An URL that is compliant to RFC 3986 (URL/URI RFC) and especially is compliant with RFC 6762 (mDNS) is rejected, even though it should not be.

In Detail:

django URLValidator only excepts Non-FQDN: "localhost".
This is a dirty fix. But actually in many scenarios (intranet, local DNS, mDNS) no FQDN is required or desirable.

When wanting to authenticate a ACS URL that matches that criteria - this is an issue.

I created a pull request for django django/django#19095.

So - maybe one of you can lobby for that change - or knows about a quicker solution in authentik itself?!

Thanks for your work!!

best regard
ludwig

EDIT: created django TRAC ticket: https://code.djangoproject.com/ticket/36131