Quick Answer
The answer is a firewall blocking port 22 because even when the SSH daemon (sshd) is running and listening, a firewall rule that drops or rejects traffic on TCP port 22 will prevent the TCP three-way handshake from completing, leaving the client stuck with a connection refused or timeout error. This scenario is a classic trap on the Linux Professional Institute Certification Level 1 LPIC-1 exam, where candidates often assume a running service guarantees connectivity, but network-layer filtering operates independently of the application layer. The exam tests your ability to distinguish between service status and network accessibility, so always verify firewall rules with commands like `iptables -L` or `firewall-cmd --list-all` when an SSH connection is refused despite the service being active. Remember the mnemonic: “Service up, port blocked? Check the firewall, don’t be shocked.”
This LPIC-1 practice question tests your understanding of essential system services and networking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 user reports that they cannot connect to a remote server using SSH. The administrator checks the SSH server status and it is running. Which of the following is the most likely cause?
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.
A firewall is blocking port 22.
Firewall blocking SSH port is a common issue.
The client's subnet mask is incorrect.
Why wrong: This would affect local network, not remote server.
The client cannot resolve the server's hostname.
Why wrong: This would cause a different error.
The SSH server is using UDP instead of TCP.
Why wrong: SSH uses TCP.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
A firewall is blocking port 22.
SSH operates over TCP port 22 by default. If the SSH server is running but the client cannot connect, a firewall blocking port 22 is the most likely cause because it would prevent the TCP handshake from completing, even though the SSH daemon (sshd) is active and listening.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
A firewall is blocking port 22.
The client's subnet mask is incorrect.
Why it's wrong here
This would affect local network, not remote server.
The client cannot resolve the server's hostname.
Why it's wrong here
This would cause a different error.
The SSH server is using UDP instead of TCP.
Common exam traps
The trap here is that candidates may assume a running SSH server guarantees connectivity, overlooking that a firewall can block the port even when the service is active, or they may confuse SSH's TCP usage with UDP-based protocols like DNS.
Detailed technical explanation
Firewalls typically filter traffic based on port numbers and protocols; a common rule is to block inbound TCP port 22 except from trusted IPs. The SSH daemon (sshd) listens on TCP port 22, and a firewall can drop SYN packets silently (no response) or reject them with an ICMP unreachable message, both of which would cause a timeout or 'Connection refused' error. In real-world scenarios, administrators use tools like `nmap -p 22 <server>` or `telnet <server> 22` to test if the port is reachable, distinguishing between a firewall block and a service failure.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Essential System Services and Networking — study guide chapter
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FAQ
Essential System Services and Networking — This question tests Essential System Services and Networking — Read the scenario before looking for a memorised answer..
The correct answer is: A firewall is blocking port 22. — SSH operates over TCP port 22 by default. If the SSH server is running but the client cannot connect, a firewall blocking port 22 is the most likely cause because it would prevent the TCP handshake from completing, even though the SSH daemon (sshd) is active and listening.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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.
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