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Manage packet captures in Virtual machine scale sets with Azure Network Watcher using the portal

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Network Watcher packet capture allows you to create capture sessions to track traffic to and from a virtual machine scale set instance/(s). Filters are provided for the capture session to ensure you capture only the traffic you want. Packet capture helps to diagnose network anomalies, both reactively, and proactively. Other uses include gathering network statistics, gaining information on network intrusions, to debug client-server communication, and much more. Being able to remotely trigger packet captures, eases the burden of running a packet capture manually on a desired virtual machine scale set instance/(s), which saves valuable time.

In this article, you learn to start, stop, download, and delete a packet capture.

Before you begin

Packet capture requires the following outbound TCP connectivity:

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

Note

The ports mentioned in the latter two cases above are common across all Network Watcher features that involve the Network Watcher extension 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 that allow the previous ports. Similarly, adding user-defined traffic routes to your network may prevent connectivity to the above mentioned IPs and ports. Ensure they're reachable.

Start a packet capture

  1. In your browser, navigate to the Azure portal and select All services, and then select Network Watcher in the Networking section.

  2. Select Packet capture under Network diagnostic tools. Any existing packet captures are listed, regardless of their status.

  3. Select Add to create a packet capture. You can select values for the following properties:

    • Subscription: The subscription that the virtual machine scale set you want to create the packet capture for is in.

    • Resource group: The resource group of the virtual machine scale set.

    • Target Type: Choose Virtual Machine Scale Set from the drop-down.

    • Target Instance: The specific instance/(s) where you want to run captures on. You can choose Select all, if you wish to run captures on all the instances.

    • Packet capture name: Name gets autopoulated and can be overwritten as per the user's convenience

    • Storage account or file: Select Storage account, File, or both. Recommended option is to choose storage account option. If you select File, the capture is written to a path within the virtual machine instance.

    • Storage accounts: Select an existing storage account, if you selected Storage account. This option is only available if you selected Storage.

    • Local file path: The local path on the virtual machine where the packet capture will be saved (valid only when File is selected). The path must be a valid path. If you're using a Linux virtual machine scale set, the path must start with /var/captures.

      Note

      Premium storage accounts are currently not supported for storing packet captures.

    • Maximum bytes per packet: The number of bytes from each packet that are captured. If left blank, all bytes are captured.

    • Maximum bytes per session: The total number of bytes that are captured. By default the value is 1.07 GB

    • Time limit (seconds): The time limit before the packet capture is stopped. The default is 18,000 seconds(5 hours).

    • Filtering (Optional). Select + Add filter

      • Protocol: The protocol to filter for the packet capture. The available values are TCP, UDP, and Any.
      • Local IP address: Filters the packet capture for packets where the local IP address matches this value.
      • Local port: Filters the packet capture for packets where the local port matches this value.
      • Remote IP address: Filters the packet capture for packets where the remote IP address matches this value.
      • Remote port: Filters the packet capture for packets where the remote port matches this value.

      Note

      Port and IP address values can be a single value, range of values, or a range, such as 80-1024, for port. You can define as many filters as you need.

  4. Select OK.

After the time limit set on the packet capture has expired, the packet capture is stopped, and can be reviewed. You can also manually stop a packet capture session.

Note

The portal automatically:

  • Creates a network watcher in the same region as the region the virtual machine scale set you selected exists in, if the region doesn't already have a network watcher.
  • Adds the AzureNetworkWatcherExtension Linux or Windows to the virtual machine scale set, if it's not already installed.

Delete a packet capture

  1. In the packet capture view, select ... on the right-side of the packet capture, or right-click an existing packet capture, and select Delete.
  2. You're asked to confirm you want to delete the packet capture. Select Yes.

Note

Deleting a packet capture does not delete the capture file in the storage account or on the virtual machine scale set instance/(s).

Stop a packet capture

In the packet capture view, select ... on the right-side of the packet capture, or right-click an existing packet capture, and select Stop.

Download a packet capture

Once your packet capture session has completed, the capture file is uploaded to blob storage or to a local file on the virtual machine scale set instance. The storage location of the packet capture is defined during creation of the packet capture. A convenient tool to access capture files saved to a storage account is Microsoft Azure Storage Explorer, which you can download.

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

If you selected File when you created the capture, you can view or download the file from the path you configured on the virtual machine scale set instance.

Next steps