Nearly all FTP connectivity problems trace back to firewall settings. FTP uses multiple network ports to transfer data between a client and a server. If any required port is blocked, file transfers fail.

You’ll often notice this problem when your FTP client connects successfully and can browse folders, but uploads or downloads never complete.

FTP communication uses two types of connections: a Command channel and one or more Data channels.

The Command channel manages the session, including authentication and commands such as listing folders or initiating transfers.

The Data channels handle the actual file content during uploads and downloads.

Command channels typically use a well-known port, such as 21 or 990, which network administrators can easily open on firewalls. The Command channel always starts from the client and connects to the server.

Data channels, however, use random ports, and their direction depends on whether you’re using Active or Passive mode. Most connectivity issues occur when the firewall blocks these Data channel connections. Understanding the difference between Active and Passive FTP modes helps you troubleshoot and configure your network correctly.

Common Troubleshooting Tips

Use Passive mode whenever possible. It avoids inbound connection problems and works best across firewalls and NAT networks. Avoid Active mode unless no firewall exists between your FTP client and Files.com.

If you can connect and browse directories but cannot upload or download files, your firewall is likely blocking the Data channel. Verify that connections to the Files.com Passive (PASV) port range are allowed through your firewall.

FTP creates files using the OPEN command over the Command channel before transferring data through the Data channel. If you see zero-byte files appearing during failed uploads or downloads, your firewall is blocking the Data channel. Confirm that the Files.com Passive port range (40000 to 50000) is open through the firewall.

Files.com uses valid, chained SSL certificates for FTPS connections. You do not need to enable insecure or self-signed certificates in your client. If your client reports invalid, self-signed, or expired SSL certificates when connecting, check your DNS configuration to ensure you’re resolving the correct Files.com IP addresses.