ftp port in aws still closed after update security group

Juan Carlos Oropeza asked:

I went to security group and add the rule for tcp 21

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But when use this tool to check the port, still say is closed.

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I know the tool works, because if I remove the line for http, then say port 80 is closed.

So why even when add the port to the security group still appear as closed?

My answer:

A service must be running on the port for it to be open. Start your FTP server.

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