The answer is an application on the internal network not completing TCP handshakes. This is the likely cause because a high half-open TCP connections troubleshooting scenario reveals that SYN packets are being received by the firewall, but the three-way handshake stalls—the internal server either fails to send the final ACK or does not respond to the SYN-ACK, leaving connections in a half-open state that consumes firewall session table resources and degrades performance. On the Palo Alto Networks Certified Network Security Administrator PCNSA exam, this concept tests your understanding of TCP state tracking and how incomplete handshakes can indicate a misbehaving internal host rather than an external attack. A common trap is to assume the issue is always a SYN flood from outside, but the question specifically points to an internal application fault. Memory tip: think “Half-open = Half the handshake; the missing ACK is the internal app’s fault.”
PCNSA Core Concepts Practice Question
This PCNSA practice question tests your understanding of core concepts. 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.
Exhibit
Refer to the exhibit.
admin@PA-500> show counter global | match tcp
tcp-conn-init 1500
tcp-conn-established 1200
tcp-conn-closed 1400
tcp-conn-failed 200
tcp-conn-reset 100
tcp-conn-half-open 50
tcp-conn-timeout 30
Refer to the exhibit. A firewall administrator is troubleshooting a performance issue. The number of half-open TCP connections is unusually high. What is a likely cause?
Refer to the exhibit.
admin@PA-500> show counter global | match tcp
tcp-conn-init 1500
tcp-conn-established 1200
tcp-conn-closed 1400
tcp-conn-failed 200
tcp-conn-reset 100
tcp-conn-half-open 50
tcp-conn-timeout 30
A
A DDoS attack is flooding the firewall with SYN packets.
Why wrong: SYN flood would show very high tcp-conn-init, not just half-open.
B
An application on the internal network is not completing TCP handshakes.
Half-open connections indicate incomplete handshakes, likely due to application failure.
C
The firewall's TCP timeout setting is too short.
Why wrong: Short timeouts would reduce half-open count, not increase it.
D
The firewall's hardware is failing.
Why wrong: Hardware failure would show more generic errors, not specific half-open count.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
An application on the internal network is not completing TCP handshakes.
A high number of half-open TCP connections indicates that SYN packets are received but the three-way handshake is never completed. Option B is correct because an internal application that fails to send the final ACK (or does not respond to SYN-ACK) leaves connections in a half-open state, consuming firewall resources and degrading performance.
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.
✗
A DDoS attack is flooding the firewall with SYN packets.
Why it's wrong here
SYN flood would show very high tcp-conn-init, not just half-open.
✓
An application on the internal network is not completing TCP handshakes.
Why this is correct
Half-open connections indicate incomplete handshakes, likely due to application failure.
Related concept
Read the scenario before looking for a memorised answer.
✗
The firewall's TCP timeout setting is too short.
Why it's wrong here
Short timeouts would reduce half-open count, not increase it.
✗
The firewall's hardware is failing.
Why it's wrong here
Hardware failure would show more generic errors, not specific half-open count.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often associate high half-open connections exclusively with DDoS SYN floods, but the question specifically asks for a 'likely cause' given the context of a performance issue, and an internal application misbehavior is a common real-world scenario that does not require an attack.
Trap categories for this question
Command / output trap
SYN flood would show very high tcp-conn-init, not just half-open.
Detailed technical explanation
How to think about this question
Half-open TCP connections occur when the firewall receives a SYN, sends a SYN-ACK, but never receives the final ACK. This can be caused by a misconfigured or buggy application that does not complete the handshake, or by asymmetric routing where the SYN-ACK is dropped. The firewall maintains a session table entry for each half-open connection; if the number exceeds the configured maximum (e.g., 65,535 by default in PAN-OS), new connections may be dropped, leading to performance degradation.
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 PCNSA 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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Core Concepts — This question tests Core Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: An application on the internal network is not completing TCP handshakes. — A high number of half-open TCP connections indicates that SYN packets are received but the three-way handshake is never completed. Option B is correct because an internal application that fails to send the final ACK (or does not respond to SYN-ACK) leaves connections in a half-open state, consuming firewall resources and degrading performance.
What should I do if I get this PCNSA question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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