Custom DNS servers are not showing up in OS

Apurva Pathak 635

Hi folks,

We've setup ASR in one of our Azure hosted RHEL 7.9 VM. Today, when we did a failover from primary to secondary region, after failover completed, we set our custom DNS servers (x.x.1.4/5) at NIC of the secondary VM, however, newly applied IPs were not showing in the 'resolve.conf' file of the OS. The file was still referring to the IPs applied at the NIC of the primary NIC (x.x.0.4/5).

We tried rebooting the server many times but that didn't change anything. I found a similar issue here which was got resolved by rebooting the VM with Az CLI command, but it didn't work for me.

To make the things work we manually added the expected IPs to the 'resolve.conf' file of the secondary server but could someone please help me understand why this is happening and how can we fix it so that we don't have to alter the resolv.conf file manually.

PFB snips for reference:

Content of resolv.conf file where it can be seen that it is referring to the primary NIC's custom DNS servers (x.x.0.4/5).

{78C463F8-D7C7-420A-8A78-F20C470BF22E}

Custom DNS servers applied at NIC of the secondary VM (x.x.1.4/5).

User's image

Thanks a lot in advance!

SadiqhAhmed-MSFT 47,841 Reputation points Microsoft Employee

2024-11-25T17:39:00.1766667+00:00

@Apurva Pathak Greetings!

The main issue is that after the failover, the network interface on the secondary region may not automatically update the DNS settings in /etc/resolv.conf due to caching or NetworkManager not detecting the change. The easiest fix is to restart the NetworkManager service, force DHCP to rebind, or use static DNS settings. If you still face issues, manually applying the DNS settings in /etc/resolv.conf is a workaround to be used.

Hope this helps!
Apurva Pathak 635 Reputation points

2024-11-25T17:57:31.3233333+00:00

Hi @SadiqhAhmed-MSFT ,

Thanks for coming back!

However, as specified above, I had tried all of these. I tried restarting the service at least 5- 6 times. I rebooted the server 2- 3 times. I even, readded the custom DNS servers to the NIC, 3 times, but nothing of them worked. I faced the similar incident while failing back the server. I had to update the resolv.conf file manually.

Please do let me know if you need any further details

Thanks!
SadiqhAhmed-MSFT 47,841 Reputation points Microsoft Employee

2024-11-26T06:20:27.43+00:00

@Apurva Pathak Thanks for confirming that the steps suggested were already tried and it did not help resolve the issue. I checked on this with our internal resources and can confirm that the issue needs further investigation. Recommend you reach out to Azure technical support team by raising a support request. The ticket enables you to work closely with the support engineers and get a quick resolution to your issue.

Here is the link - https://docs.microsoft.com/en-us/azure/azure-portal/supportability/how-to-create-azure-support-request to create support case.

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Custom DNS servers are not showing up in OS

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