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Editorial Team
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Asked: 2026-06-13T05:32:33+00:00

We have had a windows service running without any issues for some time but

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We have had a windows service running without any issues for some time but is now throwing 500, invalid port command errors when attempting to make an FTP connection.

Using Core FTP lite client from the same server I can connect to the FTP server in Active and Passive modes without any problems. The firewall rules have not changed and the application code has not changed at all.

Any ideas why only when attempting to connect from the windows app the Invalid Port Command error is thrown would be much appreciated.

Thank you,

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1 Answer

  1. Editorial Team

    I tried the second solution on this post and it solved my problem. I also had to upgrade to IIS 7.5 FTP and allow the passive ports on both my WIndows 2008 Server Firewall and my external Firewall

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