FileZilla IPv6 PASV Parsing: Zone Identifier Support

IPv6 PASV Parsing Behavior

FileZilla generally does not support zone identifiers (e.g., %eth0) within the PASV response. This is because the standard PASV command expects a comma-separated list of numeric values representing the address and port, rather than a formatted string. If a server returns a non-standard string containing a zone identifier, the client's parsing logic will likely fail, resulting in a connection timeout or a "Failed to retrieve directory listing" error.

IPv4-Mapped IPv6 Addresses

IPv4-mapped IPv6 addresses (e.g., ::ffff:192.168.1.1) are typically handled by the underlying operating system's socket layer. FileZilla passes the parsed address to the OS; if the OS supports dual-stack sockets, the connection is established. However, if the server returns these in a non-standard PASV format, the same parsing failures mentioned above apply.

Version Differences and EPSV

While versions prior to 3.52 had more limited IPv6 handling, the primary distinction is not the version number but the command used. The legacy PASV command is ill-suited for IPv6 due to its rigid numeric format. Modern FileZilla releases prioritize EPSV (Extended Passive Mode), which is the RFC-compliant method for IPv6. EPSV returns only the port, assuming the client will connect to the same IP address used for the control connection, thereby bypassing the need to parse complex address strings or zone identifiers entirely.

Verification Steps

To determine if a zone identifier is causing a connection failure, follow these steps:

1. Capture Traffic: Use a network analyzer like Wireshark to capture the raw server response to the PASV command. Check if the server is returning a string containing a % character.
2. Force EPSV: Ensure the client is using EPSV. If the server supports it, this eliminates the address parsing issue.
3. Test IPv4-Mapped: Attempt a connection using a standard IPv4 address to verify if the failure is specific to the IPv6 mapping logic.

Missing Diagnostic Detail: To provide a more specific recommendation, please clarify if the server is responding to PASV or EPSV when the connection fails.