如何获取Azure App Service的特定用户分配的托管服务标识的令牌？

Question

我正在尝试获取特定用户定义身份的msi令牌。我们的应用服务有2个用户定义的身份，我想要代表用户指定的身份之一的令牌。

这是代码：

        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(
            "http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01&resource=https://management.azure.com/&object_id=<ObjectId>&client_id=<clientId>");

        req.Headers["Metadata"] = "true";
        req.Method = "GET";

        try
        {
            // Call /token endpoint
            HttpWebResponse response = (HttpWebResponse)req.GetResponse();

            // Pipe response Stream to a StreamReader, and extract access token
            StreamReader streamResponse = new StreamReader(response.GetResponseStream());
            string stringResponse = streamResponse.ReadToEnd();
            Dictionary<string, string> list =
                 JsonConvert.DeserializeObject<Dictionary<string, string>>(stringResponse);
            string accessToken = list["access_token"];

            System.IO.File.WriteAllText(@".Log.txt", accessToken);
        }
        catch (Exception e)
        {
            string errorText = String.Format("{0} 

{1}", e.Message, e.InnerException != null ? e.InnerException.Message : "Acquire token failed");
            System.IO.File.WriteAllText(@".Log.txt", errorText);
            throw;
        }

它部署在azure app服务中。当我点击此部分时，我看到此错误：尝试以其访问权限禁止的方式访问套接字

我尝试连接到http://169.254.169.254以使用kudu控制台获取令牌。但是这个端点似乎无法访问。

我确实尝试使用Microsoft.Azure.Services.AppAuthentication中的AzureServiceTokenProvider生成msi令牌，但找不到有关如何将其用于多个用户分配的身份的任何文档。

编辑：

更新1：

我尝试使用MSI_ENDPOINT环境变量中的端点而不是169.254.169.254。但是，当我运行应用服务时，看起来没有设置MSI_ENDPOINT值。这是我尝试过的代码：

   var endpoint = Environment.GetEnvironmentVariable("MSI_ENDPOINT");
    string apiVersion = "2018-02-01";
    string resource = "https://management.azure.com/";
    string objectId = "<objectid>";
    string clientId = "<clientId>";

        // Build request to acquire managed identities for Azure resources token
        //HttpWebRequest req = (HttpWebRequest)WebRequest.Create(
        //    "http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01&resource=https://management.azure.com/&object_id=4aef1720-b3b1-4935-8d68-e330508907fa&client_id=558ecc75-8697-4419-bab9-aa2c87043cfd");

        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(
       String.Format(
            "{0}?resource={1}&api-version={2}&object_id={3}&client_id={4}",
            endpoint,
            resource,
            apiVersion,
            objectId,
            clientId));

        req.Headers["Metadata"] = "true";
        req.Method = "GET";

        try
        {
            // Call /token endpoint
            HttpWebResponse response = (HttpWebResponse)req.GetResponse();

            // Pipe response Stream to a StreamReader, and extract access token
            StreamReader streamResponse = new StreamReader(response.GetResponseStream());
            string stringResponse = streamResponse.ReadToEnd();
            Dictionary<string, string> list =
                 JsonConvert.DeserializeObject<Dictionary<string, string>>(stringResponse);
            string accessToken = list["access_token"];

            System.IO.File.WriteAllText(@".Log.txt", accessToken);
        }
        catch (Exception e)
        {
            string errorText = String.Format("{0} 

{1}", e.Message, e.InnerException != null ? e.InnerException.Message : "Acquire token failed");

            string log = "MSI_ENDPOINT : " + endpoint + "
";
            log += ("ErrorText : " + errorText + "
");
            System.IO.File.WriteAllText(@".Log.txt", errorText);
            throw;
        }