When examining the FileZilla Server logs, we can see the critical sequence where the connection fails:

(000042)admin (xxx.xxx.xxx.xxx)> PASV
(000042)admin (xxx.xxx.xxx.xxx)> 227 Entering Passive Mode (172,16,1,9,27,5)
(000042)admin (xxx.xxx.xxx.xxx)> MLSD
(000042)admin (xxx.xxx.xxx.xxx)> 425 Can't open data connection

The key problem lies in passive mode FTP implementation across different network environments. The server responds with its local IP (172.16.1.9) which isn't routable from external networks.

In FileZilla Server Interface (Edit > Settings > Passive mode settings):

1. Check "Use the following IP" and enter your public IP
2. Set "Use custom port range": 50000-51000
3. Add these ports to your firewall exceptions

For Windows Defender Firewall with Advanced Security:

netsh advfirewall firewall add rule name="FileZilla Passive Ports" 
dir=in action=allow protocol=TCP localport=50000-51000

Essential NAT forwarding rules:

External Ports: 50000-51000 → Internal IP: 172.16.1.9
TCP Protocol only
Should match FileZilla's passive port range

After implementing these changes, check the connection:

Status: Server sent passive reply with public IP (x.x.x.x)
Command:    MLSD
Response:   150 Opening data channel for directory listing
Response:   226 Successfully transferred "/"

If passive mode continues to cause issues, consider switching to active mode in FileZilla Client:

Edit > Settings > Connection > FTP > Transfer Mode
Change from "Passive" to "Active"

Here's a PowerShell script to verify your FTP server configuration:

$server = "your.server.ip"
$port = 2121
$client = New-Object System.Net.Sockets.TcpClient
$client.Connect($server, $port)
$stream = $client.GetStream()
$writer = New-Object System.IO.StreamWriter($stream)
$reader = New-Object System.IO.StreamReader($stream)

$writer.WriteLine("USER yourusername")
$writer.WriteLine("PASS yourpassword")
$writer.WriteLine("PASV")
$response = $reader.ReadToEnd()

Write-Host "Server Response:n$response"
$client.Close()

---

When implementing remote FTP connections using FileZilla Server, many administrators encounter the frustrating "425 Can't open data connection" error during passive mode transfers. The logs clearly show successful authentication but failure at the MLSD command phase.

The key evidence in your logs:

227 Entering Passive Mode (172,16,1,9,27,5)
Server sent passive reply with unroutable address

This indicates the server is advertising its internal IP (172.16.1.9) to external clients, making the data connection unreachable.

For successful passive FTP:

1. Server must know its external IP
2. Firewall must allow the passive port range
3. NAT must forward ports correctly

In FileZilla Server Interface (Admin):

1. Edit -> Settings -> Passive mode settings
2. Check "Use custom port range": 50000-50100
3. Enter external IP in "Use the following IP"
4. Configure firewall/NAT to forward 50000-50100

For automated setups, modify FileZilla Server.xml:

<PassiveMode>
  <Ports>
    <Range Min="50000" Max="50100" />
  </Ports>
  <ExternalIP>your.public.ip</ExternalIP>
</PassiveMode>

For environments where you can't control NAT:

Solution 1: Active Mode

# In FileZilla Client:
Site Manager -> Transfer Settings -> Active mode

Solution 2: SFTP Alternative
Consider using SFTP instead:

# OpenSSH configuration example (/etc/ssh/sshd_config):
Subsystem sftp /usr/lib/openssh/sftp-server
Match Group sftpusers
  ChrootDirectory /sftp/%u
  ForceCommand internal-sftp
  AllowTcpForwarding no

Verify each component:

• Test port connectivity: telnet your.server.ip 50000
• Confirm NAT rules with iptables -t nat -L -n -v
• Check Windows firewall: netsh advfirewall show currentprofile

For production environments, implement:

• Port range monitoring script:

#!/bin/bash
for port in {50000..50100}; do
  nc -zv your.server.ip $port || echo "Port $port failed"
done

Centralized logging for FTP events

Failover configuration for high availability