Wednesday, January 15, 2020

Understanding Managed Identity and using it to Call Secured function App

What is Managed Service Identity / Managed Identity

In last post we saw how to secure a Function App with Active directory and how to make call to it from another function App.

And for doing it from function we used libraries provided by Microsoft i.e., Microsoft.IdentityModel.Clients.ActiveDirectory;

So we needed to authenticate first with AD and then request for access token. For that we create an AD app, create secret/add certificate , manage secrets expiry and storing the info somewhere so as to use in our code

With this approach there are two problem areas
1. We have to create/manage the AD app and the  certificates/secrets associated with it. 
2. We have to use credentials in our code, although from app settings (they are visible to all)  or keyvault - but to access those you need credential to authenticate to it.

To avoid this, we can use Managed Service identity (MSI)/Managed Identity feature, and the Azure will do this for us automatically. To use MI/MSI, turn on Identity of the respective Azure Service(here function app) from the Azure Portal.


Note: MSI/MI feature is not available for all services. See Services that support managed identities for Azure resources

What happens when Identity is switched on ? -- When you switch on Identity of any Azure service, Azure itself creates an AD app with same name of the service thus creating an identity in Azure Active Directory.

And you don't have to worry about Tenant ID(Directory ID),
Client ID and Client Secret - as it is taken care(managed) by Azure thus the name Managed Identities.

Using Managed Identity we can authenticate to any service that supports Azure AD authentication without having credentials in your code. But we would need access token for the target app in order to access it. 

Microsoft has provided a library Microsoft.Azure.Services.AppAuthentication  which has azureServiceTokenProvider.GetAccessTokenAsync method which can be used to get access token by just providing Audience ID.

In some cases, we need to explicitly provide access to an identity apart from token e.g., in Key Vault we have to add identity in it's access policy. The access mechanism can be different for each Azure Service.

I personally feel, the access mechanism should be unified across all Azure service.

How to do it


1. Create Function App and Enable Identity (Caller AD App)


Go to Portal and create an instance on Function App and once created select it and go to Platform features and select Identity


select Identity on function App


Upon selecting Identity you will be presented with two option System assigned and User assigned - Select System assigned.

Identity On
 Make Status as On and save

identityOn Confirmation


What happens when we switch on Identity?

As mentioned earlier, an AD app gets created with same name as that of the azure service in Active directory, you can check it in Enterprise Application
check AD app in Enterprise application


Difference between System assigned and user assigned Identity -

The lifecycle of a system-assigned identity is directly tied to the Azure service instance that it's enabled on (function app here). So if this function app is deleted, Azure automatically cleans up the credentials and the identity in Azure AD

user-assigned managed identity is created as a standalone Azure resource i.e.not tied to any service.So it is same as explicitly creating AD app and can be shared by any number of service.


2. Create Function



In Visual Studio, create a project using Azure functions Template and add a http triggered function .

Now we create a function which will use the above identity to call the AD secured function app by getting access token from Active Directory and pass it in header along with request.

However here no details other than AudienceID and TargetURL is required, this we store in App settings of function app (on local machine it is to be kept in local.settings.json file). As rest is taken care by azureServiceTokenProvider.GetAccessTokenAsync method and to use it we need to install   Microsoft.Azure.Services.AppAuthentication nuget package.Microsoft has provided this library to work with Managed Identity.

Following code is used to get the access token

var azureServiceTokenProvider = new AzureServiceTokenProvider();
string accessToken= awaitazureServiceTokenProvider.GetAccessTokenAsync(Environment.GetEnvironmentVariable("AudienceID"));

And this access token is passed with request in header  as AuthenticationHeaderValue

httpClient.DefaultRequestHeaders.Authorization = newSystem.Net.Http.Headers.AuthenticationHeaderValue("Bearer", accessToken);
var content = new StringContent(body, System.Text.Encoding.UTF8, "application/json");
response = httpClient.PostAsync(Environment.GetEnvironmentVariable("TargetURL"),   content).Result;

And below is the full code

using System;
using System.IO;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;
using System.Net.Http;
using Microsoft.Azure.Services.AppAuthentication;

namespace DevT2
{
    public static class CallerFunctionMSI
    {
        [FunctionName("CallerFunctionMSI")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
            ILogger log)
        {
            log.LogInformation("C# HTTP trigger function processed a request.");

            string requestBody = await new StreamReader(req.Body).ReadToEndAsync();

            HttpResponseMessage Targetresp = await PostMessage(requestBody);
            return Targetresp != null
                ? (ActionResult)new OkObjectResult($"Hello, {Targetresp.Content.ReadAsStringAsync().Result}")
                : new BadRequestObjectResult("Please pass a name on the query string or in the request body");
        }
        public static async Task<HttpResponseMessage> PostMessage(string body)
        {
            HttpResponseMessage response;
            var azureServiceTokenProvider = new AzureServiceTokenProvider();
            string accessToken = await azureServiceTokenProvider.GetAccessTokenAsync(Environment.GetEnvironmentVariable("AudienceID"));

            using (var httpClient = new HttpClient())
            {
                httpClient.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", accessToken);
                var content = new StringContent(body, System.Text.Encoding.UTF8, "application/json");
                response = httpClient.PostAsync(Environment.GetEnvironmentVariable("TargetURL"), content).Result;
            }
            return response;
        }

    }
}


Build it and deploy it to Function App created in step1. And do not forget to add the TargetUrl and AudienceID in app settings.





Testing


The target function expects input in following form 

{"name":"Input"}

and it returns result as "Hello,Reply from target app,+input it received"
testing evidence


Grab the url of caller function, use any rest client and make call to caller function app and pass on input in expected format as above and check the response.


How it works


GetAccessTokenAsync method automatically manages the authentication part before raising an access token request to authority.And to get the details required it makes  a call internally to Managed Identity rest api. 

Thus this process eliminates the overhead of managing the app, it's credentials and helps in avoiding the credentials in code.



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