The answer is to use `--add-host host.docker.internal:host-gateway` when running the container. This approach is correct because the `host-gateway` magic value automatically maps to the host machine’s gateway IP on the default bridge network—typically 172.17.0.1 on Linux—and adds an entry to the container’s `/etc/hosts` file that resolves `host.docker.internal` to that address. On the default bridge, Docker’s internal DNS does not resolve hostnames to the host, so this explicit host entry provides a consistent, reliable way for container-host communication by hostname. For the CompTIA Linux+ XK0-005 exam, this question tests your understanding of Docker networking limitations and the `--add-host` flag as a workaround; a common trap is assuming `--link` or Docker DNS works on the default bridge. Remember the memory tip: “Gateway gets you home”—the host-gateway flag points the container back to its host.
XK0-005 Scripting, Containers and Automation Practice Question
This XK0-005 practice question tests your understanding of scripting, containers and automation. 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 container needs to communicate with a database on the host machine using the default bridge network. The container cannot resolve the host by hostname. Which approach should be used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use --add-host host.docker.internal:host-gateway
Option D is correct because `--add-host host.docker.internal:host-gateway` adds a special entry to the container's `/etc/hosts` file that resolves `host.docker.internal` to the host machine's gateway IP address, which on the default bridge network is the host itself. This allows the container to reach the host by a consistent hostname without relying on Docker's internal DNS, which does not resolve hostnames on the default bridge. The `host-gateway` magic value automatically maps to the host's IP (typically 172.17.0.1 on Linux).
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.
✗
Set --net=host
Why it's wrong here
Using host network mode reduces container isolation and is not recommended for security.
✗
Use --link db:db
Why it's wrong here
The --link flag is a legacy feature and deprecated; it also does not resolve the host itself.
✗
Create a custom bridge network
Why it's wrong here
A custom bridge network provides DNS resolution between containers but does not automatically resolve the host.
✓
Use --add-host host.docker.internal:host-gateway
Why this is correct
This adds a host entry that resolves to the host's IP via the gateway, allowing the container to reach host services.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse `--net=host` as a quick fix for hostname resolution, not realizing it sacrifices network isolation and is not the intended method for reaching the host from a container on the default bridge.
Detailed technical explanation
How to think about this question
Under the hood, the default bridge network does not support automatic service discovery via DNS; containers can only communicate with the host via the gateway IP (172.17.0.1). The `host-gateway` special value, introduced in Docker 20.10, dynamically resolves to the host's gateway address on any network, making it portable across different bridge networks. In real-world scenarios, this is commonly used when a container needs to connect to a database or API server running directly on the host without exposing it to external networks.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
Scripting, Containers and Automation — This question tests Scripting, Containers and Automation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use --add-host host.docker.internal:host-gateway — Option D is correct because `--add-host host.docker.internal:host-gateway` adds a special entry to the container's `/etc/hosts` file that resolves `host.docker.internal` to the host machine's gateway IP address, which on the default bridge network is the host itself. This allows the container to reach the host by a consistent hostname without relying on Docker's internal DNS, which does not resolve hostnames on the default bridge. The `host-gateway` magic value automatically maps to the host's IP (typically 172.17.0.1 on Linux).
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.
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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