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Secure communication between hosting and client integrations

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This article is a continuation of two previous articles demonstrating the creation of custom hosting integrations and custom client integrations.

One of the primary benefits to .NET Aspire is how it simplifies the configurability of resources and consuming clients (or integrations). This article demonstrates how to share authentication credentials from a custom resource in a hosting integration, to the consuming client in a custom client integration. The custom resource is a MailDev container that allows for either incoming or outgoing credentials. The custom client integration is a MailKit client that sends emails.

Prerequisites

Since this article continues from previous content, you should have already created the resulting solution as a starting point for this article. If you haven't already, complete the following articles:

The resulting solution from these previous articles contains the following projects:

  • MailDev.Hosting: Contains the custom resource type for the MailDev container.
  • MailDevResource.AppHost: The app host that uses the custom resource and defines it as a dependency for a Newsletter service.
  • MailDevResource.NewsletterService: An ASP.NET Core Web API project that sends emails using the MailDev container.
  • MailDevResource.ServiceDefaults: Contains the default service configurations intended for sharing.
  • MailKit.Client: Contains the custom client integration that exposes the MailKit SmtpClient through a factory.

Update the MailDev resource

To flow authentication credentials from the MailDev resource to the MailKit integration, you need to update the MailDev resource to include the username and password parameters.

The MailDev container supports basic authentication for both incoming and outgoing simple mail transfer protocol (SMTP). To configure the credentials for incoming, you need to set the MAILDEV_INCOMING_USER and MAILDEV_INCOMING_PASS environment variables. For more information, see MailDev: Usage. Update the MailDevResource.cs file in the MailDev.Hosting project, by replacing its contents with the following C# code:

// For ease of discovery, resource types should be placed in
// the Aspire.Hosting.ApplicationModel namespace. If there is
// likelihood of a conflict on the resource name consider using
// an alternative namespace.
namespace Aspire.Hosting.ApplicationModel;

public sealed class MailDevResource(
    string name,
    ParameterResource? username,
    ParameterResource password)
        : ContainerResource(name), IResourceWithConnectionString
{
    // Constants used to refer to well known-endpoint names, this is specific
    // for each resource type. MailDev exposes an SMTP and HTTP endpoints.
    internal const string SmtpEndpointName = "smtp";
    internal const string HttpEndpointName = "http";

    private const string DefaultUsername = "mail-dev";

    // An EndpointReference is a core .NET Aspire type used for keeping
    // track of endpoint details in expressions. Simple literal values cannot
    // be used because endpoints are not known until containers are launched.
    private EndpointReference? _smtpReference;

    /// <summary>
    /// Gets the parameter that contains the MailDev SMTP server username.
    /// </summary>
    public ParameterResource? UsernameParameter { get; } = username;

    internal ReferenceExpression UserNameReference =>
        UsernameParameter is not null ?
        ReferenceExpression.Create($"{UsernameParameter}") :
        ReferenceExpression.Create($"{DefaultUsername}");

    /// <summary>
    /// Gets the parameter that contains the MailDev SMTP server password.
    /// </summary>
    public ParameterResource PasswordParameter { get; } = password;

    public EndpointReference SmtpEndpoint =>
        _smtpReference ??= new(this, SmtpEndpointName);

    // Required property on IResourceWithConnectionString. Represents a connection
    // string that applications can use to access the MailDev server. In this case
    // the connection string is composed of the SmtpEndpoint endpoint reference.
    public ReferenceExpression ConnectionStringExpression =>
        ReferenceExpression.Create(
            $"Endpoint=smtp://{SmtpEndpoint.Property(EndpointProperty.Host)}:{SmtpEndpoint.Property(EndpointProperty.Port)};Username={UserNameReference};Password={PasswordParameter}"
        );
}

These updates add a UsernameParameter and PasswordParameter property. These properties are used to store the parameters for the MailDev username and password. The ConnectionStringExpression property is updated to include the username and password parameters in the connection string. Next, update the MailDevResourceBuilderExtensions.cs file in the MailDev.Hosting project with the following C# code:

using Aspire.Hosting.ApplicationModel;

// Put extensions in the Aspire.Hosting namespace to ease discovery as referencing
// the .NET Aspire hosting package automatically adds this namespace.
namespace Aspire.Hosting;

public static class MailDevResourceBuilderExtensions
{
    private const string UserEnvVarName = "MAILDEV_INCOMING_USER";
    private const string PasswordEnvVarName = "MAILDEV_INCOMING_PASS";

    /// <summary>
    /// Adds the <see cref="MailDevResource"/> to the given
    /// <paramref name="builder"/> instance. Uses the "2.0.2" tag.
    /// </summary>
    /// <param name="builder">The <see cref="IDistributedApplicationBuilder"/>.</param>
    /// <param name="name">The name of the resource.</param>
    /// <param name="httpPort">The HTTP port.</param>
    /// <param name="smtpPort">The SMTP port.</param>
    /// <returns>
    /// An <see cref="IResourceBuilder{MailDevResource}"/> instance that
    /// represents the added MailDev resource.
    /// </returns>
    public static IResourceBuilder<MailDevResource> AddMailDev(
        this IDistributedApplicationBuilder builder,
        string name,
        int? httpPort = null,
        int? smtpPort = null,
        IResourceBuilder<ParameterResource>? userName = null,
        IResourceBuilder<ParameterResource>? password = null)
    {
        var passwordParameter = password?.Resource ??
            ParameterResourceBuilderExtensions.CreateDefaultPasswordParameter(
                builder, $"{name}-password");

        // The AddResource method is a core API within .NET Aspire and is
        // used by resource developers to wrap a custom resource in an
        // IResourceBuilder<T> instance. Extension methods to customize
        // the resource (if any exist) target the builder interface.
        var resource = new MailDevResource(
            name, userName?.Resource, passwordParameter);

        return builder.AddResource(resource)
                      .WithImage(MailDevContainerImageTags.Image)
                      .WithImageRegistry(MailDevContainerImageTags.Registry)
                      .WithImageTag(MailDevContainerImageTags.Tag)
                      .WithHttpEndpoint(
                          targetPort: 1080,
                          port: httpPort,
                          name: MailDevResource.HttpEndpointName)
                      .WithEndpoint(
                          targetPort: 1025,
                          port: smtpPort,
                          name: MailDevResource.SmtpEndpointName)
                      .WithEnvironment(context =>
                      {
                          context.EnvironmentVariables[UserEnvVarName] = resource.UserNameReference;
                          context.EnvironmentVariables[PasswordEnvVarName] = resource.PasswordParameter;
                      });
    }
}

// This class just contains constant strings that can be updated periodically
// when new versions of the underlying container are released.
internal static class MailDevContainerImageTags
{
    internal const string Registry = "docker.io";

    internal const string Image = "maildev/maildev";

    internal const string Tag = "2.0.2";
}

The preceding code updates the AddMailDev extension method to include the userName and password parameters. The WithEnvironment method is updated to include the UserEnvVarName and PasswordEnvVarName environment variables. These environment variables are used to set the MailDev username and password.

Update the app host

Now that the resource is updated to include the username and password parameters, you need to update the app host to include these parameters. Update the Program.cs file in the MailDevResource.AppHost project with the following C# code:

var builder = DistributedApplication.CreateBuilder(args);

var mailDevUsername = builder.AddParameter("maildev-username");
var mailDevPassword = builder.AddParameter("maildev-password");

var maildev = builder.AddMailDev(
    name: "maildev",
    userName: mailDevUsername,
    password: mailDevPassword);

builder.AddProject<Projects.MailDevResource_NewsletterService>("newsletterservice")
       .WithReference(maildev);

builder.Build().Run();

The preceding code adds two parameters for the MailDev username and password. It assigns these parameters to the MAILDEV_INCOMING_USER and MAILDEV_INCOMING_PASS environment variables. The AddMailDev method has two chained calls to WithEnvironment which includes these environment variables. For more information on parameters, see External parameters.

Next, configure the secrets for these parameters. Right-click on the MailDevResource.AppHost project and select Manage User Secrets. Add the following JSON to the secrets.json file:

{
  "Parameters:maildev-username": "@admin",
  "Parameters:maildev-password": "t3st1ng"
}

Warning

These credentials are for demonstration purposes only and MailDev is intended for local development. These credentials are fictitious and shouldn't be used in a production environment.

Update the MailKit integration

It's good practice for client integrations to expect connection strings to contain various key/value pairs, and to parse these pairs into the appropriate properties. Update the MailKitClientSettings.cs file in the MailKit.Client project with the following C# code:

using System.Data.Common;
using System.Net;

namespace MailKit.Client;

/// <summary>
/// Provides the client configuration settings for connecting MailKit to an SMTP server.
/// </summary>
public sealed class MailKitClientSettings
{
    internal const string DefaultConfigSectionName = "MailKit:Client";

    /// <summary>
    /// Gets or sets the SMTP server <see cref="Uri"/>.
    /// </summary>
    /// <value>
    /// The default value is <see langword="null"/>.
    /// </value>
    public Uri? Endpoint { get; set; }

    /// <summary>
    /// Gets or sets the network credentials that are optionally configurable for SMTP
    /// server's that require authentication.
    /// </summary>
    /// <value>
    /// The default value is <see langword="null"/>.
    /// </value>
    public NetworkCredential? Credentials { get; set; }

    /// <summary>
    /// Gets or sets a boolean value that indicates whether the database health check is disabled or not.
    /// </summary>
    /// <value>
    /// The default value is <see langword="false"/>.
    /// </value>
    public bool DisableHealthChecks { get; set; }

    /// <summary>
    /// Gets or sets a boolean value that indicates whether the OpenTelemetry tracing is disabled or not.
    /// </summary>
    /// <value>
    /// The default value is <see langword="false"/>.
    /// </value>
    public bool DisableTracing { get; set; }

    /// <summary>
    /// Gets or sets a boolean value that indicates whether the OpenTelemetry metrics are disabled or not.
    /// </summary>
    /// <value>
    /// The default value is <see langword="false"/>.
    /// </value>
    public bool DisableMetrics { get; set; }

    internal void ParseConnectionString(string? connectionString)
    {
        if (string.IsNullOrWhiteSpace(connectionString))
        {
            throw new InvalidOperationException($"""
                    ConnectionString is missing.
                    It should be provided in 'ConnectionStrings:<connectionName>'
                    or '{DefaultConfigSectionName}:Endpoint' key.'
                    configuration section.
                    """);
        }

        if (Uri.TryCreate(connectionString, UriKind.Absolute, out var uri))
        {
            Endpoint = uri;
        }
        else
        {
            var builder = new DbConnectionStringBuilder
            {
                ConnectionString = connectionString
            };
            
            if (builder.TryGetValue("Endpoint", out var endpoint) is false)
            {
                throw new InvalidOperationException($"""
                        The 'ConnectionStrings:<connectionName>' (or 'Endpoint' key in
                        '{DefaultConfigSectionName}') is missing.
                        """);
            }

            if (Uri.TryCreate(endpoint.ToString(), UriKind.Absolute, out uri) is false)
            {
                throw new InvalidOperationException($"""
                        The 'ConnectionStrings:<connectionName>' (or 'Endpoint' key in
                        '{DefaultConfigSectionName}') isn't a valid URI.
                        """);
            }

            Endpoint = uri;
            
            if (builder.TryGetValue("Username", out var username) &&
                builder.TryGetValue("Password", out var password))
            {
                Credentials = new(
                    username.ToString(), password.ToString());
            }
        }
    }
}

The preceding settings class, now includes a Credentials property of type NetworkCredential. The ParseConnectionString method is updated to parse the Username and Password keys from the connection string. If the Username and Password keys are present, a NetworkCredential is created and assigned to the Credentials property.

With the settings class updated to understand and populate the credentials, update the factory to conditionally use the credentials if they're configured. Update the MailKitClientFactory.cs file in the MailKit.Client project with the following C# code:

using System.Net;
using MailKit.Net.Smtp;

namespace MailKit.Client;

/// <summary>
/// A factory for creating <see cref="ISmtpClient"/> instances
/// given a <paramref name="smtpUri"/> (and optional <paramref name="credentials"/>).
/// </summary>
/// <param name="settings">
/// The <see cref="MailKitClientSettings"/> settings for the SMTP server
/// </param>
public sealed class MailKitClientFactory(MailKitClientSettings settings) : IDisposable
{
    private readonly SemaphoreSlim _semaphore = new(1, 1);

    private SmtpClient? _client;

    /// <summary>
    /// Gets an <see cref="ISmtpClient"/> instance in the connected state
    /// (and that's been authenticated if configured).
    /// </summary>
    /// <param name="cancellationToken">Used to abort client creation and connection.</param>
    /// <returns>A connected (and authenticated) <see cref="ISmtpClient"/> instance.</returns>
    /// <remarks>
    /// Since both the connection and authentication are considered expensive operations,
    /// the <see cref="ISmtpClient"/> returned is intended to be used for the duration of a request
    /// (registered as 'Scoped') and is automatically disposed of.
    /// </remarks>
    public async Task<ISmtpClient> GetSmtpClientAsync(
        CancellationToken cancellationToken = default)
    {
        await _semaphore.WaitAsync(cancellationToken);

        try
        {
            if (_client is null)
            {
                _client = new SmtpClient();

                await _client.ConnectAsync(settings.Endpoint, cancellationToken)
                             .ConfigureAwait(false);

                if (settings.Credentials is not null)
                {
                    await _client.AuthenticateAsync(settings.Credentials, cancellationToken)
                                 .ConfigureAwait(false);
                }
            }
        }
        finally
        {
            _semaphore.Release();
        }       

        return _client;
    }

    public void Dispose()
    {
        _client?.Dispose();
        _semaphore.Dispose();
    }
}

When the factory determines that credentials have been configured, it authenticates with the SMTP server after connecting before returning the SmtpClient.

Run the sample

Now that you've updated the resource, corresponding integration projects, and the app host, you're ready to run the sample app. To run the sample from your IDE, select F5 or use dotnet run from the root directory of the solution to start the application—you should see the .NET Aspire dashboard. Navigate to the maildev container resource and view the details. You should see the username and password parameters in the resource details, under the Environment Variables section:

.NET Aspire Dashboard: MailDev container resource details.

Likewise, you should see the connection string in the newsletterservice resource details, under the Environment Variables section:

.NET Aspire Dashboard: Newsletter service resource details.

Validate that everything is working as expected.

Summary

This article demonstrated how to flow authentication credentials from a custom resource to a custom client integration. The custom resource is a MailDev container that allows for either incoming or outgoing credentials. The custom client integration is a MailKit client that sends emails. By updating the resource to include the username and password parameters, and updating the integration to parse and use these parameters, authentication flows credentials from the hosting integration to the client integration.