DNS CNAMEs on Root of Domain
|Applies to customers with a managed DNS service from VSHN|
CNAMEs on the root of a zone are technically not possible. For a detailed description of the issue, the post at CloudFlare "CNAME Flattening: RFC-compliant support for CNAME at the root" has good background information on the topic.
We implemented a similar behaviour and introduced a new unofficial DNS resource record type: ALIAS.
To use it, just assign this new record type to the entry:
@ IN ALIAS cust55673344.herokuapp.com
The rest is handled by our CI job and a cron job which regularly checks and updates this RR type and converts it to A and AAAA records (a single ALIAS record results in one or more A records and zero or more AAAA records).
This mechanism can only be used for the root of a zone ("@"), not for arbitrary names. For the latter, use regular CNAMEs.