The Tool Desk
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How do I find which process is sending email from Linux?
Start by inventorying active connections and attributing relevant sockets to processes. Tools such as ss, lsof, and netstat can help; which are available and what details they show depend on the distribution and privileges. MITRE ATT&CK describes these tools as ways to discover network connections, and its detection analytics also identify connection-inventory behavior. Their use is normal for administrators and is not, by itself, evidence of compromise. See MITRE ATT&CK T1049: System Network Connections Discovery and MITRE ATT&CK T1057: Process Discovery.
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For each connection worth investigating, record the process ID, executable path, account, parent process, associated service or timer, local and remote addresses, connection state, and observation time. Then compare those details with the host’s role and known-good configuration. A connection to a mail relay from an application server may be expected; the same connection from an unexplained script running under an unexpected account deserves closer scrutiny.
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Use the tools available on the host
ssreports socket information and can show process details when run with suitable permissions.lsofcan associate open network sockets with processes.netstatmay be present on some systems, although it is not installed by default on every modern distribution.
Use the command options and package sources documented for your distribution. If process ownership is not visible, rerun with appropriate privileges under your incident-response procedures; do not assume that a missing process field means the connection is harmless.
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Why is a Linux server making unexpected SMTP connections?
Linux applications and mail-transfer agents may legitimately send messages or relay mail. MITRE’s Linux detection analytic also highlights non-interactive or script-driven transmissions involving sendmail, mailx, or custom SMTP scripts, especially when initiated by background processes and associated with attachments or large payloads. Those patterns warrant investigation, but the analytic does not define a universal volume threshold or make every such transmission malicious. See MITRE ATT&CK T1566: Phishing.
Check whether the executable, account, parent process, timing, and destination make sense for the application and machine. Determine whether the destination is an approved relay, whether the process accessed files that explain its activity, and whether its traffic volume or schedule departs from the host’s baseline. A mail-related executable name or a connection on a mail-associated port is not enough to identify either the protocol or the intent.
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Compare expected and unexplained activity
| Investigation point | Expected mail service or application | Unexplained background process |
|---|---|---|
| Owner and parent | Account and launching service fit the application’s documented role. | Unexpected user, shell, script, or parent process; verify before drawing a conclusion. |
| Executable and service provenance | Path, package provenance, and configuration match the host’s trusted baseline. | Unrecognized path, package, or service definition requires validation against the distribution and host configuration. |
| Persistence | Known service or scheduled task with an expected configuration. | Recent or unexplained cron or systemd timer change, unusual interval, or unexpected user. |
| Destination and traffic pattern | Approved relay or application destination; timing and volume fit normal use. | Unknown destination, unusual timing or volume, or behavior inconsistent with the server’s purpose. |
| Related host activity | Process accesses and audit events align with the application’s work. | Unexplained file access, privileged execution, or missing audit events around the same time. |
How can I tell whether a systemd service is a backdoor?
A service name that resembles a mail component is a reason to verify it, not a verdict. Compare its unit definition, executable path, owner, package provenance, parentage, and network behavior with the machine’s trusted baseline. Familiar names can be imitated, and an unusual name alone does not establish malicious intent.
Also inspect scheduled execution. MITRE’s scheduled-task detection analytic includes changes to cron jobs through crontab and /etc/cron.*, as well as systemd timer units. Look for recent creation or modification, unexpected users, non-standard intervals, and commands whose outbound network activity does not match the host’s role. See MITRE ATT&CK T1053: Scheduled Task/Job.
Verify the service in context
- Read the unit and identify the exact executable and arguments it launches.
- Check the file’s ownership, location, package provenance, and modification history using the distribution’s trusted package and configuration tools.
- Trace which account and parent process start it, and whether a timer or other persistence mechanism is involved.
- Compare its start times and network destinations with the connection and audit timeline.
Package-verification commands and expected file locations vary among Linux distributions. Use the host’s own package manager and known-good configuration as the reference rather than treating a generic path or service name as universal.
Can malware hide command-and-control in email traffic?
Protocol tunneling can encapsulate one protocol inside another, helping communications evade filtering, blend into existing traffic, or use an outer layer of encryption. MITRE notes that tunneling may be combined with proxying or protocol impersonation. Therefore, a port-based rule alone can miss activity, and a connection using a mail-associated port does not prove that its payload is ordinary email. See MITRE ATT&CK T1572: Protocol Tunneling.
CISA’s Truebot advisory, published July 6, 2023, describes campaign activity that blended exfiltrated data with network traffic and used application-layer protocols and command-and-control channels. It is an example of observed behavior—not evidence that a particular Linux server is infected with Truebot or that email tunneling is common. See CISA advisory AA23-187A: Truebot Malware.
Where packet or flow visibility is available, compare destinations, timing, volumes, and protocol behavior with normal mail-relay and application patterns. Encryption or encapsulation may prevent payload inspection; process attribution and surrounding host events can still provide useful evidence.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should I correlate network activity with audit and process telemetry?
Build one timeline that connects process execution, service or timer changes, network connections, and audit events. MITRE identifies suspicious Python execution from non-standard contexts or cron jobs as a pattern to examine when it makes outbound connections or accesses sensitive files. It also describes audit tampering behaviors such as killing auditd, stopping its service, changing audit rules, or a sudden absence of audit logs correlated with privileged execution. See MITRE ATT&CK T1059.006: Python and MITRE ATT&CK T1562.012: Disable or Modify Linux Audit System.
Preserve the relevant process, persistence, network, and audit evidence before changing the host, following your organization’s incident-response procedures. A missing log stream can result from configuration or logging failure as well as deliberate tampering; use surrounding events to distinguish those possibilities rather than treating absence alone as proof.
Quick Recap
What should not be treated as proof?
- A mail-associated port does not prove that traffic is SMTP or that the process is malicious.
- A mail-related binary, unfamiliar executable location, or service name is contextual evidence that needs validation.
- Use of
ss,lsof, ornetstatis ordinary administrative activity as well as a behavior that adversaries may use; the command alone is not a compromise indicator. - MITRE’s techniques and CISA’s Truebot example describe behaviors and detection patterns, not the prevalence of Linux backdoors masquerading as email traffic.
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