Add configuration overrides for BaseEncoding, ForceFileSystem, and ServerCertificateValidationCallback (#46, #41, #33) (#53)

* Add configuration overrides for BaseEncoding, ForceFileSystem, and ServerCertificateValidationCallback (#46, #41, #33)

* Address PR feedback: Use switch for filesystem force, fix MLSD fallback bug, fix parser probe on single parsers, and clarify cert callback docstring

* Update README.md with advanced configuration examples

---------

Co-authored-by: Nick Briscoe <nick.briscoe@razor.co.uk>

Author: sparkeh9

README.md

@@ -80,5 +80,33 @@ using ( var ftpClient = new FtpClient( new FtpClientConfiguration
 
 ```
 
+### Advanced Configuration ###
+CoreFTP provides several configuration overrides in `FtpClientConfiguration` to assist with connecting to legacy, obfuscated, or non-compliant FTP servers:
+
+```csharp
+using ( var ftpClient = new FtpClient( new FtpClientConfiguration
+{
+    Host = "legacy-server.local",
+    Username = "user",
+    Password = "password",
+    
+    // Force the control stream encoding for servers that don't support UTF8 (e.g. Chinese GBK or Japanese Shift_JIS)
+    BaseEncoding = System.Text.Encoding.GetEncoding("GBK"),
+    
+    // Force the directory listing parser if the server hides its OS in the FEAT response
+    ForceFileSystem = FtpFileSystemType.Windows,
+    
+    // Provide a custom callback to validate specific self-signed certificates
+    IgnoreCertificateErrors = false,
+    ServerCertificateValidationCallback = (cert, chain, errors) => 
+    {
+        return cert.GetCertHashString() == "EXPECTED_THUMBPRINT_HERE";
+    }
+} ) )
+{
+    await ftpClient.LoginAsync();
+}
+```
+
 ### Integration Tests ###
 Integration tests can be run against most FTP servers with passive mode enabled, credentials can be configured in appsettings.json of CoreFtp.Tests.Integration.

src/CoreFtp/Components/DirectoryListing/ListDirectoryProvider.cs

@@ -26,6 +26,16 @@ public ListDirectoryProvider( FtpClient ftpClient, ILogger logger, FtpClientConf
             };
         }
 
+        internal void ClearParsers()
+        {
+            directoryParsers.Clear();
+        }
+
+        internal void AddParser(IListDirectoryParser parser)
+        {
+            directoryParsers.Add(parser);
+        }
+
         private void EnsureLoggedIn()
         {
             if ( !ftpClient.IsConnected || !ftpClient.IsAuthenticated )
@@ -112,7 +122,9 @@ private IEnumerable<FtpNodeInformation> ParseLines( IReadOnlyList<string> lines
             if ( !lines.Any() )
                 yield break;
 
-            var parser = directoryParsers.FirstOrDefault( x => x.Test( lines[ 0 ] ) );
+            var parser = directoryParsers.Count == 1 
+                ? directoryParsers[ 0 ]
+                : directoryParsers.FirstOrDefault( x => x.Test( lines[ 0 ] ) );
 
             if ( parser == null )
                 yield break;

src/CoreFtp/Enum/FtpFileSystemType.cs

@@ -0,0 +1,8 @@
+namespace CoreFtp.Enum
+{
+    public enum FtpFileSystemType
+    {
+        Windows,
+        Unix
+    }
+}

src/CoreFtp/FtpClient.cs

@@ -60,8 +60,10 @@ public void Configure( FtpClientConfiguration configuration )
                 configuration.Host = new Uri( configuration.Host ).Host;
             }
 
-
-            ControlStream = new FtpControlStream( Configuration, new DnsResolver() );
+            ControlStream = new FtpControlStream( Configuration, new DnsResolver() )
+            {
+                Encoding = Configuration.BaseEncoding
+            };
             Configuration.BaseDirectory = $"/{Configuration.BaseDirectory.TrimStart( '/' )}";
         }
 
@@ -490,9 +492,30 @@ public async Task<long> GetFileSizeAsync( string fileName )
         private IDirectoryProvider DetermineDirectoryProvider()
         {
             Logger?.LogTrace( "[FtpClient] Determining directory provider" );
+
             if ( this.UsesMlsd() )
                 return new MlsdDirectoryProvider( this, Logger, Configuration );
 
+            if ( Configuration.ForceFileSystem.HasValue )
+            {
+                var forcedProvider = new ListDirectoryProvider( this, Logger, Configuration );
+                forcedProvider.ClearParsers();
+                
+                switch ( Configuration.ForceFileSystem.Value )
+                {
+                    case FtpFileSystemType.Windows:
+                        forcedProvider.AddParser( new Components.DirectoryListing.Parser.DosDirectoryParser( Logger ) );
+                        break;
+                    case FtpFileSystemType.Unix:
+                        forcedProvider.AddParser( new Components.DirectoryListing.Parser.UnixDirectoryParser( Logger ) );
+                        break;
+                    default:
+                        throw new NotSupportedException( $"Unsupported file system type: {Configuration.ForceFileSystem.Value}" );
+                }
+                
+                return forcedProvider;
+            }
+
             return new ListDirectoryProvider( this, Logger, Configuration );
         }
 

src/CoreFtp/FtpClientConfiguration.cs

@@ -1,5 +1,7 @@
 namespace CoreFtp
 {
+    using System;
+    using System.Net.Security;
     using System.Security.Authentication;
     using System.Security.Cryptography.X509Certificates;
     using Enum;
@@ -26,5 +28,24 @@ public class FtpClientConfiguration
 
         public X509CertificateCollection ClientCertificates { get; set; } = new X509CertificateCollection();
         public SslProtocols SslProtocols { get; set; } = SslProtocols.None;
+
+        /// <summary>
+        /// Base encoding to use for the control stream. Useful for legacy servers that use Shift_JIS, GBK, etc.
+        /// </summary>
+        public System.Text.Encoding BaseEncoding { get; set; } = System.Text.Encoding.ASCII;
+
+        /// <summary>
+        /// Allows overriding the server certificate validation logic (e.g., verifying a specific self-signed certificate thumbprint).
+        /// Note: This callback is only invoked when <see cref="IgnoreCertificateErrors"/> is set to <c>false</c>.
+        /// When <see cref="IgnoreCertificateErrors"/> is <c>true</c> (the default), certificate errors are ignored and this callback is not used.
+        /// </summary>
+        public Func<X509Certificate, X509Chain, SslPolicyErrors, bool> ServerCertificateValidationCallback { get; set; }
+
+        /// <summary>
+        /// Allows overriding the detected server file system / directory listing format.
+        /// Useful when the server does not advertise MLSD in its FEAT response (causing a fallback to LIST),
+        /// and the automatic detection of the LIST output format fails (e.g. force FtpFileSystemType.Windows).
+        /// </summary>
+        public FtpFileSystemType? ForceFileSystem { get; set; }
     }
 }

src/CoreFtp/Infrastructure/Stream/FtpControlStream.cs

@@ -469,6 +469,9 @@ private bool OnValidateCertificate(X509Certificate certificate, X509Chain chain,
             if (Configuration.IgnoreCertificateErrors)
                 return true;
 
+            if (Configuration.ServerCertificateValidationCallback != null)
+                return Configuration.ServerCertificateValidationCallback(certificate, chain, errors);
+
             return errors == SslPolicyErrors.None;
         }