FTP/FTPS - Prefer EPSV over PASV for passive data connections

Current Situation

• For FTP passive mode:
  • The control connection currently sends PASV exclusively.
    • PASV requests the server to open a passive data port and return both the server IP address and port to the client.
    • This is an IPv4-specific mechanism.
  • The actual data connection is then established using the address returned by the server.
    • Apache Commons Net handles the data connection itself and generally uses the appropriate mechanism where supported.
• Consequences
  • Using PASV exclusively creates problems in environments where the server or the network path uses IPv6.
    • The client cannot reliably control whether the address returned by the server is IPv4.
    • For example, the hostname used for the connection may resolve to an IPv6 address, or a proxy may resolve the destination and return/use an IPv6 address.
    • This can result in an address-family mismatch and subsequently cause connection failures or exceptions.
  • EPSV does not return an IP address.
    • It only returns the passive port, allowing the existing connection target to determine the address family.
    • This makes EPSV suitable for both IPv4 and IPv6.

Desired Behaviour

• Prefer EPSV over PASV for passive FTP/FTPS connections, including IPv4.
• Fall back to PASV if EPSV is not supported by the server.

Test Instructions

• Test FTP/FTPS using the default Docker FTPS containers, which are typically configured to use IPv4 only.
• Test against an FTP/FTPS server that supports IPv6, e.g. a locally installed FileZilla Server.
  • Make sure that the FileZilla Server is configured to accept IPv6 connections:
    • Open FileZilla Server Administration.
    • Go to Server - Configure - Server Listeners.
      • Make sure the following listeners are configured:
        • :: 21 Explicit FTP over TLS and insecure plain FTP
        • :: 990 Implicit FTP over TLS (deprecated)
  • Configure the test to use the server's IPv6 address instead of its hostname.
    • Use ipconfig to determine the server's IPv6 address.
  • Verify in the FileZilla Server interface/log that the connection was received over IPv6 and processed successfully.
• Expected Result:
  • FTP/FTPS connections work successfully with both IPv4 and IPv6.