This 200-901 practice question tests your understanding of application deployment and security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Which TWO are valid methods to secure a Docker container?
Use read-only filesystem
Read-only filesystem prevents container from modifying files.
Expose all ports
Why wrong: Exposing all ports increases attack surface.
Set resource limits
Resource limits prevent container from consuming excessive resources.
Run containers as root
Why wrong: Running as root increases security risk.
Disable network isolation
Why wrong: Disabling network isolation reduces security.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Use read-only filesystem
Option A is correct because mounting the container's filesystem as read-only prevents any process inside the container from writing to the filesystem, which blocks malware persistence, log tampering, and unauthorized configuration changes. This is enforced by the Linux kernel's mount namespace and can be set with the `--read-only` flag in `docker run`. It is a key principle of immutable infrastructure for containers.
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.
Use read-only filesystem
Why this is correct
Read-only filesystem prevents container from modifying files.
Related concept
Read the scenario before looking for a memorised answer.
Expose all ports
Why it's wrong here
Exposing all ports increases attack surface.
Set resource limits
Why this is correct
Resource limits prevent container from consuming excessive resources.
Related concept
Read the scenario before looking for a memorised answer.
Run containers as root
Why it's wrong here
Running as root increases security risk.
Disable network isolation
Why it's wrong here
Disabling network isolation reduces security.
Common exam traps
Cisco often tests the misconception that 'running as root inside a container is safe because the container is isolated,' but the trap here is that root inside a container is the same UID 0 on the host if the container is not run with a user namespace remapping or `--user` flag, making it a direct privilege escalation vector.
Detailed technical explanation
Under the hood, the `--read-only` flag works by remounting the container's root filesystem as `MS_RDONLY` in the mount namespace, while still allowing writes to tmpfs volumes (e.g., `/tmp`, `/run`) if explicitly mounted. In a real-world CI/CD pipeline, combining read-only root with resource limits (option C) and dropping Linux capabilities (e.g., `--cap-drop=ALL`) creates a defense-in-depth posture that satisfies compliance frameworks like PCI DSS or SOC 2 for containerized workloads.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 200-901 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.
What to study next
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FAQ
Application Deployment and Security — This question tests Application Deployment and Security — Read the scenario before looking for a memorised answer..
The correct answer is: Use read-only filesystem — Option A is correct because mounting the container's filesystem as read-only prevents any process inside the container from writing to the filesystem, which blocks malware persistence, log tampering, and unauthorized configuration changes. This is enforced by the Linux kernel's mount namespace and can be set with the `--read-only` flag in `docker run`. It is a key principle of immutable infrastructure for containers.
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 24, 2026
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