Active vs Passive FTP
No single topic causes more FTP confusion — or more support tickets — than the difference between active and passive mode. The distinction is actually simple: it is entirely about who opens the data connection. But that one detail collides with firewalls, NAT, load balancers, and decades of default settings in ways that keep catching administrators out.
This series covers the two modes end to end: the definitive explanation, the firewall story, systematic troubleshooting, proper server-side passive configuration, the four commands on the wire, and the advanced case of FTP behind load balancers. Start with the first article if the modes have never quite clicked; jump straight to the diagnosis or configuration guides if something is broken right now.
Articles in This Series
- Active vs Passive FTP, Explained Until It Finally Sticks
The definitive plain-English explanation of FTP's two connections, who opens what in each mode, and why “switch it to passive” fixes so much. - Why Firewalls Block Active FTP (and What to Do About It)
Stateful firewalls, NAT, and connection helpers — why active mode's callback fails on modern networks, and the fixes that actually work. - Diagnosing FTP Mode Failures from Logs and Captures
The symptom patterns (hangs at LIST, 425/426 errors), how to read session logs and decode PASV replies, and three fully worked incidents. - Configuring Passive Port Ranges Properly on Your FTP Server
Sizing the range, choosing the numbers, announcing the right external IP behind NAT, matching firewall rules, and verifying from outside. - PORT, PASV, EPRT, and EPSV: FTP's Four Data-Connection Commands
What each command puts on the wire, decoding the six-number replies by hand, and how IPv6 changed the negotiation. - Running FTP Behind Load Balancers and Proxies Without Breaking It
Session affinity, passive-range strategies per backend, health checks that don't lie, and DMZ publishing patterns that survive real load.
Explore More Topics
This series is part of the Sysax file transfer topic library, which covers the protocols, security practices, automation techniques, and operational skills behind reliable file transfer. The library pairs well with the practical tools we build: Sysax Multi Server, a secure FTP, FTPS, SFTP, and HTTPS server for Windows, and Sysax FTP Automation, which schedules and scripts secure transfers so the routine ones run themselves.