Linux & service basics: logs, systemd/PM2, permissions, Nginx reverse proxy, DNS checks

Source note: imported from linux-service-triage by kowl64, with provenance previously recorded in release-only _meta.json metadata.

PURPOSE

Diagnoses common Linux service issues using logs, systemd/PM2, file permissions, Nginx reverse proxy checks, and DNS sanity checks.

WHEN TO USE

• TRIGGERS:
  • Show me why this service is failing using logs, then give the exact fix commands.
  • Restart this app cleanly and confirm it is listening on the right port.
  • Fix the permissions on this folder so the service can read and write safely.
  • Set up Nginx reverse proxy for this port and verify DNS and TLS are sane.
  • Create a systemd service for this script and make it survive reboots.
• DO NOT USE WHEN...
  • You need kernel debugging or deep performance profiling.
  • You want to exploit systems or bypass access controls.

INPUTS

• REQUIRED:
  • Service type: systemd unit name or PM2 process name.
  • Observed symptom: error message, status output, or logs (pasted by user).
• OPTIONAL:
  • Nginx config snippet, domain name, expected upstream port.
  • Filesystem paths used by the service.
• EXAMPLES:
  • systemctl status myapp output + journalctl excerpt
  • Nginx server block + domain + upstream port

OUTPUTS

• Default: triage report (likely cause, evidence from logs, minimal fix plan).
• If explicitly requested and safe: exact shell commands to apply the fix. Success = service runs, listens on expected port, and reverse proxy/DNS path is correct.

WORKFLOW

1. Confirm scope and safety:
   • identify the host OS, service manager, process runner, reverse proxy, and whether changes are permitted.
   • do not assume systemd, PM2, Nginx, DNS control, or TLS tooling until the user confirms they apply.
2. Gather evidence:
   • status output + recent logs (see references/triage-commands.md).
3. Classify failure:
   • config error, dependency missing, permission denied, port conflict, upstream unreachable, DNS mismatch.
4. Propose minimal fix + verification steps.
5. Validate network path (if web service):
   • app listens -> Nginx proxies -> DNS resolves -> (TLS sanity if applicable).
6. Provide restart/reload plan and confirm health checks.
7. STOP AND ASK THE USER if:
   • logs/status output are missing,
   • actions require privileged access not confirmed,
   • TLS/cert management is required but setup is unknown.

OUTPUT FORMAT

TRIAGE REPORT
- Symptom:
- Evidence (what you provided):
- Most likely cause:
- Fix plan (minimal steps):
- Exact commands (ONLY if user approved changes):
- Verification:
- Rollback:

SAFETY & EDGE CASES

• Read-only by default: diagnose from provided outputs; do not assume you can run commands.
• Treat systemd, PM2, and Nginx commands as stack-specific examples; adapt for other Linux init systems, process managers, or proxies.
• Avoid destructive changes; require explicit confirmation for anything risky.
• Prefer nginx -t before reload and verify ports with ss.

EXAMPLES

• Input: "journal shows permission denied on /var/app/uploads." Output: path permission analysis + safe chown/chmod plan + verification.

• Input: "App works locally but domain returns 502." Output: upstream port checks + nginx error log interpretation + proxy_pass fix plan.