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Private endpoint resolves wrong with custom DNS

Problem

vm-spoke1 can't resolve stlogs01 to its private endpoint at 10.0.3.4.

Your job
  • Make stlogs01.blob.core.windows.net resolve to 10.0.3.4 from vm-spoke1
  • Keep vnet-spoke1 using the company DNS server
Background

vnet-spoke1 uses the company DNS server in the hub instead of Azure DNS. That server, dns-hub, was moved to 10.0.4.4 last week. It already forwards privatelink.blob.core.windows.net to Azure DNS. vm-spoke1 can't resolve stlogs01.blob.core.windows.net to its private endpoint at 10.0.3.4.

How to work it
  1. 1See it fail

    Prove the problem yourself before you change anything. Reports are often wrong.

    VM commands forVMs
    1. ssh vm-spoke1
      What's this?

      Opens a shell on vm-spoke1, so the next commands run on that VM. In this lab it's a shortcut. In real Azure you'd connect through something like Azure Bastion, or SSH to an address you can reach.

    2. nslookup stlogs01.blob.core.windows.net
      What's this?

      Asks the VM's DNS server what IP address a name points to. Run it on the VM that has the problem, because each VM can get a different answer.

    3. nc -zv stlogs01.blob.core.windows.net 443
      What's this?

      netcat, a standard Linux tool. Tries to open a TCP connection and reports whether it worked.

      • -z only check whether the port answers. Send no data
      • -v print the result in words
      • stlogs01.blob.core.windows.net where to connect
      • 443 the port to try

    Ticks off when the test runs on vm-spoke1.

  2. 2Find where it stops
  3. 3Name the cause
  4. 4Fix it and test again
  5. 5Explain it
Sources on Microsoft Learn (3)
Troubleshoot private endpoint DNS resolutionPrivate endpoint DNS integrationChange DNS servers of a virtual network
0/2 changes
Network map
?
  • Resource group, the outer dashed frame
  • Virtual network, with its address space top right
  • Subnet, with range and usable IPs (5 reserved)
  • Network security group. A small shield on a VM's NIC means the NIC has one too
  • Route table on the subnet
  • NICVM Appliance Private endpoint
  • Peering Connected Initiated or access off
  • Goal working Goal broken, X marks where it stops
  • Private DNS zone, dotted lines go to linked VNets
  • Orange pulse: what your last command changed. Drag to pan, Ctrl and scroll to zoom.
Resource grouprg-labvnet-spoke1: tap to inspectVirtual networkvnet-spoke110.1.0.0/16subnet-app: tap to inspectsubnet-app10.1.1.0/24 · 251 usablevm-spoke1 10.1.1.4: tap to inspectNICvm-spoke110.1.1.4vnet-hub: tap to inspectVirtual networkvnet-hub10.0.0.0/16subnet-pe: tap to inspectsubnet-pe10.0.3.0/24 · 251 usablepe-stlogs01 10.0.3.4: tap to inspectpe-stlogs0110.0.3.4subnet-dns: tap to inspectsubnet-dns10.0.4.0/24 · 251 usabledns-hub 10.0.4.4: tap to inspectNICdns-hub10.0.4.4Connected: tap to inspectpeeredprivatelink.blob.core.windows.net: tap to inspectDNSPrivate DNS zone · privatelink.blob.core.windows.net · 1 link · stlogs01 → 10.0.3.4stlogs01.blob.core.windows.net?:443 ✗
  • vm-spoke1 → stlogs01.blob.core.windows.net:443should work: not working
Goals 0/3
Cloud Shell · bashVMsrg-lab · eastus2
cloudshell:~$