- A
The firewall is blocking UDP port 53
Why wrong: The technician verified port 53 is allowed, but this option doesn't specify protocol; it's likely not the issue.
- B
The DNS server is using port 5353 instead of 53
Why wrong: Port 5353 is used for mDNS (Multicast DNS), not standard DNS.
- C
The firewall is blocking TCP port 53
DNS requires TCP for zone transfers and large responses; blocking TCP port 53 can cause resolution failures.
- D
Clients are using the wrong DNS server IP
Why wrong: This could be an issue, but the scenario implies the server is internal and configured correctly.
Question 584 of 1,020
DNS Port: 53 (TCP and UDP)
This 220-1201 practice question tests your understanding of tcp & udp ports. 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 company's internal DNS server is not resolving hostnames for clients. The technician verifies that the DNS service is running and the firewall allows traffic on port 53. However, clients still cannot resolve names. What is the most likely issue?
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.
Quick Answer
The answer is that the firewall is blocking TCP port 53, not just UDP. DNS uses both UDP and TCP on port 53, but for different purposes: UDP handles standard queries and responses, while TCP is required for zone transfers and any response exceeding 512 bytes. If a firewall only permits UDP traffic on port 53, larger DNS responses or critical zone transfers will fail, leaving clients unable to resolve hostnames even though the service appears to be running. On the CompTIA A+ Core 1 220-1201 exam, this is a classic trick—many students remember port 53 but forget that DNS relies on both protocols, so the question tests your understanding of when TCP becomes necessary. A common trap is assuming that allowing port 53 alone is sufficient, when in reality the protocol type matters just as much. To remember this, think of the mnemonic: “UDP for quick queries, TCP for transfers and big replies.”
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 firewall is blocking TCP port 53
DNS uses both UDP and TCP on port 53. While initial queries typically use UDP, responses that are truncated (e.g., due to large DNSSEC records or many resource records) require TCP to complete the resolution. Since the technician verified that the firewall allows traffic on port 53 (likely only UDP) and the DNS service is running, the most likely issue is that the firewall is blocking TCP port 53, preventing the fallback TCP connection needed for successful name resolution.
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.
- ✗
The firewall is blocking UDP port 53
Why it's wrong here
The technician verified port 53 is allowed, but this option doesn't specify protocol; it's likely not the issue.
- ✗
The DNS server is using port 5353 instead of 53
Why it's wrong here
Port 5353 is used for mDNS (Multicast DNS), not standard DNS.
- ✓
The firewall is blocking TCP port 53
- ✗
Clients are using the wrong DNS server IP
Why it's wrong here
This could be an issue, but the scenario implies the server is internal and configured correctly.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that DNS only uses UDP port 53, leading candidates to overlook the necessity of TCP port 53 for truncated responses and zone transfers.
Trap categories for this question
Scenario analysis trap
This could be an issue, but the scenario implies the server is internal and configured correctly.
Detailed technical explanation
How to think about this question
DNS resolution relies on UDP for most queries due to its low overhead, but the DNS protocol (RFC 1035) mandates that when a UDP response is truncated (TC flag set), the resolver must retry over TCP to retrieve the full response. In real-world scenarios, this is critical for DNSSEC-signed zones, large SRV records, or environments with many A/AAAA records; if TCP/53 is blocked, queries that exceed the 512-byte UDP limit (or 4096 bytes with EDNS0) will silently fail, causing intermittent resolution failures that are hard to diagnose without packet capture.
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 220-1201 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.
Visual reference
Quick reference
Common DNS Record Types
| Record | Purpose | Example |
|---|---|---|
| A | IPv4 address mapping | example.com → 93.184.216.34 |
| AAAA | IPv6 address mapping | example.com → 2606:2800::1 |
| CNAME | Alias to another hostname | www → example.com |
| MX | Mail server for domain | example.com → mail.example.com (priority 10) |
| TXT | Text data (SPF, DKIM, verification) | v=spf1 include:_spf.example.com ~all |
| NS | Authoritative name servers | example.com NS ns1.example.com |
| PTR | Reverse DNS (IP → hostname) | 34.216.184.93.in-addr.arpa → example.com |
| SOA | Zone authority record | Primary NS, admin email, serial, TTL defaults |
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.
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TCP & UDP Ports — study guide chapter
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FAQ
Questions learners often ask
What does this 220-1201 question test?
TCP & UDP Ports — This question tests TCP & UDP Ports — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall is blocking TCP port 53 — DNS uses both UDP and TCP on port 53. While initial queries typically use UDP, responses that are truncated (e.g., due to large DNSSEC records or many resource records) require TCP to complete the resolution. Since the technician verified that the firewall allows traffic on port 53 (likely only UDP) and the DNS service is running, the most likely issue is that the firewall is blocking TCP port 53, preventing the fallback TCP connection needed for successful name resolution.
What should I do if I get this 220-1201 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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