After updating ProFTPd on a server running FreeBSD, certain clients (FireFTP specifically) were able to log in, but not transfer files or retrieve a directory listing. The following error was shown to the client user:
425 Unable to build data connection: Operation not permitted
It appears that a recent change in ProFTPd's default TLS handling rules requires SSL sessions to be re-used, which FireFTP does not do. Looking at the TLS log file shows these kinds of errors:
Sep 17 12:31:43 mod_tls/2.4.1: client did not reuse SSL session, rejecting data connection (see TLSOption NoSessionReuseRequired)
The fix is simple, add the following to proftpd.conf and restart ProFTPd:
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