I'd like to interject for a moment. What you're referring to as FTP is, in fact, smelly hot garbage.
For context, I wrote this while waiting for a migraine to pass. I was angry at my brain for ruining my morning, and I like to shit on FTP. It's fun to be hyperbolic. I don't intend for this to be an attack on you, I was just bored and decided to write this ridiculous rant to pass the time.
I must once again rant about FTP. I've no idea if you're serious about liking it or you're just taking the piss, but seeing those three letters surrounded by whitespace reminds me of all the bad things in the world.
FTP is, as I've said, smelly hot garbage, and the infrastructure built to support FTP is even worse. Why? Well, one reason is that FTP has the most idiotic networking model conceivable. To see how crazy it is, let's compare to a more sane protocol, like HTTP (for simplicity's sake, I'll do HTTP/1.1). First, you get the underlying transport protocol stuff and probably SSL. The HTTP client opens a connection from some local ephemeral port to the destination server on port 80/443/whatever and does all the normal protocol things (so syn->synack->ack and Client Hello -> Server Hello+server cert -> client kex+change cipher -> change cipher -> encrypted data). FTP does TCP too! Same same so far (minus SSL, unless you're using FTPS). Next, the HTTP client goes like this:
GET /index.html HTTP/1.1
Host: www.whatever.the.fuck
# a bunch of other headers
and you know what fucking happens here? The fucking server responds with the data and a response code on the same goddamn TCP connection. You get a big, glorious response over the nice connection you established:
200 OK
# a bunch of headers and shit
HERE'S YOUR DAMN DATA NERD
So that's nice, and the client you're using to read this used that flow (or an evolution of that flow if you're using HTTP/2 or HTTP/3). So what does FTP do? It does one of two really stupid things depending on whether you're using active or passive mode. Active mode is the default for the protocol (although not the default for most clients), so let's analyze that! First, your FTP client initiates a TCP connection to your server on port 21 (by default), and then the server just sends this:
<--- 220 Rebex FTP Server ready.
ok, that kinda came out of nowhere. You're probably using a modern client that saves you from all of the godawful footguns, so it then asks the server what it supports:
---> FEAT
<--- 211-Supported extensions:
<--- AUTH TLS;SSL;
<--- CDUP
<--- CLNT
# A whole bunch of other 4 letter acronyms. If I was writing an FTP server, I'd make it swear at the user since there are a lot of fun 4 letter words
There's some other bullshit we don't care about right now, although highlights include sending the username and password in plain text. There's also ASCII vs binary mode. WE'LL GET BACK TO THAT. :|
So then we want to do a LIST. You know what happens in active mode? Your computer opens up some random fucking TCP port. It then instructs the FTP server to CONNECT TO YOUR GODDAMN COMPUTER. Your computer is the server, and the other side is now the client. I would post a more detailed overview of the FTP commands, but most servers on the internet disable active mode because it's a goddamn liability. All of the sudden, your computer has to be internet facing with open firewall ports, and that's just a whole heap of shit.
I'm probably not blowing many minds right now because people know about this shit. I just want to mention that this is how FTP was built. The data plane and control plane are separate, and back in 19XX when this shit was invented, you could trust your fellows on ARPANET and NAT didn't exist and sure HAM radio operators here's the entire goddamn 44.0.0.0/8 block for you to do packet switched radio. A simple protocol for simple times, back before we knew what was good and what was bad.
So, active mode sucks! PASV is the future, and is the default on basically all modern clients and servers! Passive mode works exactly the same as the above, except when the client goes to LIST, the server opens some random TCP port (I've often seen something like 44000-44010) and tells the client, "hey you, connect to 1.2.3.4:44000 to get you your tasty data." Sounds great, right? Well, there's a problem that I actually touched on in my last paragraph. Back when this dogshit was first squeezed out in the 70s, everyone had a public address. There were SO MANY addresses! 4 billion addresses? We'll never use all of those! That is clearly not the case anymore. We don't have enough addresses, and now we have this wonderful thing called NAT.