I need to whitelist Heroku dynos (e.g. on a corporate firewall, on AWS RDS) - what are IP address ranges in use at Heroku?
Heroku Dynos use a subset of the IP range of the underlying AWS EC2 instances.
You can find the underlying AWS region for your app here: https://devcenter.heroku.com/articles/regions
The IPs in use by Heroku at any given time are highly dynamic, meaning that the published ranges may cover other IP addresses not currently in use by Heroku. This means that it is often not desirable to open up your firewall to the whole of the AWS region for security reasons. If you wanted to do this regardless you can find the published IP ranges from AWS here: http://docs.aws.amazon.com/general/latest/gr/aws-ip-ranges.html
A better approach would be to use an add-on to provide a static outbound IP address https://elements.heroku.com/addons/categories/network or to rely on secure communication via TLS.