SFTP Server

A lightweight, enterprise-ready SFTP server built on .NET 8 using the FxSsh library. Compatible with popular SFTP clients like FileZilla, WinSCP, and OpenSSH.

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Installation

dotnet add package SFTPServerLib

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Features

• Full SFTP v3 Support - Upload, download, rename, delete files and directories
• User Management - Built-in user authentication with granular permissions
• Audit Logging - Track all file operations for compliance and debugging
• Connection Limits - Configure maximum concurrent connections
• Cross-Platform - Runs on Windows, macOS, and Linux
• Secure - RSA and ECDSA host key support with SHA-2 algorithms

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Quick Start

Basic Usage

using SFTPServer.Services;

// Create configuration
var config = new SftpServerConfiguration
{
    ListenAddress = "127.0.0.1",
    Port = 22,
    EnableLogging = true,
    MaxConnections = 100
};

// Create user manager and configure users
var userManager = new UserManager();

// Add a user with full permissions
userManager.AddOrUpdateUser(new UserAccount("admin", "admin123", config.RootDirectory)
{
    IsEnabled = true,
    CanUpload = true,
    CanDownload = true,
    CanDelete = true,
    CanCreateDirectory = true
});

// Create and start SFTP server
var server = new SftpServer(config, userManager);
server.Start();

Console.WriteLine("Press Ctrl+C to stop the server...");
await Task.Delay(-1);

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Configuration

SftpServerConfiguration Properties

Property	Type	Default	Description
ListenAddress	string?	null	IP address to listen on. null = all interfaces. Example: "127.0.0.1" for localhost only
Port	int	22	SSH server port
RootDirectory	string	OS-specific*	SFTP root directory where files are served from
MaxConnections	int	100	Maximum concurrent connections allowed
EnableLogging	bool	true	Enable debug logging to console
ConnectionTimeoutSeconds	int	300	Connection timeout in seconds (0 = no timeout)
MaxUploadSizeMB	int	0	Maximum upload file size in MB (0 = unlimited)
EnableAuditLog	bool	true	Enable audit logging for file operations
AuditLogPath	string	OS-specific**	Path to the audit log file
ServerBanner	string	"SSH-2.0-SFTP_1.0"	Server banner message (must start with "SSH-2.0-")
RsaHostKey	string	Auto-generated	RSA host key in PEM format
EcdsaP256HostKey	string	Auto-generated	ECDSA P-256 host key in PEM format

* RootDirectory defaults:

• All platforms: ~/.sftp/root (user's home directory)
• Fallback: {TempPath}/sftp/root if home directory is unavailable

** AuditLogPath defaults:

• All platforms: ~/.sftp/logs/audit.log (user's home directory)
• Fallback: {TempPath}/sftp/logs/audit.log if home directory is unavailable

Note: The server automatically detects the user's home directory using environment variables (HOME on Linux/macOS, USERPROFILE on Windows) and handles permission errors gracefully.

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User Management

UserAccount Properties

Property	Type	Default	Description
Username	string	Required	User's login name
Password	string	Required	User's password
HomeDirectory	string	Required	User's home directory path
IsEnabled	bool	true	Whether the account is active
CanUpload	bool	true	Permission to upload files
CanDownload	bool	true	Permission to download files
CanDelete	bool	true	Permission to delete files and directories
CanCreateDirectory	bool	true	Permission to create directories
MaxUploadBytes	long	0	Maximum upload size per user (0 = unlimited)

UserManager Methods

var userManager = new UserManager();

// Add or update a user
userManager.AddOrUpdateUser(new UserAccount("john", "password123", "/home/john"));

// Get a user
var user = userManager.GetUser("john");

// Remove a user
userManager.RemoveUser("john");

// Get all users
var allUsers = userManager.GetAllUsers();

// Check permissions
bool canUpload = userManager.HasPermission("john", SftpOperation.Write);

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Tested Clients

Client	Version
OpenSSH	OpenSSH_for_Windows_9.5p1, LibreSSL 3.8.2
PuTTY	Release 0.82
WinSCP	6.3.6
FileZilla	Compatible

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Underlying Technology: FxSsh

This SFTP server is built on top of the FxSsh library, a lightweight SSH server implementation.

Supported Algorithms

Category	Algorithms
Public Key	rsa-sha2-256, rsa-sha2-512, ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521
Key Exchange	diffie-hellman-group14-sha256, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521
Encryption	aes128-ctr, aes192-ctr, aes256-ctr
MAC	hmac-sha2-256, hmac-sha2-512

RFC Compliance

FxSsh adheres to the following RFC documents:

• RFC4250 - Protocol Assigned Numbers
• RFC4251 - Protocol Architecture
• RFC4252 - Authentication Protocol
• RFC4253 - Transport Layer Protocol
• RFC4254 - Connection Protocol
• RFC4344 - Transport Layer Encryption Modes
• RFC5656 - Elliptic Curve Algorithm Integration
• RFC6668 - SHA-2 Data Integrity Algorithms
• RFC8332 - Use of RSA Keys with SHA-2
• draft-ietf-secsh-filexfer-02 - SSH File Transfer Protocol (SFTP v3)

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License

The MIT license

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Target: .NET 8