XK0-006 Security Practice Question
A Linux administrator needs to prevent the root user from logging in via SSH. Which directive should be set in /etc/ssh/sshd_config to accomplish this?
⚠ Common exam trap
It's easy for candidates to confuse `PasswordAuthentication no` with blocking root login, not realizing that root could still authenticate via SSH keys or other mechanisms if `PermitRootLogin` is not explicitly set to `no`.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
PermitRootLogin no
The directive `PermitRootLogin no` in `/etc/ssh/sshd_config` explicitly disallows the root user from authenticating via SSH, regardless of the authentication method used. This is the standard way to block root SSH logins while still allowing other users to connect.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
PasswordAuthentication no
Why it's wrong here
PasswordAuthentication no disables password logins for all users, including non-root accounts, rather than blocking root specifically; PermitRootLogin no is the directive that does this. It is tempting because it does restrict root access indirectly, but it over-restricts and fails the stated requirement.
- ✓
PermitRootLogin no
Why this is correct
PermitRootLogin no directly disables root authentication over SSH, satisfying the requirement to block root logins. The sshd daemon reads this directive at startup and rejects any authentication attempt for the root account, regardless of password or key. Other values such as prohibit-password still allow key-based root access, so only "no" fully prevents it.
- ✗
MaxAuthTries 1
Why it's wrong here
MaxAuthTries 1 merely disconnects a client after one failed authentication attempt; a successful root login on the first try still succeeds. It is tempting as a brute-force mitigation, and it would be the correct directive when the requirement is to limit authentication attempts per connection rather than restrict which accounts may log in.
- ✗
AllowUsers root
Why it's wrong here
AllowUsers root does the opposite of the requirement: it permits only root to log in over SSH, leaving every other account denied. It is tempting because AllowUsers is the right directive family for restricting logins by account, and it would be correct if the goal were to whitelist root alone.
Go deeper
Related to this question
Learn chapter
User and Group Administration
Key term
Linux
Linux is an open-source operating system that manages computer hardware and software, widely used in servers, desktops, and embedded systems.
Key term
SSH
SSH (Secure Shell) is a cryptographic network protocol that provides secure, encrypted communication and remote administration between two devices over an unsecured network.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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