This LPIC-2 practice question tests your understanding of advanced networking configuration. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 security analyst wants to capture only TCP packets with the SYN flag set to identify connection attempts. Which tcpdump filter expression accomplishes this?
tcp[tcpflags] & tcp-syn != 0
Why wrong: tcpflags and tcp-syn are not predefined in tcpdump.
tcp[13] & 2 != 0
Why wrong: This is correct but the question expects a single best answer; however, both B and C are correct.
tcp[13] & 0x02 != 0
Why wrong: This is also correct.
Both B and C are correct.
Both expressions correctly filter for the SYN flag.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Both B and C are correct.
Option D is correct because both B and C correctly filter TCP packets with the SYN flag set. In tcpdump, the TCP header's 14th byte (offset 13) contains the flags, with the SYN flag represented by bit 1 (value 2). Both `tcp[13] & 2 != 0` and `tcp[13] & 0x02 != 0` check if the SYN bit is set, making them equivalent. Option A is syntactically incorrect because `tcp-syn` is not a valid tcpdump primitive; the correct form uses the numeric offset and bitmask.
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.
tcp[tcpflags] & tcp-syn != 0
Why it's wrong here
tcpflags and tcp-syn are not predefined in tcpdump.
tcp[13] & 2 != 0
Why it's wrong here
This is correct but the question expects a single best answer; however, both B and C are correct.
tcp[13] & 0x02 != 0
Why it's wrong here
This is also correct.
Both B and C are correct.
Why this is correct
Both expressions correctly filter for the SYN flag.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often assume `tcp-syn` is a valid tcpdump filter keyword (like `icmp` or `arp`), but tcpdump does not define symbolic names for individual TCP flags, requiring the use of raw byte offsets and bitmasks instead.
Detailed technical explanation
The TCP header flags are stored in the 14th byte (offset 13 from the start of the TCP header), with bits assigned as: FIN (0x01), SYN (0x02), RST (0x04), PSH (0x08), ACK (0x10), URG (0x20). The expression `tcp[13] & 0x02 != 0` performs a bitwise AND to isolate the SYN bit; if the result is non-zero, the SYN flag is set. This filter is commonly used to detect TCP connection initiation (SYN packets) in network troubleshooting and security monitoring, but note it also matches SYN-ACK packets since the ACK flag is independent — to match only pure SYN packets, you would need `tcp[13] & 0x12 == 0x02` (SYN set, ACK not set).
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
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FAQ
Advanced Networking Configuration — This question tests Advanced Networking Configuration — Read the scenario before looking for a memorised answer..
The correct answer is: Both B and C are correct. — Option D is correct because both B and C correctly filter TCP packets with the SYN flag set. In tcpdump, the TCP header's 14th byte (offset 13) contains the flags, with the SYN flag represented by bit 1 (value 2). Both `tcp[13] & 2 != 0` and `tcp[13] & 0x02 != 0` check if the SYN bit is set, making them equivalent. Option A is syntactically incorrect because `tcp-syn` is not a valid tcpdump primitive; the correct form uses the numeric offset and bitmask.
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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