Active and Passive
The low-level details of FTP are ridiculously complex. It opens a control channel on port 21, simple enough. But then when you want to transfer a file the server side makes a request to port 20 on the client... Which modern firewalls (or NAT) don't allow. So PASV mode was invented, which lets the client make a second connection to the server, but on a RANDOM PORT, so that makes it a nightmare to reasonably firewall the FTP server systems. Most firewalls get around this by running an FTP proxy right on the firewall, because there's just no other way... No other protocol in use today is so crazy and convoluted. The old rsh/rlogin/rcp commands/protocols are a good candidate, though, but almost extinct, where FTP keeps going.
It's a shame nobody ever put together a command-line file transfer client for HTTP transfers. HTTP has got more error/status codes than FTP, supports uploads and downloads, authenticated and anonymous connections, etc. HTTP/1.0 was a perfectly simple protocol, too. Would love to have command-line file transfers over HTTP, without crufty old FTP design decisions.
Biting the hand that feeds IT
