- A
The network cable is faulty.
Why wrong: Incorrect. If the network cable were faulty, ping would likely fail as well.
- B
The app server's firewall is blocking outgoing connections to port 3306.
Correct. The app server's firewall may allow ICMP but block outbound TCP to port 3306, explaining why telnet fails while ping succeeds.
- C
The router does not have a route to 192.168.1.0/24 from the DB server's subnet, so return packets are dropped.
Why wrong: Incorrect. A missing return route would typically cause ping to fail because ICMP reply packets would also be dropped, but ping succeeds here.
- D
The database service is only listening on the loopback interface (127.0.0.1).
Why wrong: Incorrect. The question states MySQL listens on 0.0.0.0:3306, which includes all interfaces.
LFCS ICMP vs TCP filtering Practice Question
This LFCS practice question tests your understanding of networking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: iCMP vs TCP filtering. 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.
You are a Linux administrator for a small company. A developer has deployed a web application on a server with IP 192.168.1.50. The application needs to connect to a database server at 10.0.0.100 on TCP port 3306. Both servers are on the same physical network but different subnets (192.168.1.0/24 and 10.0.0.0/24) connected by a router. The default gateway for the app server is 192.168.1.1, and for the DB server is 10.0.0.1. You have verified that the app server can ping the DB server by IP address successfully. However, the application fails to connect to the database. You have used telnet from the app server to test connectivity on port 3306 and it fails (connection refused). On the DB server, you check that the MySQL service is listening on 0.0.0.0:3306 and that the local firewall (firewalld) allows incoming connections on port 3306. What is the most likely cause of the connection failure?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The app server's firewall is blocking outgoing connections to port 3306.
The app server can ping the DB server, confirming Layer 3 connectivity via ICMP. However, the telnet test fails on port 3306, indicating the TCP SYN packet is not reaching the DB server or the SYN-ACK is not returning. While ping uses ICMP, which may be allowed by the app server's firewall, outbound TCP traffic to port 3306 could be blocked by the local firewall (firewalld) on the app server. This is the most plausible cause because the DB server is configured correctly and the network path is functional for ICMP. Option B is correct.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The network cable is faulty.
Why it's wrong here
Incorrect. If the network cable were faulty, ping would likely fail as well.
- ✓
The app server's firewall is blocking outgoing connections to port 3306.
- ✗
The router does not have a route to 192.168.1.0/24 from the DB server's subnet, so return packets are dropped.
Why it's wrong here
Incorrect. A missing return route would typically cause ping to fail because ICMP reply packets would also be dropped, but ping succeeds here.
- ✗
The database service is only listening on the loopback interface (127.0.0.1).
Why it's wrong here
Incorrect. The question states MySQL listens on 0.0.0.0:3306, which includes all interfaces.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that successful ping does not guarantee TCP connectivity. ICMP and TCP are handled separately by firewalls; blocking outbound TCP on a specific port will still allow ICMP echo requests and replies. Candidates often overlook host-based outbound firewall rules.
Detailed technical explanation
How to think about this question
When a TCP connection is initiated, the SYN packet reaches the DB server, but the SYN-ACK reply must be routed back to the app server's subnet. If the router does not have a route for 192.168.1.0/24 on its interface connected to 10.0.0.0/24, the return packet is dropped, causing the app server to see a 'connection refused' or timeout, depending on whether an ICMP unreachable is generated. This scenario is common in multi-subnet environments where static routes are misconfigured, and it highlights the importance of verifying bidirectional routing with tools like traceroute or checking the routing table on the router.
KKey Concepts to Remember
- ICMP vs TCP filtering
- Outbound firewall rules
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
ICMP vs TCP filtering
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review iCMP vs TCP filtering, then practise related LFCS questions on the same topic to reinforce the concept.
- →
Networking — study guide chapter
Learn the concepts, then practise the questions
- →
Networking practice questions
Targeted practice on this topic area only
- →
All LFCS questions
513 questions across all exam domains
- →
Linux Foundation Certified System Administrator LFCS study guide
Full concept coverage aligned to exam objectives
- →
LFCS practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related LFCS practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
User and Group Management practice questions
Practise LFCS questions linked to User and Group Management.
Operation of Running Systems practice questions
Practise LFCS questions linked to Operation of Running Systems.
Essential Commands practice questions
Practise LFCS questions linked to Essential Commands.
Networking practice questions
Practise LFCS questions linked to Networking.
Service Configuration practice questions
Practise LFCS questions linked to Service Configuration.
Storage Management practice questions
Practise LFCS questions linked to Storage Management.
LFCS fundamentals practice questions
Practise LFCS questions linked to LFCS fundamentals.
LFCS scenario practice questions
Practise LFCS questions linked to LFCS scenario.
LFCS troubleshooting practice questions
Practise LFCS questions linked to LFCS troubleshooting.
Practice this exam
Start a free LFCS practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this LFCS question test?
Networking — This question tests Networking — ICMP vs TCP filtering.
What is the correct answer to this question?
The correct answer is: The app server's firewall is blocking outgoing connections to port 3306. — The app server can ping the DB server, confirming Layer 3 connectivity via ICMP. However, the telnet test fails on port 3306, indicating the TCP SYN packet is not reaching the DB server or the SYN-ACK is not returning. While ping uses ICMP, which may be allowed by the app server's firewall, outbound TCP traffic to port 3306 could be blocked by the local firewall (firewalld) on the app server. This is the most plausible cause because the DB server is configured correctly and the network path is functional for ICMP. Option B is correct.
What should I do if I get this LFCS question wrong?
Review iCMP vs TCP filtering, then practise related LFCS questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
ICMP vs TCP filtering
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Keep practising
More LFCS practice questions
- A system administrator needs to ensure the Apache httpd service starts automatically on system boot. Which command shoul…
- Which THREE of the following are valid methods to mount an NFS filesystem on a client?
- Refer to the exhibit. The administrator wants to create a RAID 1 array using /dev/sdb1 and /dev/sdc1. Which command shou…
- A system administrator needs to list all files in the current directory, including hidden files, in a long listing forma…
- A user wants to find all files in /var/log that have been modified within the last 2 days. Which command should they use…
- A user needs to view the contents of a compressed log file /var/log/syslog.gz without first decompressing it. Which comm…
Last reviewed: Jun 25, 2026
This LFCS practice question is part of Courseiva's free Linux Foundation 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 LFCS exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.