Defensive Controls and Hardening

Purpose

This document provides a defense-first hardening baseline for enterprise guest networks, with a primary objective of protecting the Guest network itself (availability, fairness, user safety), while also preventing Guest from becoming a pivot into internal environments.

Key principle: Guest is an untrusted, attacker-accessible zone. Security must be explicit, layered, and validated. (Zero Trust aligns with protecting resources over trusting network location.)

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1) Threats this hardening must address

Guest networks are vulnerable to:

• Guest-to-Guest abuse: scanning, opportunistic sniffing attempts, ARP-based AiTM attempts

• Infrastructure abuse: attacks on DHCP, DNS, WLC/AP/controller resources, firewall state tables

• DoS / resource exhaustion: airtime abuse, association floods, DHCP starvation, DNS query floods

• Onboarding/portal attacks: phishing lookalike portals, redirect manipulation, TLS trust degradation

• Recon: learning topology via DNS leakage, service discovery via multicast, etc.

MITRE ATT&CK correlations commonly seen in Guest scenarios include:

• Network Service Discovery (T1046)

• Adversary-in-the-Middle (T1557)

• Network Denial of Service (T1498)

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2) Layered defensive model (what “secure guest” actually means)

Layer A — Wireless/Edge isolation (reduce Guest-to-Guest harm)

Goal: protect guests from each other and reduce local attack surface.

Controls

1. Enable client isolation / peer-to-peer blocking on the Guest SSID

   • Cisco Catalyst 9800: peer-to-peer blocking is not enabled by default and does not apply to multicast traffic.

   • Meraki: client isolation for Bridge mode SSIDs is disabled by default.

   • Aruba: “Deny Intra VLAN Traffic / Client Isolation” disables peer-to-peer communication and is configured explicitly.

2. Explicit multicast/broadcast stance

   • Do not assume client isolation handles multicast; for Cisco 9800 peer-to-peer blocking, multicast is explicitly not covered.

   • If multicast discovery is not needed, restrict it. If needed, scope it tightly and rate-limit where possible.

Common failure modes

• Isolation enabled on SSID but broken by upstream routing/VLAN pooling assumptions

• Multicast still allows service discovery patterns (use-case dependent)

• “Temporary exceptions” for printers/Chromecast/etc. silently reintroduce lateral surfaces

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Layer B — L3/L4 segmentation and policy enforcement (prevent pivot and reduce blast radius)

Goal: make Guest a contained security domain with minimal exposure.

Controls

1. Dedicated Guest VRF/VLAN + strict boundary firewall policy

   • Default deny Guest → RFC1918/internal

   • Allowlist only explicitly required services (e.g., portal VIP, Guest DNS resolvers)

2. Enforce DNS egress

   • Allow Guest clients to reach only approved Guest DNS resolvers on UDP/TCP 53

   • Block direct DNS to arbitrary resolvers unless your policy explicitly allows it

3. State-table and connection protection

   • Apply per-client connection rate limits to protect firewalls and gateways from state exhaustion (practical DoS resilience)

   • Prefer “abuse-tolerant” policy design for Guest (shorter timeouts, thresholds)

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Layer C — Dedicated Guest DNS design (stop recon, reduce outage coupling)

Goal: protect Guest users and infrastructure by treating DNS as a security control, not just plumbing.

Controls

1. Dedicated Guest recursive resolvers

   • Do not use the same internal DNS resolvers used by corporate endpoints.

   • This reduces internal naming disclosure and blast radius.

2. Minimal split-horizon usage

   • If you require split-horizon for portal FQDN reachability, scope it to the minimum set of names required.

Standards-backed guidance

• DNS core architecture is defined in RFC 1034 and RFC 1035.

• DNS terminology (to avoid design confusion) is consolidated in RFC 8499.

• NIST provides enterprise DNS security deployment guidance in SP 800-81.

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Layer D — Portal/certificate trust (protect users from phishing and trust downgrade)

Goal: preserve trust in onboarding and prevent portal mechanics from becoming an attack surface.

Controls

• Use a dedicated portal FQDN that guests can resolve without internal DNS access.

• Use a public CA-signed certificate that matches the portal FQDN (avoid “click through” culture).

• Inspect redirect chains to ensure they do not expose internal hostnames/IPs.

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Layer E — DoS resilience (keep Guest usable under attack)

Goal: keep the guest network stable and fair under hostile conditions.

Controls

1. DHCP protections

   • Rate limits, pool sizing, monitoring for starvation patterns

2. DNS protections

   • Resolver capacity planning, rate limiting, caching strategy

   • Avoid coupling Guest DNS to internal DNS availability

3. Wireless protections

   • Monitor association/authentication anomalies (flood patterns)

   • Enable management frame protection where feasible (e.g., PMF/802.11w)

MITRE mapping: Network Denial of Service (T1498).

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3) Hardening checklist (design + configuration acceptance criteria)

Wireless / SSID

• Client isolation / P2P blocking enabled on Guest SSID

• Multicast behavior explicitly defined (allowed/blocked/limited)

• VLAN pooling reviewed for bypass risk

• Guest SSID policy does not rely on “defaults” (vendor-specific defaults)

Network boundary

• Dedicated Guest VLAN/VRF

• Guest → internal/RFC1918 denied by default

• Only allowlisted infra reachable from Guest (DNS, portal VIP, etc.)

DNS

• Guest uses dedicated recursive resolvers

• DNS egress is enforced (block arbitrary resolvers)

• No internal zone leakage to Guest

• DNS hardening aligned with NIST guidance

Portal and certificate

• Portal FQDN resolvable from Guest

• TLS certificate matches portal FQDN

• Redirect chain does not leak internal hostnames/IPs

Monitoring and detection

• Alerts for DHCP anomalies (starvation, spikes)

• Alerts for DNS query rate spikes

• Alerts for WLC/AP association/auth spikes

• Guest firewall state usage monitored

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4) Practical “secure guest” definition

A Guest network is “secure enough” when:

• Guest-to-guest abuse is materially reduced (isolation works)

• Infrastructure services are resilient to abuse (DNS/DHCP/WLC not fragile)

• The boundary policy is enforceable and auditable

• The onboarding flow is trustworthy

• All of the above is validated continuously (see validation chapter)