The correct command is `iptables -I INPUT 1 -s 203.0.113.5 -j DROP`, because inserting the rule at position 1 ensures it is evaluated before the existing ESTABLISHED,RELATED rule, which would otherwise accept packets from that IP if a related connection existed. This sequential processing is critical: iptables stops at the first match, so placing the block early prevents the malicious IP from bypassing the drop via an established session. On the CompTIA Linux+ XK0-005 exam, this scenario tests your understanding of rule ordering and the `-I` flag for insertion versus `-A` for append—a common trap is using `-A` instead of `-I`, which would place the rule after the ACCEPT for established traffic, rendering the block ineffective. Remember the memory tip: “Insert first to block the worst”—always use `-I 1` when you need to deny a specific IP before any stateful acceptance rules.
XK0-005 Security Practice Question
This XK0-005 practice question tests your understanding of security. 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.
Scenario: A financial services company runs a critical application on a Linux server that stores sensitive customer data. The server is configured with a firewall (iptables) that only allows SSH (port 22) and HTTPS (port 443) from the internal network (10.0.0.0/8). Recently, the security team detected unauthorized access attempts from an external IP address (203.0.113.5) targeting port 22. The administrator needs to block this specific IP while maintaining current access rules. The existing iptables rules are:
- INPUT chain policy ACCEPT
- Rule 1: -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
- Rule 2: -A INPUT -p tcp --dport 22 -s 10.0.0.0/8 -j ACCEPT
- Rule 3: -A INPUT -p tcp --dport 443 -s 10.0.0.0/8 -j ACCEPT
- Rule 4: -A INPUT -j DROP
The administrator wants to block 203.0.113.5 from any access. Which command should be added?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
iptables -I INPUT 1 -s 203.0.113.5 -j DROP
Option A is correct because inserting the DROP rule at position 1 with `-I INPUT 1` ensures it is evaluated before the existing ESTABLISHED,RELATED rule (Rule 1). Since iptables processes rules sequentially, placing the block early prevents the malicious IP from being matched by the ESTABLISHED,RELATED rule, which would otherwise accept its packets if a related connection existed. This maintains the existing SSH and HTTPS access rules for the internal network while explicitly dropping all traffic from 203.0.113.5.
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.
✓
iptables -I INPUT 1 -s 203.0.113.5 -j DROP
Why this is correct
Inserts a DROP rule at the top, blocking the IP before any ACCEPT rules.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
iptables -I INPUT 5 -s 203.0.113.5 -j DROP
Why it's wrong here
Inserting at position 5 is after the final DROP rule; still ineffective.
✗
iptables -A INPUT -s 203.0.113.5 -j DROP
Why it's wrong here
Appending after the final DROP rule is ineffective.
✗
iptables -I INPUT 1 -s 203.0.113.5 -j ACCEPT
Why it's wrong here
This would allow the IP instead of blocking.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often append a DROP rule with `-A` or insert it after the default DROP rule, not realizing that rules added after a final DROP are never processed, or they mistakenly use `-j ACCEPT` thinking it will override the default policy, when in fact it would allow the unwanted IP.
Detailed technical explanation
How to think about this question
The ESTABLISHED,RELATED rule (Rule 1) matches packets that are part of an existing connection (tracked by conntrack) or associated with it (e.g., ICMP errors). If the attacker had previously established an SSH session before the block was added, the ESTABLISHED rule would continue to accept their packets unless the block is placed before it. This highlights the importance of rule ordering in iptables: connection tracking rules must be carefully positioned to avoid inadvertently allowing malicious traffic that is part of a tracked flow.
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.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: iptables -I INPUT 1 -s 203.0.113.5 -j DROP — Option A is correct because inserting the DROP rule at position 1 with `-I INPUT 1` ensures it is evaluated before the existing ESTABLISHED,RELATED rule (Rule 1). Since iptables processes rules sequentially, placing the block early prevents the malicious IP from being matched by the ESTABLISHED,RELATED rule, which would otherwise accept its packets if a related connection existed. This maintains the existing SSH and HTTPS access rules for the internal network while explicitly dropping all traffic from 203.0.113.5.
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.
Are there clue words in this question I should notice?
Yes — watch for: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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