Diagnose a virtual machine network routing problem using PowerShell

Bài viết
10/29/2024

In this article, you learn how to use Azure Network Watcher next hop tool to troubleshoot and diagnose a virtual machine (VM) routing problem that's preventing it from correctly communicating with other resources.

Prerequisites

An Azure account with an active subscription. Create an account for free.
Azure Cloud Shell or Azure PowerShell.

The steps in this article run the Azure PowerShell cmdlets interactively in Azure Cloud Shell. To run the cmdlets in the Cloud Shell, select Open Cloud Shell at the upper-right corner of a code block. Select Copy to copy the code and then 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 PowerShell locally to run the cmdlets. Run Get-Module -ListAvailable Az to find the installed version. If you run PowerShell locally, sign in to Azure using the Connect-AzAccount cmdlet.

Create a virtual machine

Before you can create a VM, you must create a resource group to contain the VM. Create a resource group with New-AzResourceGroup. The following example creates a resource group named myResourceGroup in the eastus location.

New-AzResourceGroup -Name myResourceGroup -Location EastUS

Create the VM with New-AzVM. When running this step, you are prompted for credentials. The values that you enter are configured as the user name and password for the VM.

$vM = New-AzVm `
    -ResourceGroupName "myResourceGroup" `
    -Name "myVm" `
    -Location "East US"

The VM takes a few minutes to create. Don't continue with remaining steps until the VM is created and PowerShell returns output.

Test network communication

To test network communication with Network Watcher, you must first enable a network watcher in the region the VM that you want to test is in, and then use Network Watcher's next hop capability to test communication.

Enable network watcher

If you already have a network watcher enabled in the East US region, use Get-AzNetworkWatcher to retrieve the network watcher. The following example retrieves an existing network watcher named NetworkWatcher_eastus that is in the NetworkWatcherRG resource group:

$networkWatcher = Get-AzNetworkWatcher `
  -Name NetworkWatcher_eastus `
  -ResourceGroupName NetworkWatcherRG

If you don't already have a network watcher enabled in the East US region, use New-AzNetworkWatcher to create a network watcher in the East US region:

$networkWatcher = New-AzNetworkWatcher `
  -Name "NetworkWatcher_eastus" `
  -ResourceGroupName "NetworkWatcherRG" `
  -Location "East US"

Use next hop

Azure automatically creates routes to default destinations. You may create custom routes that override the default routes. Sometimes, custom routes can cause communication to fail. To test routing from a VM, use the Get-AzNetworkWatcherNextHop command to determine the next routing hop when traffic is destined for a specific address.

Test outbound communication from the VM to one of the IP addresses for www.bing.com:

Get-AzNetworkWatcherNextHop `
  -NetworkWatcher $networkWatcher `
  -TargetVirtualMachineId $VM.Id `
  -SourceIPAddress 192.168.1.4 `
  -DestinationIPAddress 13.107.21.200

After a few seconds, the output informs you that the NextHopType is Internet, and that the RouteTableId is System Route. This result lets you know that there is a valid route to the destination.

Test outbound communication from the VM to 172.31.0.100:

Get-AzNetworkWatcherNextHop `
  -NetworkWatcher $networkWatcher `
  -TargetVirtualMachineId $VM.Id `
  -SourceIPAddress 192.168.1.4 `
  -DestinationIPAddress 172.31.0.100

The output returned informs you that None is the NextHopType, and that the RouteTableId is also System Route. This result lets you know that, while there is a valid system route to the destination, there is no next hop to route the traffic to the destination.

View details of a route

To analyze routing further, review the effective routes for the network interface with the Get-AzEffectiveRouteTable command:

Get-AzEffectiveRouteTable `
  -NetworkInterfaceName myVm `
  -ResourceGroupName myResourceGroup |
  Format-table

Output that includes the following text is returned:

Name State  Source  AddressPrefix           NextHopType NextHopIpAddress
---- -----  ------  -------------           ----------- ----------------
     Active Default {192.168.0.0/16}        VnetLocal   {}              
     Active Default {0.0.0.0/0}             Internet    {}              
     Active Default {10.0.0.0/8}            None        {}              
     Active Default {100.64.0.0/10}         None        {}              
     Active Default {172.16.0.0/12}         None        {}

As you can see in the previous output, the route with the AddressPrefix of 0.0.0.0/0 routes all traffic not destined for addresses within other route's address prefixes with a next hop of Internet. As you can also see in the output, though there is a default route to the 172.16.0.0/12 prefix, which includes the 172.31.0.100 address, the nextHopType is None. Azure creates a default route to 172.16.0.0/12, but doesn't specify a next hop type until there is a reason to. If, for example, you added the 172.16.0.0/12 address range to the address space of the virtual network, Azure changes the nextHopType to Virtual network for the route. A check would then show Virtual network as the nextHopType.

Clean up resources

When no longer needed, you can use Remove-AzResourceGroup to remove the resource group and all of the resources it contains:

Remove-AzResourceGroup -Name myResourceGroup -Force

Next steps

In this article, you created a VM and diagnosed network routing from the VM. You learned that Azure creates several default routes and tested routing to two different destinations. Learn more about routing in Azure and how to create custom routes.

For outbound VM connections, you can also determine the latency and allowed and denied network traffic between the VM and an endpoint using Network Watcher's connection troubleshoot capability. You can monitor communication between a VM and an endpoint, such as an IP address or URL over time using the Network Watcher connection monitor capability. For more information, see Monitor a network connection.

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