The answer is to add iptables rules that allow forwarding and enable masquerading, because the default Docker bridge network relies on iptables NAT and FORWARD rules to route container traffic to the internet. When the FORWARD policy is set to DROP, the host silently discards all forwarded packets from the container, breaking outbound connectivity even though the host itself has internet access. This scenario is a classic CompTIA Linux+ XK0-005 exam trap: candidates often focus on container configuration or DNS, but the real issue is the host’s firewall blocking packet forwarding between the docker0 bridge and the external interface. The fix involves adding `-A FORWARD -i docker0 -j ACCEPT` to permit forwarding and `-t nat -A POSTROUTING -s 172.17.0.0/16 -o eth0 -j MASQUERADE` to perform source NAT, restoring the container’s ability to reach external networks. Remember the mnemonic “Docker Drops, Masquerade Moves” — when Docker containers have no internet, check that FORWARD isn’t dropping and that masquerading is moving traffic out.
XK0-005 Troubleshooting Practice Question
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.
A developer reports that a Docker container on a CentOS 7 host cannot connect to the internet. The host itself can access the internet. The container is started with default bridge network. The administrator checks iptables and sees the FORWARD policy is DROP. What is the most likely cause and solution?
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.
The container needs to be run with --network host.
Why wrong: Host network mode bypasses bridge but does not fix the iptables issue.
B
The container's DNS configuration is incorrect.
Why wrong: DNS issues would affect name resolution, not connectivity to internet IPs.
C
Add iptables rules to allow forwarding and enable masquerading.
Docker manages iptables, but if the FORWARD policy is DROP without proper rules, container traffic is blocked. Adding rules or restarting Docker restores connectivity.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add iptables rules to allow forwarding and enable masquerading.
The default Docker bridge network relies on iptables NAT (masquerading) and FORWARD rules to allow containers to reach external networks. When the FORWARD policy is set to DROP, the host drops all forwarded packets from the container, blocking outbound internet access. Adding iptables rules to allow forwarding (e.g., `-A FORWARD -i docker0 -j ACCEPT`) and enabling masquerading (e.g., `-t nat -A POSTROUTING -s 172.17.0.0/16 -o eth0 -j MASQUERADE`) restores connectivity.
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 container needs to be run with --network host.
Why it's wrong here
Host network mode bypasses bridge but does not fix the iptables issue.
✗
The container's DNS configuration is incorrect.
Why it's wrong here
DNS issues would affect name resolution, not connectivity to internet IPs.
✓
Add iptables rules to allow forwarding and enable masquerading.
Why this is correct
Docker manages iptables, but if the FORWARD policy is DROP without proper rules, container traffic is blocked. Adding rules or restarting Docker restores connectivity.
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.
✗
AppArmor is blocking outbound connections.
Why it's wrong here
CentOS uses SELinux, not AppArmor.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume DNS or network mode is the issue, but the explicit mention of the FORWARD policy being DROP directly points to a missing iptables forwarding rule, which is a classic Linux networking troubleshooting scenario.
Detailed technical explanation
How to think about this question
Docker's default bridge network creates a virtual interface (docker0) and relies on iptables rules for NAT and forwarding. The `FORWARD` chain's default policy of DROP is common in hardened CentOS 7 installations; Docker normally adds its own ACCEPT rules for the docker0 interface, but if those rules are missing or overridden, traffic stops. In production, administrators often use `iptables -I FORWARD -i docker0 -j ACCEPT` and ensure `net.ipv4.ip_forward=1` is set in sysctl to enable routing.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
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: Add iptables rules to allow forwarding and enable masquerading. — The default Docker bridge network relies on iptables NAT (masquerading) and FORWARD rules to allow containers to reach external networks. When the FORWARD policy is set to DROP, the host drops all forwarded packets from the container, blocking outbound internet access. Adding iptables rules to allow forwarding (e.g., `-A FORWARD -i docker0 -j ACCEPT`) and enabling masquerading (e.g., `-t nat -A POSTROUTING -s 172.17.0.0/16 -o eth0 -j MASQUERADE`) restores connectivity.
What should I do if I get this XK0-005 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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Question Discussion
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