can be shared between multiple
## systems).
## Syntax:
## The COMMANDS section may have other options added to it.
## Allow root to run any commands anywhere 
# root    ALL=(ALL)     ALL
shadowman ALL=/sbin/groupadd

Test it out by switching to that user's account and running the command:

$ su - shadowman
$ sudo groupadd qa
[sudo] password for shadowman: 
$ sudo ls
Sorry, user bogan is not allowed to execute '/bin/ls' 
as root on darkstar.sysadmin.local.

Sudo aliases

You can create aliases in the sudoers file to group hosts, commands, and users together. You could have, for instance, an admin group that could use commands such as useradd, groupadd, and usermod, and a software group that could use commands like dnf and rpm.

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The sudoers file provides example syntax, but here's an example of allowing all users in the Unix group softadmins to run all commands in the sudoer alias SOFTWARE:

## Installation and management of software
Cmnd_Alias SOFTWARE = /usr/bin/dnf, /usr/bin/rpm
softadmins ALL=SOFTWARE

Protect your admin privileges

Everything from mission-critical machines to computers with customer data, and even your own humble personal laptop, are too important to casually lend the keys out to anyone who needs an escalation of privilege. Disable the root account to avoid broad permission escalation, and use the sudo command and a well-tended sudoers file to manage who can manage your computers.

Topics:   Certification   Linux administration   Security  
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Seth Kenlon

Seth Kenlon is a UNIX geek and free software enthusiast. More about me

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