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Manage packet captures for virtual machines with Azure Network Watcher using the Azure CLI

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The Network Watcher packet capture tool allows you to create capture sessions to record network traffic to and from an Azure virtual machine (VM). Filters are provided for the capture session to ensure you capture only the traffic you want. Packet capture helps in diagnosing network anomalies both reactively and proactively. Its applications extend beyond anomaly detection to include gathering network statistics, acquiring insights into network intrusions, debugging client-server communication, and addressing various other networking challenges. Network Watcher packet capture enables you to initiate packet captures remotely, alleviating the need for manual execution on a specific virtual machine.

In this article, you learn how to remotely configure, start, stop, download, and delete a virtual machine packet capture using Azure PowerShell. To learn how to manage packet captures using the Azure portal or Azure CLI, see Manage packet captures for virtual machines using the Azure portal or Manage packet captures for virtual machines using PowerShell.

Prerequisites

  • An Azure account with an active subscription. Create an account for free.

  • Azure Cloud Shell or Azure CLI.

    The steps in this article run the Azure CLI commands interactively in Azure Cloud Shell. To run the commands in the Cloud Shell, select Open Cloud Shell at the upper-right corner of a code block. Select Copy to copy the code, and paste it into Cloud Shell to run it. You can also run the Cloud Shell from within the Azure portal.

    You can also install Azure CLI locally to run the commands. If you run Azure CLI locally, sign in to Azure using the az login command.

  • A virtual machine with the following outbound TCP connectivity:

    • to the storage account over port 443
    • to 169.254.169.254 over port 80
    • to 168.63.129.16 over port 8037

Note

  • Azure creates a Network Watcher instance in the the virtual machine's region if Network Watcher wasn't enabled for that region. For more information, see Enable or disable Azure Network Watcher.
  • Network Watcher packet capture requires Network Watcher agent VM extension to be installed on the target virtual machine. For more information, see Install Network Watcher agent.
  • The last two IP addresses and ports listed in the Prerequisites are common across all Network Watcher tools that use the Network Watcher agent and might occasionally change.

If a network security group is associated to the network interface, or subnet that the network interface is in, ensure that rules exist to allow outbound connectivity over the previous ports. Similarly, ensure outbound connectivity over the previous ports when adding user-defined routes to your network.

Install Network Watcher agent

Step 1

Run the az vm extension set command to install the packet capture agent on the guest virtual machine.

For Windows virtual machines:

az vm extension set --resource-group resourceGroupName --vm-name virtualMachineName --publisher Microsoft.Azure.NetworkWatcher --name NetworkWatcherAgentWindows --version 1.4

For Linux virtual machines:

az vm extension set --resource-group resourceGroupName --vm-name virtualMachineName --publisher Microsoft.Azure.NetworkWatcher --name NetworkWatcherAgentLinux --version 1.4

Step 2

To ensure that the agent is installed, run the vm extension show command and pass it the resource group and virtual machine name. Check the resulting list to ensure the agent is installed.

For Windows virtual machines:

az vm extension show --resource-group resourceGroupName --vm-name virtualMachineName --name NetworkWatcherAgentWindows

For Linux virtual machines:

az vm extension show --resource-group resourceGroupName --vm-name virtualMachineName --name AzureNetworkWatcherExtension

The following sample is an example of the response from running az vm extension show

{
  "autoUpgradeMinorVersion": true,
  "forceUpdateTag": null,
  "id": "/subscriptions/aaaa0a0a-bb1b-cc2c-dd3d-eeeeee4e4e4e/resourceGroups/{resourceGroupName}/providers/Microsoft.Compute/virtualMachines/{vmName}/extensions/NetworkWatcherAgentWindows",
  "instanceView": null,
  "location": "westcentralus",
  "name": "NetworkWatcherAgentWindows",
  "protectedSettings": null,
  "provisioningState": "Succeeded",
  "publisher": "Microsoft.Azure.NetworkWatcher",
  "resourceGroup": "{resourceGroupName}",
  "settings": null,
  "tags": null,
  "type": "Microsoft.Compute/virtualMachines/extensions",
  "typeHandlerVersion": "1.4",
  "virtualMachineExtensionType": "NetworkWatcherAgentWindows"
}

Start a packet capture

Once the preceding steps are complete, the packet capture agent is installed on the virtual machine.

Step 1

Retrieve a storage account. This storage account is used to store the packet capture file.

az storage account list

Step 2

At this point, you are ready to create a packet capture. First, let's examine the parameters you may want to configure. Filters are one such parameter that can be used to limit the data that is stored by the packet capture. The following example sets up a packet capture with several filters. The first three filters collect outgoing TCP traffic only from local IP 10.0.0.3 to destination ports 20, 80 and 443. The last filter collects only UDP traffic.

az network watcher packet-capture create --resource-group {resourceGroupName} --vm {vmName} --name packetCaptureName --storage-account {storageAccountName} --filters "[{\"protocol\":\"TCP\", \"remoteIPAddress\":\"1.1.1.1-255.255.255.255\",\"localIPAddress\":\"10.0.0.3\", \"remotePort\":\"20\"},{\"protocol\":\"TCP\", \"remoteIPAddress\":\"1.1.1.1-255.255.255.255\",\"localIPAddress\":\"10.0.0.3\", \"remotePort\":\"80\"},{\"protocol\":\"TCP\", \"remoteIPAddress\":\"1.1.1.1-255.255.255.255\",\"localIPAddress\":\"10.0.0.3\", \"remotePort\":\"443\"},{\"protocol\":\"UDP\"}]"

The following example is the expected output from running the az network watcher packet-capture create command.

{
  "bytesToCapturePerPacket": 0,
  "etag": "W/\"b8cf3528-2e14-45cb-a7f3-5712ffb687ac\"",
  "filters": [
    {
      "localIpAddress": "10.0.0.3",
      "localPort": "",
      "protocol": "TCP",
      "remoteIpAddress": "1.1.1.1-255.255.255.255",
      "remotePort": "20"
    },
    {
      "localIpAddress": "10.0.0.3",
      "localPort": "",
      "protocol": "TCP",
      "remoteIpAddress": "1.1.1.1-255.255.255.255",
      "remotePort": "80"
    },
    {
      "localIpAddress": "10.0.0.3",
      "localPort": "",
      "protocol": "TCP",
      "remoteIpAddress": "1.1.1.1-255.255.255.255",
      "remotePort": "443"
    },
    {
      "localIpAddress": "",
      "localPort": "",
      "protocol": "UDP",
      "remoteIpAddress": "",
      "remotePort": ""
    }
  ],
  "id": "/subscriptions/aaaa0a0a-bb1b-cc2c-dd3d-eeeeee4e4e4e/resourceGroups/NetworkWatcherRG/providers/Microsoft.Network/networkWatchers/NetworkWatcher_westcentralus/packetCaptures/packetCaptureName",
  "name": "packetCaptureName",
  "provisioningState": "Succeeded",
  "resourceGroup": "NetworkWatcherRG",
  "storageLocation": {
    "filePath": null,
    "storageId": "/subscriptions/aaaa0a0a-bb1b-cc2c-dd3d-eeeeee4e4e4e/resourceGroups/{resourceGroupName}/providers/Microsoft.Storage/storageAccounts/gwteststorage123abc",
    "storagePath": "https://gwteststorage123abc.blob.core.windows.net/network-watcher-logs/subscriptions/aaaa0a0a-bb1b-cc2c-dd3d-eeeeee4e4e4e/resourcegroups/{resourceGroupName}/providers/microsoft.compute/virtualmachines/{vmName}/2017/05/25/packetcapture_16_22_34_630.cap"
  },
  "target": "/subscriptions/aaaa0a0a-bb1b-cc2c-dd3d-eeeeee4e4e4e/resourceGroups/{resourceGroupName}/providers/Microsoft.Compute/virtualMachines/{vmName}",
  "timeLimitInSeconds": 18000,
  "totalBytesPerSession": 1073741824
}

Get a packet capture

Running the az network watcher packet-capture show-status command, retrieves the status of a currently running, or completed packet capture.

az network watcher packet-capture show-status --name packetCaptureName --location {networkWatcherLocation}

The following example is the output from the az network watcher packet-capture show-status command. The following example is when the capture is Stopped, with a StopReason of TimeExceeded.

{
  "additionalProperties": {
    "status": "Succeeded"
  },
  "captureStartTime": "2016-12-06T17:20:01.5671279Z",
  "id": "/subscriptions/aaaa0a0a-bb1b-cc2c-dd3d-eeeeee4e4e4e/resourceGroups/NetworkWatcherRG/providers/Microsoft.Network/networkWatchers/NetworkWatcher_westcentralus/packetCaptures/packetCaptureName",
  "name": "packetCaptureName",
  "packetCaptureError": [],
  "packetCaptureStatus": "Stopped",
  "stopReason": "TimeExceeded"
}

Stop a packet capture

By running the az network watcher packet-capture stop command, if a capture session is in progress it is stopped.

az network watcher packet-capture stop --name packetCaptureName --location westcentralus

Note

The command returns no response when ran on a currently running capture session or an existing session that has already stopped.

Delete a packet capture

az network watcher packet-capture delete --name packetCaptureName --location westcentralus

Note

Deleting a packet capture does not delete the file in the storage account.

Download a packet capture

Once your packet capture session has completed, the capture file can be uploaded to blob storage or to a local file on the VM. The storage location of the packet capture is defined at creation of the session. A convenient tool to access these capture files saved to a storage account is Microsoft Azure Storage Explorer, which can be downloaded here: https://storageexplorer.com/

If a storage account is specified, packet capture files are saved to a storage account at the following location:

https://{storageAccountName}.blob.core.windows.net/network-watcher-logs/subscriptions/{subscriptionId}/resourcegroups/{storageAccountResourceGroup}/providers/microsoft.compute/virtualmachines/{VMName}/{year}/{month}/{day}/packetCapture_{creationTime}.cap

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