Azure Functions (Node.js 22 & 24) fails SRV DNS resolution for Azure DocumentDB(with MongoDB compatibility) unless dns.setServers() is used

Environment

• Service: Azure Functions (JavaScript/TypeScript, Node.js v4 programming model)
• Package: @azure/functions v4
• Runtime: Node.js 22 and 24
• Hosting: Azure Function App (default configuration)
• Networking:
  • No VNet integration
  • No Private Endpoint
  • No custom DNS configured
• Database: Azure Cosmos DB for MongoDB vCore
• Authentication tested:
  • Microsoft Entra ID (RBAC) using MONGODB-OIDC with DefaultAzureCredential
  • Native MongoDB (DocumentDB/Cosmos DB) username and password authentication

Expected behavior:

The Function App should successfully connect using the official SRV connection string:

mongodb+srv://<clusterName>.global.mongocluster.cosmos.azure.com/

This should work regardless of whether authentication is performed using:

• Microsoft Entra ID (RBAC / OIDC), or
• Native MongoDB credentials,

without requiring any custom DNS configuration.

Actual behavior:

On both Node.js 22 and Node.js 24, the MongoDB Node.js driver fails during SRV lookup before authentication begins.

Error:

{ 
	"error": { 
		"code": "ECONNREFUSED", 
		"syscall": "querySrv", 
		"hostname": "_mongodb._tcp.<clusterName>.global.mongocluster.cosmos.azure.com" 
	} 
}

The Function App starts successfully and the HTTP trigger is reachable. The failure occurs only during DNS SRV resolution.

The exact same behavior is observed when using:

• Microsoft Entra ID (RBAC / MONGODB-OIDC)
• Native MongoDB username/password authentication

Since both authentication mechanisms fail with the same DNS error, the issue appears to occur before authentication is attempted.

Workaround

Adding the following at the beginning of the function resolves the issue immediately:

import dns from "node:dns";
dns.setServers(['8.8.8.8', '8.8.4.4']);

After overriding the DNS servers:

• SRV lookup succeeds.
• The same mongodb+srv:// connection string works.
• Connections succeed using both:
  • Microsoft Entra ID (RBAC / OIDC)
  • Native MongoDB credentials
• No other code or configuration changes are required.

Why this appears to be a platform/runtime issue ?

• The Function App uses the default Azure networking configuration.
• No custom DNS, VNet, or Private Endpoint is configured.
• The Cosmos DB endpoint and connection string are verified to be correct because they work immediately after changing only the DNS resolver.
• The failure occurs during querySrv, before any TCP connection or authentication attempt.
• The issue is reproducible with both Node.js 22 and Node.js 24.
• The issue is reproducible with both Microsoft Entra ID (RBAC) and native MongoDB authentication.
• This suggests the default DNS resolver available in the Azure Functions/App Service environment may be refusing or failing SRV queries for *.global.mongocluster.cosmos.azure.com.

Additional Observation:

1. The same Function App code and the same mongodb+srv:// connection string work successfully when the function is run locally using Azure Functions Core Tools.
2. The connection succeeds locally with both:
   • Microsoft Entra ID (RBAC / MONGODB-OIDC)
   • Native MongoDB username/password authentication
3. No DNS override (dns.setServers()) is required when running locally.
4. The issue is only observed after deploying the Function App to Azure.
5. Since the application code, connection string, MongoDB driver, and Cosmos DB resource are identical between the local and hosted environments, the behavior appears to be specific to the DNS resolution path within the Azure Functions/App Service hosting environment.

Questions

1. Is this a known issue with Azure Functions running Node.js 22 or Node.js 24 when resolving SRV records for mongodb+srv connections?
2. Is there any known limitation or issue in the Azure Functions/App Service DNS infrastructure that could cause querySrv requests to return ECONNREFUSED?
3. Is using dns.setServers() considered a supported workaround, or is there a recommended platform-level configuration or runtime update?
4. Has the Cosmos DB for MongoDB vCore team published any guidance regarding DNS resolution or SRV lookups when connecting from Azure Functions using mongodb+srv?
5. Could Microsoft confirm whether this behavior is expected, a known regression, or a platform issue affecting DNS resolution within the Azure Functions runtime?

Any clarification or recommended long-term solution would be greatly appreciated.