Implemented smtp client certificate validation and introduced 2 new events if smtp server fails to bind to a socket

Examples/SampleApp/Examples/CustomEndpointListenerExample.cs

@@ -1,6 +1,7 @@
 using System;
 using System.IO;
 using System.IO.Pipelines;
+using System.Net.Security;
 using System.Security.Authentication;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
@@ -80,7 +81,7 @@ public CustomSecurableDuplexPipe(ISecurableDuplexPipe securableDuplexPipe)
                 _securableDuplexPipe = securableDuplexPipe;
             }
 
-            public Task UpgradeAsync(X509Certificate certificate, SslProtocols protocols, CancellationToken cancellationToken = default)
+            public Task UpgradeAsync(X509Certificate certificate, SslProtocols protocols, CancellationToken cancellationToken = default, RemoteCertificateValidationCallback remoteCertificateValidationCallback = null)
             {
                 return _securableDuplexPipe.UpgradeAsync(certificate, protocols, cancellationToken);
             }

Examples/SampleApp/Examples/SecureServerExample.cs

@@ -10,11 +10,11 @@
 
 namespace SampleApp.Examples
 {
-    public static class SecureServerExample
+    public static class UnsecureServerExample
     {
         public static void Run()
         {
-            // this is important when dealing with a certificate that isnt valid
+            // this is important when dealing with a certificate that isn't valid
             ServicePointManager.ServerCertificateValidationCallback = IgnoreCertificateValidationFailureForTestingOnly;
 
             var cancellationTokenSource = new CancellationTokenSource();
@@ -23,7 +23,9 @@ public static void Run()
                 .ServerName("SmtpServer SampleApp")
                 .Endpoint(builder =>
                     builder
-                        .Port(9025, true)
+                        .Port(9025)
+                        .IsSecure(true)
+                        .AuthenticationRequired(true)
                         .AllowUnsecureAuthentication(false)
                         .Certificate(CreateCertificate()))
                 .Build();
@@ -33,7 +35,7 @@ public static void Run()
 
             var server = new SmtpServer.SmtpServer(options, serviceProvider);
             server.SessionCreated += OnSessionCreated;
-            
+
             var serverTask = server.StartAsync(cancellationTokenSource.Token);
 
             SampleMailClient.Send(user: "user", password: "password", useSsl: true);

Examples/SampleApp/Examples/UnsecureServerExample.cs

@@ -0,0 +1,79 @@
+using System;
+using System.IO;
+using System.Net;
+using System.Net.Security;
+using System.Security.Cryptography.X509Certificates;
+using System.Threading;
+using SmtpServer;
+using SmtpServer.ComponentModel;
+using SmtpServer.Tracing;
+
+namespace SampleApp.Examples
+{
+    public static class SecureServerExample
+    {
+        public static void Run()
+        {
+            // this is important when dealing with a certificate that isn't valid
+            ServicePointManager.ServerCertificateValidationCallback = IgnoreCertificateValidationFailureForTestingOnly;
+
+            var cancellationTokenSource = new CancellationTokenSource();
+
+            var options = new SmtpServerOptionsBuilder()
+                .ServerName("SmtpServer SampleApp")
+                .Endpoint(builder =>
+                    builder
+                        .Port(9025)
+                        .IsSecure(false)
+                        .AuthenticationRequired(true)
+                        .AllowUnsecureAuthentication(false)
+                        .Certificate(CreateCertificate()))
+                .Build();
+
+            var serviceProvider = new ServiceProvider();
+            serviceProvider.Add(new SampleUserAuthenticator());
+            // Client certificate validation will only be performed if the endpoint is not secure by default.
+            serviceProvider.Add(new SampleClientCertificateValidator());
+
+            var server = new SmtpServer.SmtpServer(options, serviceProvider);
+            server.SessionCreated += OnSessionCreated;
+
+            var serverTask = server.StartAsync(cancellationTokenSource.Token);
+
+            SampleMailClient.Send(user: "user", password: "password", useSsl: true);
+
+            cancellationTokenSource.Cancel();
+            serverTask.WaitWithoutException();
+        }
+
+        static void OnSessionCreated(object sender, SessionEventArgs e)
+        {
+            Console.WriteLine("Session Created.");
+
+            e.Context.CommandExecuting += OnCommandExecuting;
+        }
+
+        static void OnCommandExecuting(object sender, SmtpCommandEventArgs e)
+        {
+            Console.WriteLine("Command Executing.");
+
+            new TracingSmtpCommandVisitor(Console.Out).Visit(e.Command);
+        }
+
+        static bool IgnoreCertificateValidationFailureForTestingOnly(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
+        {
+            return true;
+        }
+
+        static X509Certificate2 CreateCertificate()
+        {
+            // to create an X509Certificate for testing you need to run MAKECERT.EXE and then PVK2PFX.EXE
+            // http://www.digitallycreated.net/Blog/38/using-makecert-to-create-certificates-for-development
+
+            var certificate = File.ReadAllBytes(@"C:\Users\caino\Dropbox\Documents\Cain\Programming\SmtpServer\SmtpServer.pfx");
+            var password = File.ReadAllText(@"C:\Users\caino\Dropbox\Documents\Cain\Programming\SmtpServer\SmtpServerPassword.txt");
+
+            return new X509Certificate2(certificate, password);
+        }
+    }
+}

Examples/SampleApp/SampleClientCertificateValidator.cs

@@ -0,0 +1,16 @@
+using System.Net.Security;
+using SmtpServer.Authentication;
+
+namespace SampleApp
+{
+    public sealed class SampleClientCertificateValidator : ClientCertificateValidator
+    {
+        public override RemoteCertificateValidationCallback RemoteClientCertificateValidationCallback { get; set; } =
+            (sender, certificate, chain, sslPolicyErrors) =>
+            {
+                // Provide your custom client certificate validation logic here.
+                // Return true if valid; otherwise, false.
+                return true;
+            };
+    }
+}

README.md

@@ -36,14 +36,15 @@ await smtpServer.StartAsync(CancellationToken.None);
 
 # What hooks are provided?
 
-There are three hooks that can be implemented; IMessageStore, IMailboxFilter, and IUserAuthenticator.
+There are four hooks that can be implemented; IMessageStore, IMailboxFilter, IUserAuthenticator and IClientCertificateValidator.
 
 ```cs
 var options = new SmtpServerOptionsBuilder()
     .ServerName("localhost")
     .Endpoint(builder =>
         builder
-            .Port(9025, true)
+            .Port(9025)
+            .IsSecure(true)
             .AllowUnsecureAuthentication(false)
             .Certificate(CreateCertificate()))
     .Build();
@@ -52,6 +53,7 @@ var serviceProvider = new ServiceProvider();
 serviceProvider.Add(new SampleMessageStore());
 serviceProvider.Add(new SampleMailboxFilter());
 serviceProvider.Add(new SampleUserAuthenticator());
+serviceProvider.Add(new SampleClientCertificateValidator());
 
 var smtpServer = new SmtpServer.SmtpServer(options, serviceProvider);
 await smtpServer.StartAsync(CancellationToken.None);
@@ -130,3 +132,20 @@ public class SampleUserAuthenticator : IUserAuthenticator, IUserAuthenticatorFac
     }
 }
 ```
+
+```cs
+public sealed class SampleClientCertificateValidator : IClientCertificateValidator, IClientCertificateValidatorFactory
+{
+    public RemoteCertificateValidationCallback RemoteClientCertificateValidationCallback { get; set; } =
+        (sender, certificate, chain, sslPolicyErrors) =>
+        {
+            // Provide your custom client certificate validation logic here.
+            // Return true if valid; otherwise, false.
+            return true;
+        };
+
+    public IClientCertificateValidator CreateInstance(ISessionContext context)
+    {
+        return new SampleClientCertificateValidator();
+    }
+```

Src/SmtpServer.Tests/MailClient.cs

@@ -1,4 +1,6 @@
-using System.Threading;
+using System;
+using System.Threading;
+using MailKit;
 using MailKit.Net.Smtp;
 using MailKit.Security;
 using MimeKit;
@@ -46,16 +48,23 @@ public static MimeMessage Message(
             return message;
         }
 
-        public static SmtpClientEx Client(string host = "localhost", int port = 9025, SecureSocketOptions options = SecureSocketOptions.Auto)
+        public static SmtpClientEx Client()
         {
-            var client = new SmtpClientEx();
-
-            client.Connected += (sender, args) =>
-            {
+            return new SmtpClientEx();
+        }
 
-            };
+        public static SmtpClientEx Connect(this SmtpClientEx client,
+            EventHandler<ConnectedEventArgs> onConnected = null,
+            string host = "localhost",
+            int port = 9025,
+            SecureSocketOptions options = SecureSocketOptions.Auto)
+        {
+            if (onConnected is not null)
+            {
+                client.Connected += onConnected;
+            }
 
-            client.Connect("localhost", 9025, options);
+            client.Connect(host, port, options);
 
             return client;
         }
@@ -67,7 +76,7 @@ public static void Send(
         {
             message ??= Message();
 
-            using var client = Client();
+            using var client = Client().Connect();
 
             if (user != null && password != null)
             {
@@ -82,17 +91,17 @@ public static void Send(
 
         public static void NoOp(SecureSocketOptions options = SecureSocketOptions.Auto)
         {
-            using var client = Client(options: options);
+            using var client = Client().Connect(options: options);
 
-            client.NoOp();            
+            client.NoOp();
         }
     }
 
     internal class SmtpClientEx : SmtpClient
     {
         public SmtpResponse SendUnknownCommand(string command, CancellationToken cancellationToken = default)
         {
-            return SendCommand(command, cancellationToken); 
+            return SendCommand(command, cancellationToken);
         }
     }
 }