Azure Firewall

Azure Firewall

Azure Firewall is a managed, highly available, cloud-native stateful network firewall used to centrally govern and log traffic, typically deployed in the hub of a hub-spoke network. It is the choke point that enforces a default-deny egress posture across an Azure estate.

When to use

Centralising egress (and east-west / ingress) traffic control and inspection across an Azure network estate. Use this skill to pick the SKU, design rules, and deploy in the hub.

Do not use this skill for L7 web app protection (WAF on Front Door / App Gateway), subnet microsegmentation (NSGs - see azure-network-security-design), or third-party NVA design.

Pick the SKU by capability need

Rule of thumb: Standard SKU is right for most enterprise hubs in 2026. Move to Premium only when you have a concrete TLS-inspection or IDPS requirement - the certificate / PKI overhead of TLS inspection is real and breaks some apps. Basic only for SMB or sandbox.

Approach

1. Deploy in the hub — Single firewall per region in the hub VNet (with at least two availability zones for resilience). All spokes peer to the hub; egress and east-west traffic flows through. Verify: firewall has public IP for egress, deployed across ≥ 2 AZs, capacity baseline established.

2. Use Firewall Policy (not classic rules) — Author rules in Azure Firewall Policy with hierarchical parent + child policies (e.g. parent = enterprise baseline, child = region-specific). Don't author rules per firewall. Verify: policy hierarchy mapped; one base policy reused across regional firewalls.

3. Default-deny + structured rule collections — Order: DNAT (inbound) → Network rules (IP/port/protocol) → Application rules (FQDN/URL). Inside each collection group, priority controls evaluation. Default action = deny. Verify: a probe from a spoke to an unallowed FQDN is logged as deny.

4. Application rules with FQDN tags — Use built-in FQDN tags (WindowsUpdate, AzureKubernetesService, Office365) for Microsoft service egress instead of hand-curated lists. Custom FQDNs for third-party SaaS.

5. Force spoke egress through firewall via UDRs — User-defined routes on spoke subnets: 0.0.0.0/0 next hop = firewall private IP. Without UDRs, traffic bypasses the firewall. Verify: Get-AzEffectiveRouteTable on a spoke NIC shows next hop = firewall.

6. Enable threat intel + IDPS (Premium) — Threat-intel-based filtering in Deny mode for known-bad IPs / FQDNs. Premium: enable IDPS in Alert+Deny for high-severity signatures; tune false positives.

7. Stream logs and monitor capacity — Send to Log Analytics / Sentinel using structured firewall logs. Use the workbook for top denied flows, capacity / throughput. Verify: firewall workbook populated; alert on > 80% throughput sustained for 15 minutes.

Guardrails

• Azure Firewall complements, not replaces, NSGs (subnet/NIC microsegmentation) and WAF (L7 web). Layered defence; each does a different job.
• TLS inspection (Premium) requires certificate / PKI planning. Internal CA cert chain trusted on clients; some apps (cert-pinning, mTLS) break under inspection. Pilot first.
• Plan capacity and availability zones for resilient hub deployments. Single-zone = single zone outage takes the hub down.
• UDRs are the enforcement. Without 0.0.0.0/0 → firewall on spokes, traffic doesn't go through. Audit UDRs as part of landing zone.
• Firewall Policy is hierarchical for a reason. Don't author per-firewall rules at scale - they drift.
• Don't enable Premium features speculatively. Cost is meaningfully higher; only enable when there's a use case.

Common anti-patterns

• "Allow * for outbound to start" - Defeats the firewall. Default-deny + named FQDN rules from day one.
• "Premium SKU for everyone" - Cost without value if no IDPS / TLS inspection in scope.
• "TLS inspection on day one for all traffic" - Breaks half the apps. Pilot per workload.
• "No UDR - we assume traffic goes through the firewall" - It doesn't. Routes are the enforcement.
• "One single firewall, single zone" - AZ outage = hub down. Multi-AZ from day one.
• "Don't log - it's noisy" - First incident, no forensics. Structured logs + Sentinel always.

Example prompts

• Design Azure Firewall egress filtering with FQDN and application rules in a hub-spoke topology.
• When should I choose Firewall Premium with IDPS and TLS inspection?
• Set up Firewall Policy hierarchy and force spoke egress through UDRs.
• How do I protect Azure Virtual Desktop traffic with Azure Firewall?
• Build a Sentinel workbook for top denied flows from Azure Firewall logs.
• Plan capacity, availability zones, and DNS proxy for the hub firewall.

Microsoft Learn

• Azure Firewall overview: https://learn.microsoft.com/azure/firewall/overview
• Firewall Policy: https://learn.microsoft.com/azure/firewall/policy-rule-sets
• Premium features (IDPS / TLS): https://learn.microsoft.com/azure/firewall/premium-features
• Hub-spoke + firewall: https://learn.microsoft.com/azure/architecture/networking/architecture/hub-spoke
• FQDN tags: https://learn.microsoft.com/azure/firewall/fqdn-tags
• Threat intelligence: https://learn.microsoft.com/azure/firewall/threat-intel
• Structured logs: https://learn.microsoft.com/azure/firewall/firewall-structured-logs
• Forced tunneling: https://learn.microsoft.com/azure/firewall/forced-tunneling