FTP passive mode issues.

Passive mode in FTP (File Transfer Protocol) is commonly used to work around issues related to firewalls and network configurations. If you're experiencing issues with FTP passive mode, here are steps to troubleshoot and resolve the problem:

1. Understand Passive Mode:
   • In FTP passive mode, the client opens a random port for data transfer. The server then connects to this port. If passive mode is failing, it often indicates a problem with the client-server communication.
2. Verify Firewall Settings:
   • Ensure that both the client and server firewalls allow the necessary ports for passive FTP (usually a range of ports). This range is configurable on the server and should be allowed in the firewall.
3. Check Server Configuration:
   • Confirm that the FTP server is configured to use passive mode and that it is set up to use the correct range of ports. Additionally, verify that the server's external IP address is correctly set.
4. Use a Passive FTP Tester:
   • There are online tools available that can help diagnose passive FTP issues by simulating a passive FTP connection. They can provide valuable insights into what might be causing the problem.
5. Verify NAT and Routing:
   • If the FTP server is behind a router performing Network Address Translation (NAT), ensure that the NAT device is configured to forward passive FTP ports to the server.
6. Check for Load Balancers or Reverse Proxies:
   • If there are load balancers or reverse proxies in the network architecture, make sure they are configured to handle passive FTP traffic correctly.
7. Use Explicit FTPS (FTP over SSL/TLS):
   • If possible, consider using explicit FTPS. It encrypts the control and data channels, which can sometimes help bypass firewall issues.
8. Verify FTP Client Settings:
   • Check the FTP client settings to ensure it is configured to use passive mode. Most modern FTP clients default to passive mode, but it's worth confirming.
9. Check Passive Mode Ports:
   • Verify that the passive mode port range on the server does not overlap with any other services or applications using those ports.
10. Test Connectivity Using Active Mode:
    • Test if FTP connections work using active mode. If active mode works but passive mode does not, it may indicate a passive mode-specific issue.
11. Check Passive Mode Address Configuration:
    • Ensure that the FTP server is configured to provide the correct external IP address to clients in passive mode responses. This may involve setting an "external IP" or "masquerade address" in the FTP server configuration.
12. Verify NAT Reflection/Loopback:
    • If the FTP server is accessed from within the same network, ensure that NAT reflection or loopback is correctly configured on the router/firewall.
13. Consult FTP Server Documentation:
    • Refer to the documentation of your FTP server software for specific troubleshooting steps related to passive mode.
14. Check for Known Issues and Updates:
    • Research if there are any known issues or updates related to passive mode for your specific FTP server software.

Remember, passive mode issues often involve a combination of network configurations, firewalls, and FTP server settings. It may require some trial and error to pinpoint the exact cause and solution.