LPIC-1 Essential System Services and Networking Practice Question
A sysadmin is troubleshooting a connectivity issue between two servers in different subnets. The output of 'traceroute 192.168.2.10' shows packets reaching a router but not the destination. The router's firewall uses iptables. Which rule would prevent the traceroute from completing?
⚠ Common exam trap
The trap here is that candidates often focus on blocking the UDP probes themselves (e.g., via -p udp -j DROP) rather than understanding that traceroute completion depends on the ICMP response from the destination, not just the probe packets.
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
✓
-A INPUT -p icmp --icmp-type port-unreachable -j DROP
Traceroute relies on ICMP Time Exceeded messages from intermediate routers and an ICMP Port Unreachable message from the destination to signal completion. Dropping ICMP type 3 (Destination Unreachable) packets, specifically port-unreachable (code 3), prevents the final response from reaching the source, causing traceroute to hang after reaching the last hop.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
-A INPUT -i lo -j LOG
Why it's wrong here
Logs loopback traffic, does not drop.
- ✗
-A INPUT -p udp -j ACCEPT
Why it's wrong here
Accepts all UDP, would not block traceroute.
- ✗
-A INPUT -p tcp --dport 80 -j REJECT
Why it's wrong here
Affects TCP port 80, not traceroute (UDP).
- ✓
-A INPUT -p icmp --icmp-type port-unreachable -j DROP
Why this is correct
Blocks ICMP Port Unreachable, which stops traceroute responses.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This LPIC-1 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-1 exam.