A lightweight, simple FTP server. Pure Java, no libraries.

Although it's named "minimal", it supports a bunch of features:

• 100% Java, no libraries
• Lightweight
• Supports 57 FTP commands
• TLS/SSL support
• Custom File System support
• Custom User Authentication support
• Custom Commands support
• Support to obsolete commands (some FTP clients might still use them)

The required minimum implementation is already done, however, there are still commands that can be implemented.

• RFC 959 - File Transfer Protocol (33/33)
• RFC 697 - CWD Command of FTP (1/1) (Obsolete)
• RFC 737 - FTP Extension: XSEN (0/3) (Obsolete)
• RFC 743 - FTP extension: XRSQ/XRCP (0/4) (Obsolete)
• RFC 775 - Directory oriented FTP commands (5/5) (Obsolete)
• RFC 1123 - Requirements for Internet Hosts
• RFC 1639 - FTP Operation Over Big Address Records (2/2) (Obsolete)
• RFC 2228 - FTP Security Extensions (3/8)
• RFC 2389 - Feature negotiation mechanism for the File Transfer Protocol (2/2)
• RFC 2428 - FTP Extensions for IPv6 and NATs (2/2)
• RFC 2640 - Internationalization of the File Transfer Protocol (0/1)
• RFC 2773 - Encryption using KEA and SKIPJACK (0/1)
• RFC 3659 - Extensions to FTP (4/4)
• RFC 4217 - Securing FTP with TLS
• RFC 5797 - FTP Command and Extension Registry
• RFC 7151 - File Transfer Protocol HOST Command for Virtual Hosts (1/1)
• draft-twine-ftpmd5-00 - The "MD5" and "MMD5" FTP Command Extensions (2/2) (Obsolete)
• draft-somers-ftp-mfxx-04 - The "MFMT", "MFCT", and "MFF" Command Extensions for FTP (1/3)
• draft-bryan-ftpext-hash-02 - File Transfer Protocol HASH Command for Cryptographic Hashes (1/1)
• draft-bryan-ftp-range-08 - File Transfer Protocol RANG Command for Octet Ranges (0/1)

MinimalFTP is published on Maven Central.

<dependency>
  <groupId>com.guichaguri</groupId>
  <artifactId>minimalftp</artifactId>
  <version>1.0.7</version>
</dependency>

implementation 'com.guichaguri:minimalftp:1.0.7'

Check out more examples here :)

// Uses the current working directory as the root
File root = new File(System.getProperty("user.dir"));
// Creates a native file system
NativeFileSystem fs = new NativeFileSystem(root);
// Creates a noop authenticator, which allows anonymous authentication
NoOpAuthenticator auth = new NoOpAuthenticator(fs);
// Creates the server with the authenticator
FTPServer server = new FTPServer(auth);
// Start listening synchronously
server.listenSync(21);

The FTP protocol has two concepts of TCP connections:

This is the main FTP server, the one that usually listens on port 21.

The control connection is the one that receives commands for authentication and file manipulation. It also negotiates the data connection for file listing and transfer.

This is a secondary connection that is used for file list and transfer (both downloading and uploading).

The FTP protocol supports two modes:

• The active mode, in which the client creates a TCP server for the FTP server to connect into
• The passive mode, in which the server creates another TCP server for the client to connect into

Those passive mode servers that MinimalFTP creates each have a random port in the ephemeral range. You should either configure your firewall to allow incoming TCP connections to the ephemeral port range, or disable passive mode:

// Keeps only the Active Mode enabled (not recommended)
server.setPassiveModeEnabled(false);

Keeping only the active mode enabled is not recommended, as it can cause connectivity problems due to the user having firewall issues.