If you are trying to connect to AWS RDS Database let’s say from an EC2 instance or a client and you are getting the error ‘Can’t connect to Mysql Server (110)’, then the most probable cause is the security group configuration in AWS RDS. Check whether the incoming rule is defined to allow instances to connect on the AWS RDS port. If its mysql, then the port is most likely 3306 or as defined in your RDS configuration.
If in case, you have defined your rule but still can’t connect, try putting in the default IP 0.0.0.0/ and check whether you are able to connect. If you are able to connect, then the most likely cause is that you had configured your custom IP wrongly. I was trying to connect to RDS and wasted so much of my time assuming that my EC2’s instance IP is a certain IP and that was wrong.
A Security Consultant's Diary 