DNS Registration failing for workstations running in Azure?

Question

Hi,

We have:

• DNS servers running in Azure. Scavenging is enabled with default settings 7/7
• AVD workstations running Windows 11 in Azure. These machines pick up their IP address via DHCP from Azure

According to the docs Windows is supposed to re-register DNS every 24 hours, however we are finding that DNS entries are not being refreshed, and scavenging eventually removes them from DNS.

If we run ipconfig /registerdns on the machine then it re-registers correctly and prevents any issue for another few weeks.

The fault appears to be intermittent, suggesting that some machines do refresh correctly.

Has anyone else seen this? Any suggestions? ipconfig output below.

Thanks,
Simon.

C:\>ipconfig /all

Windows IP Configuration
   Host Name . . . . . . . . . . . . : MYSERVERNAME
   Primary Dns Suffix  . . . . . . . : mydomain.local
   Node Type . . . . . . . . . . . . : Hybrid
   IP Routing Enabled. . . . . . . . : No
   WINS Proxy Enabled. . . . . . . . : No
   DNS Suffix Search List. . . . . . : mydomain.local
                                       reddog.microsoft.com
Ethernet adapter Ethernet 2:
   Connection-specific DNS Suffix  . : reddog.microsoft.com
   Description . . . . . . . . . . . : Microsoft Hyper-V Network Adapter #2
   Physical Address. . . . . . . . . : 00-0D-3A-D1-57-4F
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes
   IPv4 Address. . . . . . . . . . . : 10.146.8.6(Preferred)
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Lease Obtained. . . . . . . . . . : Wednesday, 7 August 2024 4:20:28 PM
   Lease Expires . . . . . . . . . . : Saturday, 13 September 2160 11:42:55 PM
   Default Gateway . . . . . . . . . : 10.146.8.1
   DHCP Server . . . . . . . . . . . : 168.63.129.16
   DNS Servers . . . . . . . . . . . : 10.144.8.40
                                       10.160.8.40
   NetBIOS over Tcpip. . . . . . . . : Enabled


PS C:\> Get-DnsServerResourceRecord -ZoneName mydomain.local -ComputerName dnservername -Name MYSERVERNAME | fl

DistinguishedName : DC=MYSERVERNAME,DC=mydomain.local,cn=MicrosoftDNS,DC=DomainDnsZones,DC=mydomain,DC=local
HostName          : MYSERVERNAME
RecordType        : A
Type              : 1
RecordClass       : IN
TimeToLive        : 00:20:00
Timestamp         : 24/07/2024 12:00:00 AM
RecordData        : 10.146.8.6

Answer 1

@Simon Gadsby ,

Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.

Running DHCP Servers on Azure is more of a recent addition.

• See : Can I deploy a DHCP server in a virtual network?
• DHCP Server in Azure was previously marked not feasible since the traffic to port UDP/67 was rate limited in Azure. However, recent platform updates have removed the rate limitation, enabling this capability.
• Also, You can't use DHCP via Unicast (source port UDP/68, destination port UDP/67). UDP source port 65330 is reserved for the host.
  • Can you please confirm if the above condition is met?

From your verbatim,

• I see the DHCP server is listed as "168.63.129.16"
  • This is Azure's Wireserver IP which enables the VM to obtain a dynamic IP address from the DHCP service in Azure.
• While you may run the DHCP VMs , did you also update the VMs to use these VMs for DHCP?
  • See : To enable DHCP or change other TCP/IP settings
  • Can you confirm this has been updated?

Cheers,

Kapil

Answer 2

We have seen this in the past same as yourself.

we noticed any server that reboots within the 2 weeks doesn't have this issue and I believe the error is due to DHCP lease being set to 100+ years. When dhcp leases renew, it kicks off a re registration of the DNS record, but these DHCP records never renew because the lease period is so long and scavenging does exactly what you're telling it to do, cleanup records older then 14 days.

our workaround was to setup a script to do the following

Backup DNSZone

Backup local Hosts file

Get Server1 IP address

add IP and name to local hosts file

delete record of a server from all DNS servers (if not timestamp may not change)

remotely run ipconfig /registerdns locally on each of the servers (remote server can connect due to host entry locally)

we have ~1100 servers in domain, we do ~ 25% each day.