ok ok, all people could install easily Proftpd or alike on unix server and all people can configure MySQL to integrate the authentication (or not? Install Proftpd on debian).

But how configure proftpd with mod_sql on EC2 with RDS?

as easy as pie if you know! Set MasqueradeAddress with EC2 public DNS and SQLConnectInfo with RDS endpoint!

MasqueradeAddress ec2-xxx-xxx-xxx-xxx.eu-west-1.compute.amazonaws.com

SQLConnectInfo table_name@endpoint.eu-west-1.rds.amazonaws.com user password