Zero Trust, Segmentation, and Service-to-Service Security Questions

Difference — short

• VLAN: Layer 2 construct that segments switch ports into separate broadcast domains regardless of physical cabling.
• IP subnet: Layer 3 addressing scope; defines IP host ranges and routing boundaries. VLANs often map 1:1 to subnets, but not required.

When to use VLAN segmentation without changing subnets

• Example: Multi-tenant switches where same management subnet spans multiple floors but you want port isolation per floor for security/traffic shaping. Use VLANs for isolation and apply same subnet on routed firewall interface or use proxy ARP. Keeps addressing consistent during migrations.

When to separate by subnets

• Example: Separate server farm (10.0.1.0/24) from user workstations (10.0.2.0/24) to enforce routing policies, ACLs, and monitoring. Use different subnets when you need routing controls, different DHCP scopes, or different security zones.

Implications

• Broadcast domains: VLAN = one broadcast domain. Subnet normally aligns with broadcast domain; different subnets imply different broadcast domains.
• Inter-VLAN routing: Required when VLANs/subnets need to communicate—done on L3 switch or router (SVI) or firewall. Adds latency/inspection point and enables ACL enforcement.
• ACL placement:
  • VLAN-only (same subnet): enforce ACLs at access switches or host firewall/edge device since L2 doesn't route. Use port ACLs/MAC ACLs or private VLANs.
  • Separate subnets: apply ACLs on the L3 device (SVI/router) or firewall between subnets for centralized policy, and also at access layer for defense in depth.

Best practice

• Prefer 1:1 VLAN-to-subnet mapping for simplicity. Use ACLs at the routing boundary for inter-subnet control and at edge/access for micro-segmentation.

Goal & assumptions
Segregate prod/non-prod while minimizing downtime. Assumes same Kubernetes distribution (EKS/GKE/AKS), RBAC in place, CI/CD pipelines, and observability.

Phased migration plan

1. Discovery & design (week 0–1)

• Inventory namespaces, services, ingress, network flows (netflow, kube-bench, kube-state-metrics).
• Design segmentation: separate namespaces + NetworkPolicies + dedicated node pools or clusters for prod/non-prod as risk budget allows.
• Risks: incomplete inventory → missed dependencies. Mitigation: passive traffic capture + tagging; rollback: freeze changes, continue discovery.

2. Implement baseline NetworkPolicies (week 1–3)

• Start with default-deny egress/ingress per namespace; progressively add allow rules using label-based selectors.
• Use policy engine (Calico) for advanced selectors and global policies for infra.
• Test with canaries in non-prod; staged rollout to prod.
• Risks: broken service communication. Mitigation: policy staging, automated policy simulator (calicoctl check), CI tests. Rollback: revert to permissive policy (apply known-good ClusterNetworkPolicy).

3. Deploy service mesh (week 3–6)

• Install mesh in non-prod (Istio/linkerd) with sidecar injection for canaries; enable mTLS, traffic shifting.
• Use mesh policies to enforce inter-service auth and observability.
• Risks: latency/CPU on sidecars. Mitigation: resource limits, gradual percentage rollout, perf testing. Rollback: disable sidecar injection or scale mesh control plane down and revert traffic routing.

4. Web Application Firewall & ingress hardening (week 5–8)

• Place WAF (ModSecurity/Cloud WAF) in front of ingress; enforce OWASP rules, rate limits, IP allowlists for prod.
• Test in detection mode then enforcement in staging.
• Risks: false positives blocking users. Mitigation: run in monitor mode, whitelist known clients, staged enforcement. Rollback: switch WAF to monitor or bypass for specific paths.

5. Continuous compliance & verification (ongoing)

• Integrate policy-as-code: OPA/Gatekeeper for admission, CIS/Kubernetes benchmarks via periodic scans, runtime policy checks (Falco).
• CI gates: reject manifests violating NetworkPolicy/ingress/WAF rules.
• Risks: developer friction. Mitigation: clear docs, pre-commit checks, staged enforcement windows. Rollback: relax admission policies temporarily.

Rollback & runbook

• Maintain versioned manifests and a playbook: step identifier, kubectl/apply rollback manifest, rollback window, stakeholders, monitoring checklist.
• Use feature flags for traffic, and circuit breakers in mesh to redirect to stable deployments.

Monitoring & verification

• KPIs: pod-to-pod latency, policy deny rate, 4xx/5xx spikes, deployment failure rate.
• Post-mortem and incremental hardening.

This plan minimizes blast radius by progressive enforcement, testing in non-prod, and provides clear rollback actions for each phase.

Plan Overview
Validate segmentation with an automated, layered testing program: policy-as-code + CI, passive flow verification, active scans, host checks, scheduled pen tests, and safe remediation playbooks.

Policy-as-code (automated)

• Encode segmentation rules in Terraform/CloudFormation + OPA/Rego or Sentinel.
• CI pipeline runs unit tests (policy evaluations), and integration tests against a staging VPC using mocked flows.
• Example: PR blocked if OPA detects a security group allowing DB access from 0.0.0.0/0.

Passive flow verification

• Collect VPC Flow Logs/NetFlow/sFlow and centralize (ELK/Splunk).
• Daily automated queries to confirm only allowed source-dest-port tuples appear.
• Use baselined alerting for unexpected flows (anomaly scoring).

Active scanning

• Weekly targeted scans with nmap/Nessus from bastion/jump hosts to verify ports unreachable where blocked.
• Use authenticated scans on hosts to check internal listener exposure.

Host-based verification

• OS agents (osquery/Wazuh) validate iptables/ACL state, routing table, and service bindings; run nightly checks and report drift to CMDB.

Scheduled pen-testing

• Quarterly internal/external pen-tests focusing on lateral movement and segmentation bypass; capture TTPs and remediation tickets.

Safe remediation

• Use canary patches: apply rule changes to a small subnet, enable traffic mirroring to validate, then roll out.
• Implement automated rollback in config management if health checks (connectivity, application KPIs) degrade.
• Schedule disruptive remediations in maintenance windows and coordinate with owners; use staged firewall rule insertions (deny with logging first, then deny without logging).

Observability & Governance

• Dashboards for policy compliance and recent remediation actions.
• Change control/RBAC and runbooks for emergency rollback.
• Metric-driven SLIs: segmentation-compliance %, unauthorized-flow-count, time-to-remediate.

This approach ensures continuous validation with low risk to production availability.

Approach (brief)
I’ll show a BGP/routing sketch for Direct Connect + TGW, then specific firewall / Security Group / NACL rules to enforce: only Internet→10.1 DMZ web allowed, 10.1→10.2 allowed via inspection gateway, and block direct VPC-to-VPC lateral movement.

Topology & BGP/routing sketch

• On-prem: 10.0.0.0/16
• VPC-DMZ: 10.1.0.0/16
• VPC-App: 10.2.0.0/16
• Transit/Inspection GW (NGFW) in 10.1 or separate inspection VPC (10.3.0.0/24)

ASCII:

Internet -> IGW -> ALB / ELB -> SG(DMZ web) -> 10.1.0.0/16
10.1 <-> TGW <-> DX Gateway <-> OnPrem (BGP)
10.2 attached to TGW but routes to 10.1 go via NGFW: 10.2 -> TGW -> Route to 10.3(inspection) -> 10.1
BGP: advertise 10.1/16, 10.2/16, 10.0/16 between TGW and on-prem

Routing rules:

• On TGW, set route table for 10.2 -> direct to VPC-Attachment; 10.1 -> via inspection attachment (10.3)
• On VPC attachments, disable route propagation so traffic between 10.1 and 10.2 hits TGW rules

Firewall / SG / NACL rules

1. Internet → 10.1 DMZ (only web)

• ALB/NAT/IGW security group (allow inbound TCP 80,443 from 0.0.0.0/0)
• DMZ instances SG: allow inbound TCP 80,443 from ALB SG; deny other inbound
• NACL on 10.1 subnet: allow ephemeral responses; explicit deny other inbound ports if required

2. 10.1 → 10.2 only via inspection gateway

• TGW routing forces 10.1→10.2 traffic to inspection VPC (10.3)
• Inspection gateway NGFW rules:
  • Inbound (from 10.1): allow TCP/UDP required app ports (e.g., 443, 8080) to 10.2; apply IDS/IPS, URL/App controls
  • Outbound (to 10.2): allow same
  • Drop any traffic sourced from 10.1 destined to 10.2 that fails inspection
• On 10.2 SG: allow inbound from NGFW ENI CIDR (10.3) on app ports; deny inbound from 10.1 CIDR
• On 10.1 SG: allow outbound to NGFW only on required ports; do NOT allow direct outbound to 10.2 CIDR

3. Prevent direct VPC-to-VPC lateral movement

• 10.2 SG: deny/omit rules allowing 10.1 CIDR. Only permit traffic from NGFW IP(s).
• 10.1 SG: restrict outbound to NGFW IP(s) and Internet (for web).
• NACLs optional: add stateless rules to block 10.1→10.2 and 10.2→10.1 except via NGFW subnets.

Validation & Notes

• Ensure TGW route table attachment separation: one table forcing 10.1→inspection and 10.2→local; disallow peering that bypasses TGW inspection.
• BGP: advertise VPC CIDRs via TGW to on-prem; use route prioritization / blackhole for accidental direct paths.
• Audit: verify flow logs / NGFW logs to confirm flows traverse inspection.

Situation & Goal
Contain lateral movement from App-zone host A (10.10.20.45) to internal C2 IP in Management zone M (10.20.5.12) using segmentation controls to stop traffic quickly while minimizing business impact.

Short‑term (Immediate containment — minutes to 2 hours)

• Isolate flow via ACL changes at zone border router/firewall:
  • Add explicit deny for App -> Management TCP/UDP to the C2 IP, allow other Management services.

# Example ACL (Cisco-like)
ip access-list extended BLOCK_C2
 deny   ip host 10.10.20.45 host 10.20.5.12
 permit ip any any
interface Gig1/0
 ip access-group BLOCK_C2 in

• Apply host-based firewall rule on App host (Windows/iptables) to block outbound to 10.20.5.12.

# Windows Defender Firewall
New-NetFirewallRule -DisplayName "Block_C2" -Direction Outbound -RemoteAddress 10.20.5.12 -Action Block

• If microsegmentation (SDN/NSX) available, create a temporary policy denying App zone egress to Management zone IPs matching C2 signature.
• Preserve forensic data: take memory image, network capture (span/pcap) of App host and gateway, snapshot ACL diffs and timestamps.

Validation (hours)

• Confirm via netflow/IDS that beacons stop to 10.20.5.12.
• From a secured analyst VM, attempt controlled connection to confirm blocks.
• Verify no collateral service outages by sampling legitimate Management services from App host.

Medium-term (24–72 hours)

• Quarantine host A, rebuild or forensically analyze, rotate credentials used by service accounts.
• Harden microsegmentation policies: implement least‑privilege east‑west rules (deny by default, allow explicit flows).
• Deploy host-based baseline rules via configuration management.

Long-term (weeks)

• Implement centralized microsegmentation policy engine (NSX/Illumio) with service tags and intent-based rules.
• Add automated ACL/Microseg change playbooks integrated with SIEM for rapid mitigation.
• Review network design to reduce Management zone privileged access surface; enforce jump hosts and bastion controls.
• Patch, retrain ops, and run tabletop exercises.

Validation & Monitoring (ongoing)

• Continuous netflow, DNS anomaly detection, and IDS signatures for fallback C2 patterns.
• Regular audits of ACLs, microsegmentation rules, and host firewall baselines.
• Post‑incident review documenting exact ACL/host rule changes, rollback plan, and lessons learned.