Quick Answer
The answer is net.ipv4.tcp_max_syn_backlog. This kernel parameter controls the maximum number of half-open SYN requests the kernel will hold in the connection queue before dropping new ones; when the log shows "possible SYN flooding," it means this backlog is full, and increasing the value gives the server more room to handle incoming connection bursts without timing out. On the CompTIA Linux+ XK0-005 exam, this scenario tests your ability to interpret kernel logs and apply network tuning under the System Operation and Maintenance domain—a common trap is confusing this with tcp_syncookies, which is a complementary defense but not the direct fix for a full backlog. To remember: think "SYN backlog = SYN queue size," so when the queue overflows, you increase the backlog.
This XK0-005 practice question tests your understanding of troubleshooting. 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.
During peak hours, a web server experiences timeouts. The kernel log shows 'possible SYN flooding'. Which kernel parameter should be increased to mitigate this?
net.ipv4.tcp_syncookies
Why wrong: Disabling syncookies would worsen the problem.
net.core.somaxconn
Why wrong: This is for the maximum number of connections queued for acceptance, not SYN backlog.
net.ipv4.tcp_max_syn_backlog
Increasing this value allows more half-open connections, reducing SYN flooding issues.
net.core.rmem_default
Why wrong: This affects receive buffer sizes, not SYN handling.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
net.ipv4.tcp_max_syn_backlog
The kernel log message 'possible SYN flooding' indicates that the system's SYN backlog queue is full, causing new connection requests to be dropped. Increasing `net.ipv4.tcp_max_syn_backlog` expands the maximum number of SYN requests that can be queued before the kernel starts dropping them, directly mitigating the issue.
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.
net.ipv4.tcp_syncookies
Why it's wrong here
Disabling syncookies would worsen the problem.
net.core.somaxconn
Why it's wrong here
This is for the maximum number of connections queued for acceptance, not SYN backlog.
net.ipv4.tcp_max_syn_backlog
Why this is correct
Increasing this value allows more half-open connections, reducing SYN flooding issues.
Related concept
Read the scenario before looking for a memorised answer.
net.core.rmem_default
Why it's wrong here
This affects receive buffer sizes, not SYN handling.
Common exam traps
The trap here is confusing the SYN backlog queue (`tcp_max_syn_backlog`) with the completed connection backlog (`somaxconn`), leading candidates to choose `net.core.somaxconn` even though it only affects fully established connections, not the SYN flood protection mechanism.
Detailed technical explanation
The SYN backlog queue (controlled by `tcp_max_syn_backlog`) holds incoming SYN segments that have not yet completed the three-way handshake. When the queue is full, the kernel drops new SYNs, leading to timeouts. In modern Linux kernels, the effective backlog size is also influenced by `net.core.somaxconn` and the application's `listen()` backlog argument, but `tcp_max_syn_backlog` specifically governs the kernel's internal SYN queue, which is critical under high connection rates.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the XK0-005 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
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Troubleshooting — This question tests Troubleshooting — Read the scenario before looking for a memorised answer..
The correct answer is: net.ipv4.tcp_max_syn_backlog — The kernel log message 'possible SYN flooding' indicates that the system's SYN backlog queue is full, causing new connection requests to be dropped. Increasing `net.ipv4.tcp_max_syn_backlog` expands the maximum number of SYN requests that can be queued before the kernel starts dropping them, directly mitigating the issue.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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