FTP — File Transfer Protocol

FTP (RFC 959, 1985) is one of the oldest internet protocols. Designed for transferring files between machines.

Architecture:

Client                          Server
   |                                |
   | Control connection (port 21)  |
   |<------------------------------>|
   |   Commands + responses         |
   |                                |
   | Data connection (port 20 or  |
   |  ephemeral port)               |
   |<------------------------------>|
   |   File contents                |

Two TCP connections per session:

1. Control: persistent, text-based commands.
2. Data: per file/listing transfer; opened, transfer, closed.

Why two? FTP designed when networks were slow and packet-switched: control bytes shouldn't get stuck behind data.

Modern verdict: FTP is largely deprecated for new deployments. Reasons:

• Unencrypted by default (FTPS adds TLS, but messy).
• Two ports = firewall + NAT nightmares.
• Complex protocol (active vs passive modes).
• Modern alternatives: SFTP (SSH-based), HTTPS upload, S3 pre-signed URLs.

Still in use:

• Legacy systems (mainframes, embedded).
• File distribution (sftp.gnu.org, kernel.org).
• IoT firmware updates.
• Some ISPs use FTP for log delivery.

Real implementations:

• vsftpd (Very Secure FTP Daemon).
• Pure-FTPd.
• ProFTPD.
• FileZilla Server.
• IIS FTP (Microsoft).

We'll build a basic FTP server, then discuss FTPS, SFTP, and modern alternatives.