Enable VCN flow logs on the subnets or network resources you need to monitor, then send the logs to a destination you can retain and analyse centrally. Use them to review accepted and rejected traffic, spot unusual east-west movement, and investigate exposure. Ensure logging scope matches the VCNs that carry sensitive workloads.
What you need to configure in OCI
VCN flow logs are enabled at the subnet or network-resource level, so the first decision is scope: which VCN segments need traffic visibility and which can stay quiet. The practical goal is to place logging where it will show accepted and rejected flows, without turning every network segment into a noisy, low-value sink.
In OCI, that usually means starting with the subnets that carry internet-facing services, east-west dependencies, or sensitive workloads. If you are already using broader cloud network controls, the same design discipline applies: define what should be observable, then align logging to that boundary rather than treating flow logs as a blanket switch. NHIMG’s Cloud Network Security Guide is a useful companion for the surrounding VCN, subnet, and egress-control decisions that make flow logs actionable.
Flow logs are only useful when they are delivered to a destination you can retain and analyse centrally. That matters because the log stream is not the control itself, it is the evidence source for review, triage, and later investigation. If the destination is fragmented, short-lived, or hard to query, the logging effort will not translate into usable visibility.
How flow logs help you operate the network
Once enabled, flow logs give you a record of traffic decisions that helps you answer basic operational questions: what talked to what, what was allowed, what was denied, and whether a path appeared that you did not expect. That is especially useful for confirming whether security rules, route design, and workload placement match the architecture you intended.
They are also one of the simplest ways to detect movement that looks abnormal inside the VCN. East-west traffic that appears suddenly, repeated denies against a service that should not be probed, or a workload communicating with unfamiliar destinations can all justify deeper review. NHIMG’s NDR Guide covers how flow logs fit into broader network detection and response, including the limits of metadata-only visibility.
In practice, flow logs work best as a baseline and investigation aid rather than as a stand-alone answer. They tell you that traffic happened and whether policy allowed it, but they do not by themselves explain intent, payload, or application behavior. If the question is whether a path is expected, the log is evidence; if the question is whether a system is compromised, the log is one input among several.
When setup usually goes wrong
The most common mistake is enabling logging too narrowly. If you only log one subnet, you can miss the lateral traffic or shared-service traffic that actually matters. The next mistake is leaving logs in a place that cannot support retention, search, and correlation, which turns a good control into a short-lived troubleshooting aid.
Another failure mode is assuming that enabled logging automatically means usable logging. You still need to confirm that the destination is receiving records consistently, that the volume is sensible for the subnets selected, and that the logs line up with the time window you need for incident review. If you cannot retain them long enough to compare before-and-after behavior, you lose much of the benefit.
There is also a scope problem at the workload boundary. If the subnet carries sensitive systems, the logging design should be deliberate enough to support investigations without exposing the team to blind spots. That is where central retention and consistent naming matter more than the checkbox itself. NHIMG’s Cloud Incident Response Guide is a practical reference for using cloud logs during containment and follow-on analysis.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
NIST CSF 2.0 and NIST SP 800-53 Rev 5 set the technical controls, while ISO/IEC 27001:2022 defines the regulatory obligations.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST CSF 2.0 | DE.CM-01 — Monitoring for Anomalies and Events | Flow logs support continuous network monitoring and anomaly review. |
| PR.PS-04 — Log Events | The question is about enabling logging for network traffic visibility. | |
| Recommendation — Collect and review VCN flow logs to detect unusual traffic patterns and denied connections. Enable logging on the VCN subnets that need visibility and preserve the records centrally. | ||
| NIST SP 800-53 Rev 5 | AU-2 — Event Logging | VCN flow logs are an event source that should be defined and retained for review. |
| AU-6 — Audit Record Review, Analysis, and Reporting | Flow logs are valuable when they are analysed for accepted, rejected, and unusual traffic. | |
| Recommendation — Define which network events to log and ensure the flow log destination preserves them. Review flow logs for rejected connections, unusual east-west movement, and investigation leads. | ||
| ISO/IEC 27001:2022 | A.8.15 — Logging | The subject is the operational use of logs for network visibility and investigation. |
| Recommendation — Configure logging so VCN traffic records are retained and available for analysis. | ||
Practitioner Guidance
What to verify: Check that the selected subnets actually represent the traffic you care about, especially shared services, east-west paths, and any subnet hosting sensitive workloads. If those paths are outside scope, you will get a false sense of coverage.
What good looks like: Flow logs land in a central destination with enough retention to compare normal versus abnormal behavior, and the team can query them quickly during an investigation. The control is working when it improves decisions, not when it merely produces volume.
Decision rule: If a subnet supports production or sensitive systems, log it by default and treat exceptions as a conscious risk decision. If the logs cannot be retained or searched centrally, fix the destination design before relying on the feature operationally.
Practitioner takeaway: Treat VCN flow logs as an observability control, not a completion badge, because value comes from correct scope, central retention, and the ability to correlate traffic patterns with the systems that matter.
Related resources from NHI Mgmt Group
- How do you enable Cloud Guard in Oracle Cloud Infrastructure?
- How do you enable Data Access audit logs in Google Cloud?
- What breaks when cloud and identity logs are not correlated in one investigation flow?
- How should security teams govern infrastructure access when moving enterprise applications to Oracle Cloud Infrastructure?