ALLOWED_HOSTS breaks on subdomains that contain underscores

[jason@…]

I have allowed hosts set to * on a multitenant CMS, for which users get subdomains that are formatted like this: username.portfolios.mysite.com

However, for users with an underscore in their username, the app 500's with a "SuspiciousOperation: Invalid HTTP_HOST header (you may need to set ALLOWED_HOSTS): some_username.portfolios.mysite.com" but works fine for someusername.portfolios.mysite.com

[Claude Paroz]

The underscore character is not valid in hostnames (see also explanation in #18517). You can patch django.http.request.host_validation_re in your project, but you've been warned that you don't conform to the standard.

[Will Hardy]

I agree that underscored host names should not be accepted, but the validation is performed even when DEBUG = True (ie ALLOWED_HOSTS = ["*"]).

This was annoying because I was using a custom name in /etc/hosts to connect to a test server and it wasn't clear why it had failed (the 500 debug page could not be displayed).

[anonymous]

I gave this side issue its own ticket: #19987