The answer is to add `username hard nproc 100` in `/etc/security/limits.conf`. This configuration file, managed by the PAM (Pluggable Authentication Modules) `pam_limits.so` module, allows system administrators to enforce per-user resource limits, and the `nproc` parameter specifically caps the maximum number of processes a user can spawn. By setting a hard limit like 100, you prevent a fork bomb from exhausting system resources because the kernel will refuse to create new processes for that user once the limit is reached. On the CompTIA Linux+ XK0-005 exam, this question tests your understanding of user-level process control as a security measure; a common trap is confusing `ulimit` (a shell command) with the persistent configuration file, or mistaking `/etc/security/limits.d/` for the primary file. Remember the mnemonic: "nproc stops the fork shock"—the `nproc` parameter is your direct defense against process explosion.
XK0-005 Scripting, Containers and Automation Practice Question
This XK0-005 practice question tests your understanding of scripting, containers and automation. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A Linux system experiences high CPU usage from a process that appears to be a fork bomb. The administrator wants to prevent such attacks in the future by limiting the number of processes a user can create. Which configuration file should be modified, and what parameter should be set?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add 'username hard nproc 100' in /etc/security/limits.conf
Option C is correct because /etc/security/limits.conf is the PAM-based configuration file used to set per-user resource limits via the 'nproc' parameter. Adding 'username hard nproc 100' enforces a hard limit of 100 processes for that user, preventing a fork bomb from exhausting system resources.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Set 'kernel.pid_max=100' in /etc/sysctl.conf
Why it's wrong here
Limits total PIDs system-wide, not per user.
✗
Set 'DefaultLimitNPROC=100' in /etc/systemd/system.conf
Why it's wrong here
This limits systemd services, not all user processes.
✓
Add 'username hard nproc 100' in /etc/security/limits.conf
Why this is correct
Correctly limits the number of processes for a user via PAM.
Related concept
Read the scenario before looking for a memorised answer.
✗
Add 'ulimit -u 100' to /etc/profile
Why it's wrong here
ulimit in profile is not persistent for all sessions and can be bypassed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between system-wide PID limits (kernel.pid_max) and per-user process limits (nproc), and candidates mistakenly choose A because they confuse maximum PID number with maximum number of processes.
Detailed technical explanation
How to think about this question
The 'nproc' limit in /etc/security/limits.conf is enforced by the PAM module pam_limits.so at user login, applying to all processes spawned by that user. The 'hard' limit cannot be exceeded even by root, while a 'soft' limit can be raised up to the hard limit; for fork bomb prevention, a hard limit is essential. In real-world scenarios, administrators often combine this with cgroups (e.g., systemd's user slice) to also limit memory and CPU, providing defense in depth.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the XK0-005 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
Scripting, Containers and Automation — This question tests Scripting, Containers and Automation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Add 'username hard nproc 100' in /etc/security/limits.conf — Option C is correct because /etc/security/limits.conf is the PAM-based configuration file used to set per-user resource limits via the 'nproc' parameter. Adding 'username hard nproc 100' enforces a hard limit of 100 processes for that user, preventing a fork bomb from exhausting system resources.
What should I do if I get this XK0-005 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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