tech stuff.

It turns out that when you (a) disable the “disable ftp-proxy” on the WAN interface and (b) create a NAT rule that uses port 21, the ftp proxy will be automatically (and silently) started in the background.  If you then modify that NAT rule, the ftp proxy application will not be updated (presumably until a reboot), but if you delete and create the rule again the ftp proxy will be started anew with the updated information.