LPIC-2 System Security Practice Question
A firewall rule set is implemented using iptables. The administrator wants to allow incoming SSH connections only from the 192.168.1.0/24 subnet, while all other incoming traffic is dropped. Which set of rules achieves this?
⚠ Common exam trap
Watch out — candidates often confuse source and destination ports (--sport vs --dport) or place rules in the wrong order, mistakenly thinking that a DROP rule placed before an ACCEPT rule can be overridden by a later rule, when in fact iptables stops processing after the first match.
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
✓
iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -j DROP
It first adds a rule to accept TCP traffic on port 22 (SSH) from the 192.168.1.0/24 subnet, then adds a default DROP rule for all other incoming traffic. Since iptables processes rules in order, the ACCEPT rule matches allowed SSH connections first, and the subsequent DROP rule catches all other incoming packets, effectively dropping everything else including SSH from other sources.
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 -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -j DROP
Why this is correct
Allows SSH from subnet, then drops all other input.
- ✗
iptables -A INPUT -p tcp --sport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -j DROP
Why it's wrong here
--sport specifies source port, which is not the port we are connecting to (SSH uses destination port 22).
- ✗
iptables -A INPUT -p tcp --dport 22 -j DROP iptables -A INPUT -s 192.168.1.0/24 -p tcp --dport 22 -j ACCEPT
Why it's wrong here
DROP rule before ACCEPT rule will drop all SSH, then the second rule is never evaluated.
- ✗
iptables -A INPUT -p tcp --dport 22 -j ACCEPT iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j DROP
Why it's wrong here
First rule allows all SSH, then second rule does not override; packets from subnet will still be allowed.
- ✗
iptables -A INPUT -p tcp --dport 22 -j ACCEPT iptables -A INPUT -j DROP
Why it's wrong here
Allows SSH from any source, then drops all other input; does not restrict SSH to subnet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This LPIC-2 practice question is part of Courseiva's free LPI certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LPIC-2 exam.