Pass through authentication with ASP Core MVC, Web API and IdentityServer4?

Question

I have been working on migrating a monolithic ASP Core MVC application to use an service architecture design. The MVC front-end website uses an HttpClient to load necessary data from the ASP Core Web API. A small portion of the front-end MVC app also requires authentication which is in place using IdentityServer4 (integrated with the back-end API). This all works great, until I put an Authorize attribute on a controller or method on the Web API. I know I need to somehow pass the user authorization from the front-end to the back-end in order for this to work, but I'm not sure how. I have tried getting the access_token: User.FindFirst("access_token") but it returns null. I then tried this method and I am able to get the token:

var client = new HttpClient("url.com");
var token = HttpContext.Authentication.GetTokenAsync("access_token")?.Result;
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);

This method gets the token but still doesn't authentication with the back-end API. I'm pretty new to this OpenId/IdentityServer concepts and any help would be appreciated!

Here is the relevant code from the MVC Client Startup class:

    private void ConfigureAuthentication(IApplicationBuilder app)
    {
        app.UseCookieAuthentication(new CookieAuthenticationOptions
        {
            AuthenticationScheme = "Cookies",
            AutomaticAuthenticate = true,
            ExpireTimeSpan = TimeSpan.FromMinutes(60)
        });
        JwtSecurityTokenHandler.DefaultInboundClaimTypeMap.Clear();
        app.UseOpenIdConnectAuthentication(new OpenIdConnectOptions
        {
            AuthenticationScheme = "oidc",
            SignInScheme = "Cookies",

            Authority = "https://localhost:44348/",
            RequireHttpsMetadata = false,

            ClientId = "clientid",
            ClientSecret = "secret",

            ResponseType = "code id_token",
            Scope = { "openid", "profile" },
            GetClaimsFromUserInfoEndpoint = true,
            AutomaticChallenge = true, // Required to 302 redirect to login
            SaveTokens = true,

            TokenValidationParameters = new Microsoft.IdentityModel.Tokens.TokenValidationParameters
            {
                NameClaimType = "Name",
                RoleClaimType = "Role",
                SaveSigninToken = true
            },


        });
    }

and the StartUp class of the API:

        // Add authentication
        services.AddIdentity<ExtranetUser, IdentityRole>(options =>
        {
            // Password settings
            options.Password.RequireDigit = true;
            options.Password.RequiredLength = 8;
            options.Password.RequireNonAlphanumeric = true;
            options.Password.RequireUppercase = true;
            options.Password.RequireLowercase = true;

            // Lockout settings
            options.Lockout.DefaultLockoutTimeSpan = TimeSpan.FromMinutes(30);
            options.Lockout.MaxFailedAccessAttempts = 10;

            // User settings
            options.User.RequireUniqueEmail = true;
        })
            .AddDefaultTokenProviders();
        services.AddScoped<IUserStore<ExtranetUser>, ExtranetUserStore>();
        services.AddScoped<IRoleStore<IdentityRole>, ExtranetRoleStore>();
        services.AddSingleton<IAuthorizationHandler, AllRolesRequirement.Handler>();
        services.AddSingleton<IAuthorizationHandler, OneRoleRequirement.Handler>();
        services.AddSingleton<IAuthorizationHandler, EditQuestionAuthorizationHandler>();
        services.AddSingleton<IAuthorizationHandler, EditExamAuthorizationHandler>();
        services.AddAuthorization(options =>
        {
            /* ... etc .... */
        });
        var serviceProvider = services.BuildServiceProvider();
        var serviceSettings = serviceProvider.GetService<IOptions<ServiceSettings>>().Value;
        services.AddIdentityServer() // Configures OAuth/IdentityServer framework
            .AddInMemoryIdentityResources(IdentityServerConfig.GetIdentityResources())
            .AddInMemoryClients(IdentityServerConfig.GetClients(serviceSettings))
            .AddAspNetIdentity<ExtranetUser>()
            .AddTemporarySigningCredential(); // ToDo: Add permanent SigningCredential for IdentityServer

Answer 1

Added the nuget package here and the following code to fix:

app.UseIdentityServerAuthentication(new IdentityServerAuthenticationOptions
{
   Authority = "https://localhost:44348/",
   ApiName = "api"
});

This allows the API to host IdentityServer4 and use itself as authentication. Then in the MvcClient the bearer token can be passed to the API.