Rare client observed with high reverse DNS lookup count

let starttime = 8d;
let endtime = 1d;
let threshold = 10;
DnsEvents
| where TimeGenerated > ago(endtime)
| where Name has "in-addr.arpa"
| summarize StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated), dcount(Name), ReverseDNSLookup_List = make_set(Name,100) by ClientIP
| where dcount_Name > threshold
| project StartTimeUtc, EndTimeUtc, ClientIP , dcount_Name, ReverseDNSLookup_List
// Filter out previously seen IPs
// Returns all the records from the left side that don't have matches from the right
| join kind=leftanti (DnsEvents
    | where TimeGenerated between(ago(starttime)..ago(endtime))
    | where Name has "in-addr.arpa"
    | summarize dcount(Name) by ClientIP, bin(TimeGenerated, 1d)
    | where dcount_Name > threshold
    | project ClientIP , dcount_Name
) on ClientIP

status: Available
queryFrequency: 1d
description: |
  'Identifies clients with a high reverse DNS counts that could be carrying out reconnaissance or discovery activity.
  Alerts are generated if the IP performing such reverse DNS lookups was not seen doing so in the preceding 7-day period.'  
severity: Medium
version: 1.0.2
relevantTechniques:
- T1046
name: Rare client observed with high reverse DNS lookup count
triggerThreshold: 0
kind: Scheduled
query: |
  let starttime = 8d;
  let endtime = 1d;
  let threshold = 10;
  DnsEvents
  | where TimeGenerated > ago(endtime)
  | where Name has "in-addr.arpa"
  | summarize StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated), dcount(Name), ReverseDNSLookup_List = make_set(Name,100) by ClientIP
  | where dcount_Name > threshold
  | project StartTimeUtc, EndTimeUtc, ClientIP , dcount_Name, ReverseDNSLookup_List
  // Filter out previously seen IPs
  // Returns all the records from the left side that don't have matches from the right
  | join kind=leftanti (DnsEvents
      | where TimeGenerated between(ago(starttime)..ago(endtime))
      | where Name has "in-addr.arpa"
      | summarize dcount(Name) by ClientIP, bin(TimeGenerated, 1d)
      | where dcount_Name > threshold
      | project ClientIP , dcount_Name
  ) on ClientIP  
triggerOperator: gt
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Windows Server DNS/Analytic Rules/DNS_HighReverseDNSCount_detection.yaml
requiredDataConnectors:
- connectorId: DNS
  dataTypes:
  - DnsEvents
tactics:
- Discovery
id: 15ae38a2-2e29-48f7-883f-863fb25a5a06
queryPeriod: 8d
entityMappings:
- fieldMappings:
  - columnName: ClientIP
    identifier: Address
  entityType: IP

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "workspace": {
      "type": "String"
    }
  },
  "resources": [
    {
      "apiVersion": "2024-01-01-preview",
      "id": "[concat(resourceId('Microsoft.OperationalInsights/workspaces/providers', parameters('workspace'), 'Microsoft.SecurityInsights'),'/alertRules/15ae38a2-2e29-48f7-883f-863fb25a5a06')]",
      "kind": "Scheduled",
      "name": "[concat(parameters('workspace'),'/Microsoft.SecurityInsights/15ae38a2-2e29-48f7-883f-863fb25a5a06')]",
      "properties": {
        "alertRuleTemplateName": "15ae38a2-2e29-48f7-883f-863fb25a5a06",
        "customDetails": null,
        "description": "'Identifies clients with a high reverse DNS counts that could be carrying out reconnaissance or discovery activity.\nAlerts are generated if the IP performing such reverse DNS lookups was not seen doing so in the preceding 7-day period.'\n",
        "displayName": "Rare client observed with high reverse DNS lookup count",
        "enabled": true,
        "entityMappings": [
          {
            "entityType": "IP",
            "fieldMappings": [
              {
                "columnName": "ClientIP",
                "identifier": "Address"
              }
            ]
          }
        ],
        "OriginalUri": "https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Windows Server DNS/Analytic Rules/DNS_HighReverseDNSCount_detection.yaml",
        "query": "let starttime = 8d;\nlet endtime = 1d;\nlet threshold = 10;\nDnsEvents\n| where TimeGenerated > ago(endtime)\n| where Name has \"in-addr.arpa\"\n| summarize StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated), dcount(Name), ReverseDNSLookup_List = make_set(Name,100) by ClientIP\n| where dcount_Name > threshold\n| project StartTimeUtc, EndTimeUtc, ClientIP , dcount_Name, ReverseDNSLookup_List\n// Filter out previously seen IPs\n// Returns all the records from the left side that don't have matches from the right\n| join kind=leftanti (DnsEvents\n    | where TimeGenerated between(ago(starttime)..ago(endtime))\n    | where Name has \"in-addr.arpa\"\n    | summarize dcount(Name) by ClientIP, bin(TimeGenerated, 1d)\n    | where dcount_Name > threshold\n    | project ClientIP , dcount_Name\n) on ClientIP\n",
        "queryFrequency": "P1D",
        "queryPeriod": "P8D",
        "severity": "Medium",
        "status": "Available",
        "subTechniques": [],
        "suppressionDuration": "PT1H",
        "suppressionEnabled": false,
        "tactics": [
          "Discovery"
        ],
        "techniques": [
          "T1046"
        ],
        "templateVersion": "1.0.2",
        "triggerOperator": "GreaterThan",
        "triggerThreshold": 0
      },
      "type": "Microsoft.OperationalInsights/workspaces/providers/alertRules"
    }
  ]
}