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Amazon EC2 Instance cannot Connect to Amazon RDS Database Server

While designing a new deployment system for my company with Amazon's AWS, I stumbled onto a problem that cost me some time - I could not get my EC2 instances to connect to our Amazon RDS database servers. I figured I'd document the solution here for the sake of those to follow.

I had created two webservers, and a MultiAZ instance of an RDS server with which they were to transact. I could connect to each webserver and the RDS server directly, but I could not get the webservers to connect to the RDS. The issue ultimately ended up being related to my security groups configuration.

Failed Configuration

My initial strategy for whitelisting the webservers was the following:

While seeming logical enough, that strategy simply didn't work.

Successful Configuration

This, however, did work:

When you're trying to whitelist the EC2 security group against the DB Security Group, you'll need two pieces of information:

I had to stumble around a bit to find that information. Here's where to locate it:

Security Group

Under the "EC2" tab, under "Security Groups" in the left-hand navigation: your Security Groups will be listed in the upper content pane. The designator you need is actually just the human-readable group name (like "default") rather than the Group ID from the lower detail pane. It took me a few tries to realize this.

AWS Account ID

Click on the "Account" link in the navigation at the very top of the page. From there, go to "Manage Your Account". On the subsequent screen, you can find your account number in very small text near the top right-hand corner of the page.

If you authorize your EC2 instances that way, they should be able to connect to your RDS.