═══════════════════════════════════════════════════════════════════
  SENTINEL SYSTEM USER - QUICK REFERENCE
═══════════════════════════════════════════════════════════════════

✅ YES - DEDICATED SYSTEM USER IS HIGHLY RECOMMENDED FOR PRODUCTION

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  WHY IT'S IMPORTANT
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SECURITY:
  • Privilege isolation - attackers limited to sentinel user
  • No login access - cannot SSH/console as sentinel
  • No sudo - cannot escalate privileges
  • Immutable code - sentinel cannot modify /opt/sentinel
  • Limited write access - only /var/lib/sentinel and /var/log/sentinel

OPERATIONS:
  • Clear ownership - ps aux | grep sentinel shows all processes
  • Professional standard - like www-data, mysql, postgres
  • Independent lifecycle - survives user logouts
  • Easy auditing - all actions traceable to sentinel user

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  HOW IT WORKS
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USER CREATION:
  useradd --system --no-create-home --shell /bin/false sentinel
  
  --system        → UID < 1000 (not a login user)
  --no-create-home → No /home/sentinel directory
  --shell /bin/false → Cannot login interactively
  No password     → Cannot authenticate

PERMISSIONS:
  /opt/sentinel/          root:root       (application - read-only)
  /etc/sentinel/          root:root       (config - read-only)
  /var/lib/sentinel/      sentinel:sentinel (data - writable)
  /var/log/sentinel/      sentinel:sentinel (logs - writable)

SYSTEMD SECURITY:
  User=sentinel                    # Run as non-root
  NoNewPrivileges=true             # No privilege escalation
  ProtectSystem=strict             # /usr, /boot read-only
  ProtectHome=read-only            # /home inaccessible
  ReadWritePaths=/var/lib/sentinel # Only these writable
  MemoryLimit=2G                   # Prevent memory exhaustion
  CPUQuota=80%                     # Prevent CPU hogging

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  VERIFICATION COMMANDS
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# Check user exists
id sentinel
# Expected: uid=997(sentinel) gid=997(sentinel)

# Verify cannot login
su - sentinel
# Expected: "This account is currently not available" ✅

# Check ownership
ls -la /opt/sentinel | head -3
# Expected: root root

ls -la /var/lib/sentinel | head -3
# Expected: sentinel sentinel

# Test write restrictions
sudo -u sentinel touch /opt/sentinel/test
# Expected: Permission denied ✅

sudo -u sentinel touch /var/lib/sentinel/test
# Expected: SUCCESS ✅

# Security score
systemd-analyze security sentinel.service
# Expected: Score < 5.0 (lower is better)

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  ATTACK PROTECTION
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SCENARIO                           WITHOUT USER     WITH USER
───────────────────────────────────────────────────────────────────
Code injection in VHDL             ❌ Full access   ✅ Isolated
Command injection via Git          ❌ Home writable ✅ Protected
Privilege escalation (setuid)     ❌ Possible      ✅ Blocked
Memory exhaustion (DoS)            ❌ System crash  ✅ Killed
Log flooding (disk full)           ❌ System crash  ✅ Rotated
Malicious file modifications       ❌ Code writable ✅ Code immutable

═══════════════════════════════════════════════════════════════════
  FILES CREATED
═══════════════════════════════════════════════════════════════════

deployment/systemd/sentinel.service           # Enhanced security
deployment/systemd/sentinel-logrotate.conf    # Log rotation (NEW)
deployment/systemd/install-system.sh          # Updated installer
deployment/SECURITY_BEST_PRACTICES.md         # Full guide (NEW)

═══════════════════════════════════════════════════════════════════
  BEST PRACTICES CHECKLIST
═══════════════════════════════════════════════════════════════════

✅ Dedicated system user (sentinel)
✅ No login capability (shell=/bin/false)
✅ No password authentication
✅ Application in /opt (FHS compliant)
✅ Config in /etc (protected)
✅ Data in /var/lib (user-owned)
✅ Logs in /var/log (user-owned, rotated)
✅ systemd security hardening
✅ Resource limits (Memory/CPU)
✅ Logrotate configured (prevent disk fill)
✅ Root owns code (immutable)
✅ Minimal write permissions

═══════════════════════════════════════════════════════════════════
  INSTALLATION
═══════════════════════════════════════════════════════════════════

SYSTEM-WIDE (Production - Recommended):
  cd /home/it_admin/Sentinel/deployment/systemd
  sudo ./install-system.sh
  
  → Creates sentinel user automatically
  → Installs to /opt/sentinel
  → Applies all security hardening
  → Installs logrotate config

USER-LEVEL (Development):
  cd /home/it_admin/Sentinel/deployment/systemd
  ./install.sh
  
  → Runs as your user
  → Installs to ~/Sentinel
  → No system user needed

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  MONITORING
═══════════════════════════════════════════════════════════════════

# Service status
systemctl status sentinel.service

# Live logs
journalctl -u sentinel.service -f

# Resource usage
systemctl show sentinel.service | grep -E 'Memory|CPU'

# Processes
ps aux | grep sentinel

# Disk usage
du -sh /var/lib/sentinel

═══════════════════════════════════════════════════════════════════
  SUMMARY
═══════════════════════════════════════════════════════════════════

Q: Should I use a dedicated system user?
A: YES - Absolutely required for production. Industry standard.

Q: What if I'm just testing?
A: Use user-level install (install.sh). Upgrade to system-wide later.

Q: Is it more secure?
A: YES - Multiple layers: user isolation + systemd hardening + 
   filesystem permissions + resource limits + audit trail

Q: Can the sentinel user login?
A: NO - Shell is /bin/false, no password, no SSH access

Q: What can sentinel write to?
A: ONLY /var/lib/sentinel (data) and /var/log/sentinel (logs)

Q: Can sentinel modify its own code?
A: NO - /opt/sentinel is owned by root (read-only)

Q: What happens if compromised?
A: Damage limited to data directory. Cannot modify code, config,
   system files, or access other users' data.

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For complete documentation, see:
  deployment/SECURITY_BEST_PRACTICES.md

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