.NET on macOS: DNS Resolution Fails for Azure CNAME, Works for A Record and System Tools | Azure AI Foundry + AgentsClient

Environment:

• MacBook Air (Apple Silicon, arm64)
• macOS 15.4.1 (Sonoma)
• .NET SDK 9.0.203, .NET Runtime 9.0.4 (osx-arm64)
• No VPN, firewall, or security software interfering
• DNS set to Google (8.8.8.8)
• Azure SDK client code (using DefaultAzureCredential)

Problem: When running a .NET app that connects to Azure’s API endpoint (eastus.api.azureml.ms), DNS resolution fails in .NET, but works in the terminal with dig and for other domains in .NET.

Sample output:

[DEBUG] .NET DNS test for eastus.api.azureml.ms
[DEBUG] .NET DNS resolution failed: nodename nor servname provided, or not known
[DEBUG] .NET DNS test for vienna-eastus-ip-ingress-nginx-02.eastus.cloudapp.azure.com
[DEBUG] .NET DNS resolved: 48.211.42.162
[DEBUG] .NET DNS test for www.google.com
[DEBUG] .NET DNS resolved: 142.250.72.100

• dig eastus.api.azureml.ms returns a CNAME chain ending in an A record, as expected.
• .NET can resolve the final A record and other domains, but not the original Azure CNAME.

Error from Azure SDK:

❌ Error occurred: Retry failed after 2 tries. Retry settings can be adjusted in ClientOptions.Retry or by configuring a custom retry policy in ClientOptions.RetryPolicy. (nodename nor servname provided, or not known (eastus.api.azureml.ms:443))
[DEBUG] Exception Type: System.AggregateException
[DEBUG] Inner Exception: nodename nor servname provided, or not known (eastus.api.azureml.ms:443)
[DEBUG] Inner Exception Type: Azure.RequestFailedException

Question:
Why does .NET on macOS fail to resolve the Azure CNAME (eastus.api.azureml.ms) when system tools and .NET can resolve the final A record and other domains?
What else can I try to get .NET DNS resolution working for this Azure endpoint?

Any help or insight would be greatly appreciated!

Code excerpt:

// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License.

#nullable disable

using Azure.Identity;
using Azure.Core;
namespace Azure.AI.Projects;

public class Sample_Agent
{
    static async Task Main()
    {
        Console.WriteLine($"Application started at: {DateTime.Now:yyyy-MM-dd HH:mm:ss}");
        Console.WriteLine("Initializing agent configuration...");
        
        // Use explicit connection string as requested
        var connectionString = "<project connection string I scrubbed, format: <HostName>;<AzureSubscriptionId>;<ResourceGroup>;<ProjectName>>";
        Console.WriteLine($"[DEBUG] Using connection string: {connectionString.Substring(0, Math.Min(connectionString.Length, 20))}... (masked)");
        Console.WriteLine($"[DEBUG] Machine Name: {Environment.MachineName}");
        Console.WriteLine($"[DEBUG] OS Version: {Environment.OSVersion}");
        Console.WriteLine($"[DEBUG] Current Directory: {Environment.CurrentDirectory}");
        try
        {
            var host = "eastus.api.azureml.ms";
            Console.WriteLine($"[DEBUG] .NET DNS test for {host}");
            var entry = System.Net.Dns.GetHostEntry(host);
            Console.WriteLine($"[DEBUG] .NET DNS resolved: {entry}");
        }
        catch (Exception ex)
        {
            Console.WriteLine($"[DEBUG] .NET DNS resolution failed: {ex.Message}");
        }
        var options = new AIProjectClientOptions();
        options.Retry.MaxRetries = 1;
        AgentsClient client = new AgentsClient(connectionString, new DefaultAzureCredential(), options);
        
        try 
        {
            Console.WriteLine("[DEBUG] Attempting to retrieve agent by ID");
            Response<Agent> agentResponse = await client.GetAgentAsync("<agent id I scrubbed>");
            Agent agent = agentResponse.Value;
            Console.WriteLine($"✓ Agent retrieved successfully (ID: {agent.Id})");

            // Verify agent in list
            Console.WriteLine("\nVerifying agent in list...");
            Response<PageableList<Agent>> agentListResponse = await client.GetAgentsAsync();
            Console.WriteLine($"✓ Found {agentListResponse.Value.Data.Count} agent(s) in total");

            // Step 2: Create a thread
            Console.WriteLine("\n=== Creating Thread ===");
            Response<AgentThread> threadResponse = await client.CreateThreadAsync();
            AgentThread thread = threadResponse.Value;
            Console.WriteLine($"✓ Thread created successfully (ID: {thread.Id})");

            // Step 3: Add a message
            Console.WriteLine("\n=== Adding Message to Thread ===");
            string userMessage = "I need to solve the equation `3x + 11 = 14`. Can you help me?";
            Console.WriteLine($"User message: {userMessage}");
            Response<ThreadMessage> messageResponse = await client.CreateMessageAsync(
                thread.Id,
                MessageRole.User,
                userMessage);
            ThreadMessage message = messageResponse.Value;
            Console.WriteLine($"✓ Message added successfully (ID: {message.Id})");

            // Verify message in thread
            Console.WriteLine("\nVerifying message in thread...");
            Response<PageableList<ThreadMessage>> messagesListResponse = await client.GetMessagesAsync(thread.Id);
            Console.WriteLine($"✓ Found {messagesListResponse.Value.Data.Count} message(s) in thread");

            // Step 4: Run the agent
            Console.WriteLine("\n=== Starting Agent Run ===");
            Response<ThreadRun> runResponse = await client.CreateRunAsync(
                thread.Id,
                agent.Id,
                additionalInstructions: "");
            ThreadRun run = runResponse.Value;
            Console.WriteLine($"✓ Run initiated (ID: {run.Id})");

            // Monitor run progress
            int pollCount = 0;
            Console.WriteLine("\nMonitoring run progress:");
            do
            {
                pollCount++;
                Console.WriteLine($"[{DateTime.Now:HH:mm:ss}] Status: {runResponse.Value.Status} (Poll #{pollCount})");
                await Task.Delay(TimeSpan.FromMilliseconds(500));
                runResponse = await client.GetRunAsync(thread.Id, runResponse.Value.Id);
            }
            while (runResponse.Value.Status == RunStatus.Queued
                || runResponse.Value.Status == RunStatus.InProgress);

            Console.WriteLine($"\n✓ Run completed with status: {runResponse.Value.Status}");
            if (runResponse.Value.LastError != null)
            {
                Console.WriteLine($"⚠ Last error: {runResponse.Value.LastError.Message}");
            }

            // Get final messages
            Console.WriteLine("\n=== Final Message History ===");
            Response<PageableList<ThreadMessage>> afterRunMessagesResponse = await client.GetMessagesAsync(thread.Id);
            IReadOnlyList<ThreadMessage> messages = afterRunMessagesResponse.Value.Data;
            Console.WriteLine($"Retrieved {messages.Count} total messages\n");

            Console.WriteLine("Message Timeline:");
            Console.WriteLine("----------------");
            foreach (ThreadMessage threadMessage in messages)
            {
                Console.WriteLine($"\n[{threadMessage.CreatedAt:yyyy-MM-dd HH:mm:ss}]");
                Console.WriteLine($"Role: {threadMessage.Role}");
                Console.WriteLine("Content:");
                foreach (MessageContent contentItem in threadMessage.ContentItems)
                {
                    if (contentItem is MessageTextContent textItem)
                    {
                        Console.WriteLine($"  {textItem.Text}");
                    }
                    else if (contentItem is MessageImageFileContent imageFileItem)
                    {
                        Console.WriteLine($"  <image from ID: {imageFileItem.FileId}>");
                    }
                }
                Console.WriteLine("----------------");
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine($"\n❌ Error occurred: {ex.Message}");
            Console.WriteLine($"[DEBUG] Exception Type: {ex.GetType().FullName}");
            Console.WriteLine($"[DEBUG] Stack trace: {ex.StackTrace}");
            if (ex.InnerException != null)
            {
                Console.WriteLine($"[DEBUG] Inner Exception: {ex.InnerException.Message}");
                Console.WriteLine($"[DEBUG] Inner Exception Type: {ex.InnerException.GetType().FullName}");
                Console.WriteLine($"[DEBUG] Inner Exception Stack trace: {ex.InnerException.StackTrace}");
            }
            if (ex is Azure.RequestFailedException reqEx)
            {
                Console.WriteLine($"[DEBUG] Azure RequestFailedException Status: {reqEx.Status}");
                Console.WriteLine($"[DEBUG] Azure RequestFailedException ErrorCode: {reqEx.ErrorCode}");
            }
        }
        
        Console.WriteLine($"\nApplication completed at: {DateTime.Now:yyyy-MM-dd HH:mm:ss}");
    }
}

Guide followed: https://learn.microsoft.com/en-us/azure/ai-services/agents/quickstart?pivots=programming-language-csharp

What I’ve tried:

• Confirmed system DNS works (dig resolves the CNAME chain).
• .NET DNS test for www.google.com and the final A record in the CNAME chain both succeed.
• Disabled all VPN, firewall, and security software.
• Used Google DNS (8.8.8.8).
• Checked /etc/hosts and /etc/resolv.conf.
• Running on latest .NET SDK and runtime for arm64.
• No global.json or custom environment variables set.
• Tried both Dns.GetHostAddresses and Dns.GetHostEntry.
• Rebooted after all changes.