Configure Web Application Firewall v2 on Application Gateway with a custom rule using Azure PowerShell

Article
04/06/2023

Custom rules allow you to create your own rules evaluated for each request that passes through the Web Application Firewall (WAF) v2. These rules hold a higher priority than the rest of the rules in the managed rule sets. The custom rules have an action (to allow or block), a match condition, and an operator to allow full customization.

This article creates an Application Gateway WAF v2 that uses a custom rule. The custom rule blocks traffic if the request header contains User-Agent evilbot.

To see more custom rule examples, see Create and use custom web application firewall rules

If you want run the Azure PowerShell in this article in one continuous script that you can copy, paste, and run, see Azure Application Gateway PowerShell samples.

Prerequisites

Azure PowerShell module

If you choose to install and use Azure PowerShell locally, this script requires the Azure PowerShell module version 2.1.0 or later.

To find the version, run Get-Module -ListAvailable Az. If you need to upgrade, see Install Azure PowerShell module.
To create a connection with Azure, run Connect-AzAccount.

If you don't have an Azure subscription, create an Azure free account before you begin.

Example script

Set up variables

$rgname = "CustomRulesTest"

$location = "East US"

$appgwName = "WAFCustomRules"

Create a resource group

$resourceGroup = New-AzResourceGroup -Name $rgname -Location $location

Create a VNet

$sub1 = New-AzVirtualNetworkSubnetConfig -Name "appgwSubnet" -AddressPrefix "10.0.0.0/24"

$sub2 = New-AzVirtualNetworkSubnetConfig -Name "backendSubnet" -AddressPrefix "10.0.1.0/24"

$vnet = New-AzvirtualNetwork -Name "Vnet1" -ResourceGroupName $rgname -Location $location `
  -AddressPrefix "10.0.0.0/16" -Subnet @($sub1, $sub2)

Create a Static Public VIP

$publicip = New-AzPublicIpAddress -ResourceGroupName $rgname -name "AppGwIP" `
  -location $location -AllocationMethod Static -Sku Standard

Create pool and frontend port

$gwSubnet = Get-AzVirtualNetworkSubnetConfig -Name "appgwSubnet" -VirtualNetwork $vnet

$gipconfig = New-AzApplicationGatewayIPConfiguration -Name "AppGwIpConfig" -Subnet $gwSubnet

$fipconfig01 = New-AzApplicationGatewayFrontendIPConfig -Name "fipconfig" -PublicIPAddress $publicip

$pool = New-AzApplicationGatewayBackendAddressPool -Name "pool1" `
  -BackendIPAddresses testbackend1.westus.cloudapp.azure.com, testbackend2.westus.cloudapp.azure.com

$fp01 = New-AzApplicationGatewayFrontendPort -Name "port1" -Port 80

Create a listener, http setting, rule, and autoscale

$listener01 = New-AzApplicationGatewayHttpListener -Name "listener1" -Protocol Http `
  -FrontendIPConfiguration $fipconfig01 -FrontendPort $fp01

$poolSetting01 = New-AzApplicationGatewayBackendHttpSettings -Name "setting1" -Port 80 `
  -Protocol Http -CookieBasedAffinity Disabled

$rule01 = New-AzApplicationGatewayRequestRoutingRule -Name "rule1" -RuleType basic `
  -BackendHttpSettings $poolSetting01 -HttpListener $listener01 -BackendAddressPool $pool -Priority 1000

$autoscaleConfig = New-AzApplicationGatewayAutoscaleConfiguration -MinCapacity 3

$sku = New-AzApplicationGatewaySku -Name WAF_v2 -Tier WAF_v2

Create two custom rules and apply it to WAF policy

# Create a User-Agent header custom rule 
$variable = New-AzApplicationGatewayFirewallMatchVariable -VariableName RequestHeaders -Selector User-Agent
$condition = New-AzApplicationGatewayFirewallCondition -MatchVariable $variable -Operator Contains -MatchValue "evilbot" -Transform Lowercase -NegationCondition $False  
$rule = New-AzApplicationGatewayFirewallCustomRule -Name blockEvilBot -Priority 2 -RuleType MatchRule -MatchCondition $condition -Action Block -State Enabled
 
# Create a geo-match custom rule
$var2 = New-AzApplicationGatewayFirewallMatchVariable -VariableName RemoteAddr
$condition2 = New-AzApplicationGatewayFirewallCondition -MatchVariable $var2 -Operator GeoMatch -MatchValue "US"  -NegationCondition $False
$rule2 = New-AzApplicationGatewayFirewallCustomRule -Name allowUS -Priority 14 -RuleType MatchRule -MatchCondition $condition2 -Action Allow -State Enabled

# Create a firewall policy
$policySetting = New-AzApplicationGatewayFirewallPolicySetting -Mode Prevention -State Enabled
$wafPolicy = New-AzApplicationGatewayFirewallPolicy -Name wafpolicyNew -ResourceGroup $rgname -Location $location -PolicySetting $PolicySetting -CustomRule $rule,$rule2

Create the Application Gateway

$appgw = New-AzApplicationGateway -Name $appgwName -ResourceGroupName $rgname `
  -Location $location -BackendAddressPools $pool `
  -BackendHttpSettingsCollection  $poolSetting01 `
  -GatewayIpConfigurations $gipconfig -FrontendIpConfigurations $fipconfig01 `
  -FrontendPorts $fp01 -HttpListeners $listener01 `
  -RequestRoutingRules $rule01 -Sku $sku -AutoscaleConfiguration $autoscaleConfig `
  -FirewallPolicy $wafPolicy

Update your WAF

After you create your WAF, you can update it using a procedure similar to the following code:

# Get the existing policy
$policy = Get-AzApplicationGatewayFirewallPolicy -Name $policyName -ResourceGroupName $RGname
# Add an existing rule named $rule
$policy.CustomRules.Add($rule)
# Update the policy
Set-AzApplicationGatewayFirewallPolicy -InputObject $policy

Next steps

Learn more about Web Application Firewall on Application Gateway

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