Unsuccessful attempt to block GitHub by Dom.ru?

Question

GitHub

Some time ago, GitHub partially stopped opening when using the provider Dom.ru (ER-Telecom). I am aware of attempts to block it by government agencies and would not be surprised to see a page with a message about blocking. However, Firefox showed something very different:

This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox only connect to it securely. As a result, it is not possible to add an exception for this certificate.
...
github.com uses an invalid security certificate.
The certificate is only valid for the following names: *.ertelecom.ru, ertelecom.ru

Other browsers were allowed to go to GitHub without question, and Firefox also opened other httpS sites without question.

Actually, the question (to a greater extent, "sporting" interest than practical): what is it? What mechanism is implemented and why only Firefox and only GitHub?

Clarification: Internet access is via public Wi-Fi Dom.ru

Answer 1

well, in principle, everything is written in the warning, when you go to github.com you get a certificate for ertelocom.ru , start digging from here, see what certificates are in other browsers, maybe there are proxy settings for ff ?

Answer 2

Firefox is known to implement its own root trust space, while other browsers simply take the list of trusted certificates from Windows. Accordingly, in the described situation, the certificate that you received turned out to be signed by one of the CAs present in the Windows list, and at the same time not in the Mozilla.ORG registry.

Answer 3

Unfortunately, today it was not possible to repeat the problem due to the absence of me near the provider's Wi-Fi point, and GitHub was unblocked in the meantime. I will try to collect as much information as possible in the next "act" of blocking.