Can I connect to services outside of Heroku?
Please note that opening up your security group will not work if your service is running inside of an Amazon VPC.
Yes. Heroku runs on Amazon EC2, so any servers you run on EC2 will have the possibility of a low-latency connection from Heroku. For optimal latency, run your services in the us-east region.
If you’d like to open your EC2 server(s) up only to Heroku, you can use a command like the following:
$ ec2-authorize YOURGROUP -P tcp -p 3306 -u 098166147350 -o default
This will open up access to TCP port 3306 in the YOURGROUP security group from Heroku.