Shifting to IaC: Writing the 183-Line Central Firewall Engine in Bicep When the Portal Hits a Wall | Day 9 & 10

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In my Day 8 lab, we successfully executed a classic data-plane traffic hijack. By forging a User Defined Route (UDR) for 0.0.0.0/0 on our Spoke network (Sec_Spoke_SEA_VNet), we successfully ripped traffic away from its default internet path and funneled it straight into a virtual appliance placeholder IP at 10.0.1.4 inside our Hub. As my terminal verification tests proved, outbound internet transit hit a hard 100% packet hole.

For Days 9 & 10, the mission was clear: replace that placeholder IP with a live, stateful Azure Native Firewall (Basic SKU) to intercept, inspect, and enforce rule collections on those hijacked packets.

Then, reality hit. Mid-sprint, my active 30-day promotional credit pool hit its expiration safety boundary. My portal dashboard flagged a paused state with an absolute balance of ₹0.00.

In a production enterprise environment, infrastructure blockers like this happen all the time. Budgets freeze, subscription terms shift, or portal access gets locked down. As a Network Security Engineer transitioning into DevSecOps, you don’t stop the roadmap; you change your delivery mechanism. Instead of manually clicking buttons inside the billing-active Azure Portal GUI, I pivoted immediately. I abstracted our entire physical network, routing states, peering hooks, and stateful security rules into a single, declarative Infrastructure-as-Code (IaC) deployment blueprint using Azure Bicep.

By moving from the GUI to local code, I built and validated the complete security engine architecture entirely for free inside Visual Studio Code with zero out-of-pocket financial risk.

The Network Logic: What Are We Architecting?

Before looking at the codebase, let’s map out exactly how data packets traverse this topology during runtime:

[ Private Spoke Subnet: 10.1.1.0/24 ]
│
▼ (User Defined Route: 0.0.0.0/0 ➔ Next Hop: 10.0.2.4)
[ AzureFirewallSubnet: 10.0.2.0/24 ] ──► Stateful Inspection Engine
│
├──► Matched Rule (UDP Port 53) ──► Allowed ──► Source NAT (SNAT) ➔ Internet
└──► Unmatched Rule (TCP 80/443) ──► Dropped ➔ Implicit Deny Posture

If you are coming from a traditional Network Security TAC background (like managing Palo Alto Prisma Access service connections or security zones), this setup mimics a centralized Next-Generation Firewall (NGFW) deployment.

The private Spoke VM does not have a public interface or an internet gateway. When it tries to talk to the outside world, our UDR acts as a mandatory signpost forcing the packet across the VNet Peering highway into the Hub's AzureFirewallSubnet at coordinate 10.0.2.4. The firewall then parses the packet from Layer 3 up to Layer 7, cross-references it with our rule policy, and decides whether to let it transit or drop it on the floor.

Complete Enterprise-Grade Bicep Security Blueprint

Below is the complete, unedited 183-line Bicep file that builds the entire baseline network fabric and trains the central firewall's inspection engine:

param location string = 'southeastasia'

// ===================================================================
// 1. CENTRAL ROUTE TABLE (THE TRAFFIC HIJACK SIGNPOST)
// ===================================================================
resource spokeRouteTable 'Microsoft.Network/routeTables@2023-09-01' = {
  name: 'Spoke_To_Hub_RT'
  location: location
  properties: {
    routes: [
      {
        name: 'Route_To_Hub'
        properties: {
          addressPrefix: '0.0.0.0/0'         // Intercepts all outbound internet traffic
          nextHopType: 'VirtualAppliance'     // Overrides default system provider routing
          nextHopIpAddress: '10.0.2.4'       // Core internal private IP of our Azure Firewall
        }
      }
    ]
  }
}

// ===================================================================
// 2. CORE HUB NETWORK (MANAGEMENT, FIREWALL, AND GATEWAY SEGMENTS)
// ===================================================================
resource hubVnet 'Microsoft.Network/virtualNetworks@2023-09-01' = {
  name: 'Sec_Hub_SEA_VNet'
  location: location
  properties: {
    addressSpace: {
      addressPrefixes: [ '10.0.0.0/16' ]
    }
    subnets: [
      {
        name: 'Management_SEA_Subnet'
        properties: {
          addressPrefix: '10.0.1.0/24'
        }
      }
      {
        name: 'AzureFirewallSubnet'         // Mandated structural name for native engine binding
        properties: {
          addressPrefix: '10.0.2.0/24'
        }
      }
      {
        name: 'GatewaySubnet'               // Reserved segment for future VPN/ExpressRoute Gateways
        properties: {
          addressPrefix: '10.0.3.0/24'
        }
      }
    ]
  }
}

// ===================================================================
// 3. SPOKE PRODUCTION NETWORK (BOUND TO TRAFFIC HIJACK UDR)
// ===================================================================
resource spokeVnet 'Microsoft.Network/virtualNetworks@2023-09-01' = {
  name: 'Sec_Spoke_SEA_VNet'
  location: location
  properties: {
    addressSpace: {
      addressPrefixes: [ '10.1.0.0/16' ]
    }
    subnets: [
      {
        name: 'App_Prod_Subnet'
        properties: {
          addressPrefix: '10.1.1.0/24'
          routeTable: {
            id: spokeRouteTable.id           // Associates the traffic-hijacking UDR table
          }
        }
      }
    ]
  }
}

// ===================================================================
// 4. BIDIRECTIONAL NETWORK PEERING: HUB TO SPOKE
// ===================================================================
resource hubToSpokePeering 'Microsoft.Network/virtualNetworks/virtualNetworkPeerings@2023-09-01' = {
  parent: hubVnet
  name: 'Hub_To_Spoke_Peer'
  properties: {
    allowVirtualNetworkAccess: true
    allowForwardedTraffic: true
    allowGatewayTransit: false
    useRemoteGateways: false
    remoteVirtualNetwork: {
      id: spokeVnet.id
    }
  }
}

// ===================================================================
// 5. BIDIRECTIONAL NETWORK PEERING: SPOKE TO HUB
// ===================================================================
resource spokeToHubPeering 'Microsoft.Network/virtualNetworks/virtualNetworkPeerings@2023-09-01' = {
  parent: spokeVnet
  name: 'Spoke_To_Hub_Peer'
  properties: {
    allowVirtualNetworkAccess: true
    allowForwardedTraffic: true
    allowGatewayTransit: false
    useRemoteGateways: false
    remoteVirtualNetwork: {
      id: hubVnet.id
    }
  }
}

// ===================================================================
// 6. CENTRAL FIREWALL PUBLIC EGRESS INTERFACE (PUBLIC IP)
// ===================================================================
resource firewallPublicIP 'Microsoft.Network/publicIPAddresses@2023-09-01' = {
  name: 'Sec_Hub_FW_PIP'
  location: location
  sku: {
    name: 'Standard'
  }
  properties: {
    publicIPAllocationMethod: 'Static'
  }
}

// ===================================================================
// 7. CENTRAL FIREWALL POLICY CONTAINER (THE SECURITY RULEBOOK)
// ===================================================================
resource firewallPolicy 'Microsoft.Network/firewallPolicies@2023-09-01' = {
  name: 'Hub_Central_FW_Policy'
  location: location
  properties: {
    sku: {
      tier: 'Basic'
    }
  }
}

// ===================================================================
// 8. CENTRAL NATIVE FIREWALL ENGINE PROVISIONING
// ===================================================================
resource azureFirewall 'Microsoft.Network/azureFirewalls@2023-09-01' = {
  name: 'Central-Security-Engine'
  location: location
  properties: {
    sku: {
      name: 'AZFW_VNet'
      tier: 'Basic'
    }
    firewallPolicy: {
      id: firewallPolicy.id
    }
    ipConfigurations: [
      {
        name: 'fw-ip-config'
        properties: {
          publicIPAddress: {
            id: firewallPublicIP.id
          }
          subnet: {
            id: '${hubVnet.id}/subnets/AzureFirewallSubnet' // Drops interface into reserved subnet
          }
        }
      }
    ]
  }
}

// ===================================================================
// 9. STATEFUL RULE COLLECTION GROUP (LAYER 4 NETWORK FILTER RULES)
// ===================================================================
resource ruleCollectionGroup 'Microsoft.Network/firewallPolicies/ruleCollectionGroups@2023-09-01' = {
  parent: firewallPolicy
  name: 'Outbound_Traffic_RCG'
  properties: {
    priority: 1000
    ruleCollections: [
      {
        ruleCollectionType: 'FirewallPolicyFilterRuleCollection'
        name: 'Allow_Core_Network'
        priority: 1100
        action: {
          type: 'Allow'
        }
        rules: [
          {
            ruleType: 'NetworkRule'
            name: 'Allow_DNS'
            ipProtocols: [
              'UDP'
            ]
            sourceAddresses: [
              '10.1.1.0/24'                  // Permits packets originating from Spoke Prod Subnet
            ]
            destinationAddresses: [
              '8.8.8.8'                      // Target: Google Primary DNS
              '8.8.4.4'                      // Target: Google Secondary DNS
            ]
            destinationPorts: [
              '53'                           // Limits traffic strictly to DNS query protocols
            ]
          }
        ]
      }
    ]
  }
}

Deep-Dive: Code Logic & Packet Inspection Mechanics

When discussing this topology in enterprise cloud security interviews, you must be able to trace how this specific code handles packet state:

The UDR Override Mechanics (Resource 1): By declaring a custom route for 0.0.0.0/0 with a nextHopType of VirtualAppliance, we explicitly override default systemic routing. The infrastructure forces the next-hop IP mapping to match 10.0.2.4, which is the exact internal interface of our upcoming firewall engine.
The API Subnet Constraint (Resource 2): The Azure resource provider imposes a strict naming policy. A native firewall cannot attach to an arbitrarily named subnet profile. It searches exclusively for a subnet token string labeled AzureFirewallSubnet. We carved out a dedicated /24 block (10.0.2.0/24) to handle the private clustering interfaces of the engine.
Stateful Layer 4 Filtering (Resource 9): This section defines our firewall policy rulebook. The engine operates on an Implicit Deny architecture—if a packet parameter profile does not explicitly match an allowed rule entry, it drops dead at the gate.
- The Allowed Execution Flow: A workload inside our Spoke network (10.1.1.0/24) generates a standard domain name lookup request toward external DNS resolver 8.8.8.8 over UDP Port 53. The firewall parses the packet headers, finds an exact match inside our Allow_DNS rule matrix, permits the transit, and executes Source NAT (SNAT) using our public IP (Sec_Hub_FW_PIP) to preserve private topology boundaries.
- The Implicit Deny Flow: A background process or a user inside that same Spoke network attempts to establish an outbound web session over HTTP (Port 80) or HTTPS (Port 443). The packet hit the firewall interface. The stateful inspector reviews the policy definitions, encounters an absolute absence of any rule mapping ports 80/443, and silently drops the connection instantly.

Code Logic & Traffic Processing Deep-Dive

To demonstrate absolute operational mastery during cloud engineering interviews, we must break down how this file behaves during active network runtime:

Resource 1 (spokeRouteTable): This object overrides default routing behavior. By creating a custom route entry for 0.0.0.0/0 (all internet traffic) with a nextHopType parameter explicitly declared as VirtualAppliance, we disrupt standard network isolation. Any server on this subnet that tries to reach an external resource is automatically pointed directly to 10.0.2.4.
Resource 2 (hubVnet) Subnet Naming Constraint: The Azure API dictates that a native firewall appliance cannot bind to an arbitrary subnet profile. It requires an exact structural string identifier named AzureFirewallSubnet. We carved out a dedicated /24 block (10.0.2.0/24) to handle the clustering interfaces of the underlying infrastructure.
Resource 9 (ruleCollectionGroup) Layer 4 Parsing: This is where our packet filtering criteria are established. The firewall runs a stateful inspection engine, matching source vectors against destination coordinates.
- The Evaluated Match: When a compute host inside our Spoke network (10.1.1.0/24) generates an upstream lookup request toward 8.8.8.8 on UDP Port 53, the firewall validates an exact parameter match in its rule collection and permits egress traffic while executing Source NAT (SNAT).
- The Implicit Deny Enforcement: If an active process on that same workload attempts to create an outbound HTTP web session on port 80 or an HTTPS connection on port 443, the data packet hits the firewall interface. The engine reviews the policy rules, encounters a total absence of any rule covering ports 80/443, and instantly drops the connection with zero response back to the client.

Compilation Verification & Local Quality Control

To ensure the technical validity, reference bindings, and compliance of this 183-line enterprise design without launching live, billing-active resources, I leveraged the built-in Microsoft Bicep language compilation workspace inside Visual Studio Code.

The compiler ran an extensive static validation analysis across our declared topography configurations, network peering structures, and rule groups, verifying a completely flawless design layout: