The answer is the FORWARD chain's default policy set to DROP with no rules allowing forwarding. This is correct because iptables separates traffic into three main chains: INPUT for packets destined for the VM itself, OUTPUT for packets leaving the VM, and FORWARD for packets that should pass through the VM between interfaces. When a Linux VM is configured as a router, forwarded traffic must be explicitly permitted in the FORWARD chain; if its default policy is DROP and no ACCEPT rules exist, the kernel silently discards all forwarded packets, breaking routing while SSH remains functional since it uses the INPUT chain. On the CompTIA Cloud+ CV0-004 exam, this tests your understanding of iptables chain behavior versus routing—a common trap is confusing INPUT rules with forwarding requirements. Remember the memory tip: "INPUT for me, FORWARD for others"—if traffic isn't destined for the VM, it must be allowed in FORWARD.
CV0-004 Troubleshooting Practice Question
This CV0-004 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.
Exhibit
Refer to the exhibit.
```
$ iptables -L -n -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
0 0 DROP all -- * * 0.0.0.0/0 0.0.0.0/0 state INVALID
123 4567 ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 state ESTABLISHED,RELATED
0 0 ACCEPT tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
Chain FORWARD (policy DROP 0 packets, 0 bytes)
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
```
A cloud administrator is configuring a Linux VM as a router. The iptables rules are shown. The administrator can SSH into the VM from the network but cannot forward traffic between interfaces. What 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The FORWARD chain's default policy is DROP and no rules allow forwarding
The FORWARD chain in iptables controls traffic that passes through the VM (i.e., traffic not destined for the VM itself). If its default policy is DROP and no explicit ACCEPT rules exist for forwarding, the kernel will drop all forwarded packets, preventing the VM from acting as a router. SSH access works because it uses the INPUT chain, which is separate from FORWARD.
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 INPUT chain has a rule dropping invalid packets
Why it's wrong here
The INPUT chain is for incoming packets to the VM itself, not for forwarded traffic.
✗
The INPUT chain is missing a rule to allow forwarded traffic
Why it's wrong here
Forwarded traffic uses the FORWARD chain, not INPUT.
✓
The FORWARD chain's default policy is DROP and no rules allow forwarding
Why this is correct
With default policy DROP and no FORWARD rules, all forwarded packets are dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The NAT table is misconfigured
Why it's wrong here
NAT is not required for simple routing between interfaces.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse the INPUT chain (for local traffic) with the FORWARD chain (for transit traffic), assuming that allowing SSH implies forwarding is also allowed, when in fact they are handled by completely separate chains.
Detailed technical explanation
How to think about this question
The Linux kernel's netfilter framework uses three built-in chains (INPUT, FORWARD, OUTPUT) for packet flow. For a packet to be forwarded, it must traverse the FORWARD chain; if the default policy is DROP and no rule matches, the packet is silently discarded. Additionally, the kernel parameter net.ipv4.ip_forward must be set to 1 to enable routing; without it, the FORWARD chain is never consulted. In real-world scenarios, administrators often forget to add explicit ACCEPT rules for forwarding after setting the default policy to DROP, breaking routing while leaving local services accessible.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
Troubleshooting — This question tests Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The FORWARD chain's default policy is DROP and no rules allow forwarding — The FORWARD chain in iptables controls traffic that passes through the VM (i.e., traffic not destined for the VM itself). If its default policy is DROP and no explicit ACCEPT rules exist for forwarding, the kernel will drop all forwarded packets, preventing the VM from acting as a router. SSH access works because it uses the INPUT chain, which is separate from FORWARD.
What should I do if I get this CV0-004 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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