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Our website is hosted with loopia at example.com, but we also have a webapp running on Google App Engine which we would like to run on subdomain.example.com.

We also have a couple of other subdomains running, but those are simply referred to via the IP address.

I have followed the steps at https://cloud.google.com/appengine/docs/standard/python/mapping-custom-domains#dns_update. The Domain Mappings consist of four A, four AAAA and one CNAME records.

When I try to add all of them to example.com (the documentation isn't clear if this should be the domain or subdomain), I get an error saying there are colliding records. When I try to add them to subdomain.example.com, I'm able to add either the As and AAAAs or the CNAME, but not both, or I'll get a colliding record error again.

In both cases, subdomain.example.com just displays a Google backed 404 error, while example.appspot.com is working without problems.

This is very similar to this question, except that we're not using Google as our registrar.

How do I properly set up the DNS records so that the subdomain points to my webapp?

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You need only to use one of the following for the subdomain:

  • A and AAAA records
  • CNAME record

You can't use both at the same time for the same subdomain. Because it is a subdomain, I'd recommend using the CNAME record. Google likes to use dynamic IP addresses for its hosting. If you use A records that specify an IP address, you might have to update them at some point. If you use a CNAME pointing to Google's domain, Google will keep the IP address up to date for you.

You don't want to touch other DNS records other than for the subdomain.

  • Don't edit the domain apex @ records. Those are for the bare domain and should usually point to the IP address that hosts your main www website. (Note this particular record cannot be a CNAME and has to be an A record.)
  • Don't edit the www records. That will break your main site.
  • Don't edit the records for your other subdomains, doing so would break them.
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