Android Tutorial [B4X] Tip: Always set FTP.PassiveMode to True

Discussion in 'Tutorials & Examples' started by Erel, May 28, 2018.

  1. Erel

    Erel Administrator Staff Member Licensed User

    I've just wasted an hour trying to understand why a desktop program that I'm using for many months failed to upload a file to a FTP server.
    I realized that the only thing that was recently changed is that B4J was running with Java 10. Mistakenly I assumed that it is related to IPv6 handling (the server is using IPv4).

    What actually happened is that after Java 10 was installed, at some point there was a windows message asking whether I allow it to open a firewall port. I didn't allow it.
    The default mode for FTP is active mode. In active mode the server needs to make an incoming connection to the client. This doesn't work on mobile / cellular networks and it doesn't work if the firewall is not configured for this.
    The solution is very simple:
    FTP.PassiveMode = True
    Passive mode doesn't require an incoming connections and should have been the default mode (it will not be changed in the Net library).
    Last edited: May 29, 2018
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