We had a difficult time achieving this (we ended up whitelisting every domain)
IP's
The problem is Dyno's are hosted on AWS' EC2 cloud - meaning they aren't actually Heroku's servers. This causes a lot of problems, as the IPs are all shrouded & change:
Because the Heroku dyno grid is dynamic in nature, the IP address that
a given dyno will be assigned over time will be both dynamic and
unpredictable. This dynamic sourcing of outbound traffic can make it
difficult to integrate with APIs or make connections through firewalls
that require IP-based whitelisting
After seeing the proximo addon, you may be able to achieve what you need using a static IP
Proximo
According to the proximo tutorial on Heroku's site, you should be able to install the add-on & receive your outbound IP relatively simply:
$ heroku addons:add proximo:development
Adding proximo to sharp-mountain-4005⦠done, v18 ($5/mo)
Your static IP address is 127.0.0.1
You should then be able to use this on your db host - to allow the IP