PART 3.
They made their STUPID MODEMS FUCK WITH THE FTP PACKETS. I have personally experienced this with Comcast Business. The stupid piece of shit DOCSIS modem they provide intercepts the FTP packet from your server saying "oh, connect to this address: x.x.x.x:44010" and they rewrite the fucking address to the public IP. There is no way to turn just this horse piss off. Now, for average business customers, this probably saved Comcast a bunch of money in support calls. However, if you're using the so-called bridge mode on that degenerate piece of shit-wrapped-silicon (where rather than allowing the modem to give you a DHCP address, you just configure your system to have one of the addresses in the /29 space and the modem detects that and says oh okay don't NAT traffic when it's going to this address, just rewrite the MAC and shunt it over the right interface), then something funny happens. The modem still rewrites the contents of the packet, but it uses the wrong fucking IP address! Because the public IP that your server is running on is no longer available to the modem, the modem just chooses another fucking address. Then, the client tries to connect to 1.2.3.5 instead of 1.2.3.4 where your server is listening, the modem says "hey I'm 1.2.3.5 and you can fuck off, I'm dropping your SYN for port 44010", and I get an angry call from the client asking why they can't download their files using this worthless protocol. I remember having a conversation like this:
Me: "Just use SFTP on port 22!"
Client: "No! FTP is faster/more secure/good enough for my grandfather good enough for me/corporate won't allow port 22."
Me: "Comcast is fucking me right now. What if we lied and served SFTP over port 21?" # we try it
Client: "It's not working! I can't even connect!"
I couldn't connect either. I couldn't connect to anything. Trying to do SFTP over port 21 caused the stupid fucking modem to CRASH.
Are you starting to see what the problem is? It's like Microsoft preserving bugs in Windows APIs so that shitty software doesn't break, and then they end up doing crazy gymnastics to accomodate old shit like the Windows 8 -> Windows 10 thing where they couldn't use "Windows 9" because that would confuse software into thinking it was running "Windows 95" or "Windows 98". FTP has some bugfuck crazy design decisions that we've collectively decided to just "work around," and it leads to fucking gymnastics.
Speaking of bugfuck crazy design decisions, FTP's default file transfer mode intentionally mangles data!