Azure Function App VNet Integration Custom DNS

Question

We are running an Azure Function App with VNet integration. The VNet that we're integrating with has custom DNS servers configured. When our application runs or I run nslookup from SCM/Kudu I'm not able to resolve the address of a zone configured in our DNS servers.

If I specify the custom DNS server in the nslookup command, then the resolution works correctly. If I don't specify the DNS server in the nslookup command, then it uses the default 168.63.129.16, and the address does not resolve

Why isn't the Function App using the custom DNS servers?

Answer 1

When you are integrating an Azure function app by default virtual network uses DNS server only.You can use custom DNS server like below:

I created function app with vnet integeration like below

You can make use of configuring DNS setting for vnet integeration:

In your virtual network -> under setting, DNS server -> select Custom and enter the IP addresses of your custom DNS servers and Save.

The Function App should use the specific DNS servers that have been set up in the VNet once you configured the DNS settings in the networking setup of the Function App

Note that The DNS settings may not reach the Function App right away. It takes few minutes, if you're still having trouble resolving the address, you might need to restart the Function App in order for it to take up the updated DNS settings

You can make use of following commands in the Kudu DebugConsole https://.scm.azurewebsites.net/DebugConsole to see if the hostname is returning the right IP address. Use nameresolver domain IPofDNSServer and Nameresolver.exe, an alternative to nslookup, if it fails.

Ensure you have your DNS cache is cleared by using this ipconfig /flushdns your debug console

Additionally, you can create a DNS zone and add a record set to authorise your server:

References:

Azure Functions networking options | Microsoft Learn

Integrate Azure DNS with your Azure resources - Azure DNS | Microsoft Learn