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When i use some websites like lets's enrcipt or ssl for free i found everytime an error. Are they a way to obtain a ssl for free? (cloudflare block websocket so i can't use it)
Please check the knowledge base first before posting. There is an article in it which specifically references both SSL providers you listed: https://infinityfree.net/support/cannot-use-or-have-problems-using-lets-encrypt-or-sslforfreecom/
Also, note that our servers don't support websockets either.
Infinityfree's service does not provide a way for either DNS-CAA or FILE-CAA check, because it blocks bots from accessing http and does not allow strange DNS records.
I suggest you to use CloudFlare SSL or point your domain to another nameserver which resolves your website to the current IP address and can also resolve TXT records(CloudFlare DNS is a great solution).
By the way, comodoca supports CNAME verification but I cannot sure whether Infinityfree can pass the verify or not(I'm not able to verify it because iFastNet seems to be debugging the cPanel and I cannot access cPanel). The TrustOcean Encryption365 CA - RSA - 2018 is based on comodoca and you can request it for free at here(This is a Chinese website, I can't guarantee that it has an English translation so you may translate it using your browser).
TrustOcean Encryption365 CA - RSA - 2018