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To monitor Linux systems for hidden malware, correlate three kinds of evidence: host telemetry that can identify the process behind a connection, DNS logs that show which host requested a name and when, and network records collected at the egress boundary. None proves malware on its own. Establish what normal traffic looks like for each host group, investigate deviations across all three layers, and preserve evidence before making changes that could disrupt a system.
What a useful monitoring plan needs to show
A suspicious domain, an unfamiliar destination, or a process making an unexpected connection is a lead—not a verdict. A useful investigation should be able to answer three questions:
- Which Linux host was involved? DNS and network records need a host identity or address and a trustworthy timestamp.
- What initiated the activity? Host-side evidence should help connect a socket or connection to a process, service, user, or scheduled task.
- What happened on the network? Egress records should show the destination, protocol, port, timing, frequency, and, where available, bytes transferred.
CISA guidance recommends examining host artifacts alongside DNS activity and connection details, including processes, services, listening ports, established connections, and DNS settings. A network sensor can show that a host communicated; it generally cannot establish which Linux process initiated that traffic.
Build visibility across host, DNS, and egress
Collect host evidence that can identify the process
Gather process and service inventories, process relationships where available, listening sockets, established or recent connections, resolver configuration, system logs, and persistence-related artifacts such as cron and systemd configuration. CISA’s investigation guidance identifies these as useful host artifacts, including Linux logs and process trees.
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Distinguish ongoing telemetry from a point-in-time inspection. A snapshot of current sockets can help answer what is connected now, but it does not provide a durable history of short-lived connections. Persistent process-to-socket attribution depends on the Linux distribution, kernel, permissions, and endpoint tooling. There is no single configuration established here as a universal auditd, eBPF, or endpoint-agent recipe.
Make DNS requests attributable
Route ordinary DNS requests through authorized organizational resolvers and enable logging that associates each query with a host identity or address and a timestamp. Where the network design permits, restrict direct external DNS and monitor for unauthorized resolvers or possible DNS tunneling. CISA recommends host-level DNS visibility and routing DNS through organizational servers so defenders can identify the machine behind a suspicious request.
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DNS records help investigate name-based command-and-control, but they do not reveal connections made directly to an IP address. Encrypted DNS or traffic carried inside other protocols can also limit resolver visibility. Pair DNS logs with host connection telemetry and network records rather than treating DNS as a complete account of egress.
Record network egress
Collect flow records at relevant network boundaries. Add protocol logs or packet capture where justified, authorized, and operationally supportable. For each connection, retain the dimensions needed to investigate it: destination, port, protocol, time, frequency, and bytes transferred. CISA recommends flow visibility, egress controls, centralized logging, and baselining.
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Packet capture can provide more detail than flow metadata, but it also increases data volume and storage demands. Choose sensor placement and retention to fit the organization’s network and investigation needs; a sensor that cannot see the relevant egress path will not fill gaps in the records.
Choose network tools for the evidence they provide
Zeek and Suricata provide complementary network visibility. Neither should be mistaken for Linux process attribution; correlate their records with endpoint telemetry.
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| Tool | Useful evidence | Role and limits |
|---|---|---|
| Zeek | Structured connection records and application-layer records such as DNS requests and replies. | Useful for passive network monitoring, investigation, and hunting. The Zeek Project’s documentation describes it as a passive, open-source network traffic analyzer and notes that dedicated IDS tools may be better suited to signature matching. |
| Suricata | Signature and anomaly detection, protocol logging, DNS query and response logs, and full packet capture support. | Useful for alerting and collecting protocol or packet evidence. The Suricata project’s features page lists version 8.0.7, released September 15, 2026; check the project’s current release and support status before choosing a deployment. |
Use both when their different roles meet a real operational need—for example, an alert can point to a flow while transaction records add context. Sensor placement, data volume, storage, tuning effort, and analyst capacity matter as much as feature lists.
Baseline behavior before setting alerts
Build expectations by host group: normal destinations, protocols, services, DNS resolver paths, and typical connection patterns. A web server, workstation, and build machine may have very different legitimate egress. CISA recommends baselining network behavior and investigating unusual ports, destinations, DNS activity, and connection patterns.
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Use the baseline to identify candidates for review, such as:
- Connections to destinations, ports, or resolvers that are unusual for the host’s role.
- Regular outbound connections with an unexplained cadence, or unexpected changes in connection frequency.
- Unusual transfer volumes or unexplained data transfers.
- DNS activity that departs from expected resolver paths or host behavior.
- Traffic associated with a process, service, listener, or systemd change that has no clear business purpose.
These observations are not proof of compromise. Legitimate software updates, monitoring agents, and workload changes can also produce new destinations or traffic patterns. Validate a finding against the host’s role, owner, process context, and surrounding records.
Quick Recap
Investigate an anomaly without losing its context
- Define the expected behavior. Identify the host group’s normal services, destinations, protocols, and resolver path before treating a difference as suspicious.
- Centralize and protect the records. Bring together host, resolver, firewall, flow, and network-monitoring logs. Keep timestamps consistent and retain records long enough for the organization’s threat model and investigation needs. CISA recommends centralized logging and adequate incident-data retention.
- Pivot from the network event to the host. Identify the source host, then use host telemetry to connect the observed connection to a process, parent process, binary, service or scheduled execution, and user context.
- Reconstruct the surrounding activity. Compare DNS requests and connection history with process and service changes, listening ports, resolver settings, system logs, and relevant cron or systemd artifacts.
- Preserve evidence before disruptive action. Retain relevant logs and volatile artifacts and coordinate the response before isolating or altering the system. CISA warns that premature mitigation may change volatile data, destroy useful evidence, or alert an adversary.
Know what the records cannot establish
- Host attribution is not automatic. Network logs ordinarily identify a host or flow, not the Linux process that opened the connection; that mapping requires suitable endpoint telemetry.
- DNS is not all egress. Direct-IP connections do not require a DNS lookup, and encrypted DNS or other protocol use can reduce visibility at a resolver.
- A single indicator is not a malware diagnosis. Interpret unusual destinations, ports, timing, byte counts, or DNS patterns in context and corroborate them with host evidence.
- More capture is not always better. Flow metadata, protocol records, and packet capture have different detail, storage, and operational costs. Collect the level of detail the investigation requires and the organization can protect and retain.
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