If I make a request to my Daphne/Django server in Postman or the Android app we're developing, Daphne serves the certificate, but it's rejected. If I first make a simple get request to https://letsencrypt.org/ and then make a request to my server, the certificate is accepted.
How can I make sure a client will trust my certificate, even if it's the first time this client is seeing a certificate issued by this CA?
Everything bellow can serve as a history of how I studied the problem.
Original title: SSL Certificate works in browser but can't be verified by Postman
I have an AWS EC2 instance running Ubuntu 18.04, with python 3, Django, a bunch of project dependencies, Daphne running with ASGI, with a certificate by Let's Encrypt. Daphne is using port 8000 for HTTP and por 4430 for HTTPS, iptables is configured to redirect requests from port 80 to 8000 and from port 443 to 4430. Django is configured to enforce secure connections with SECURE_SSL_REDIRECT=True in the settings.py file.
There's a "Site in Construction" temporary page being served, and it's properly accessible from every browser and every device I tested so far. If I explicitly type http, I get redirected to https and the certificate is accepted. Every browser I tested (Firefox, Brave, Chrome, Chrome for Android) says cert is good.
Curl outputs the HTML content returned from the server. I don't know if it accepts the certificate or ignores it.
The Problem
Postman, however, says "Error: unable to verify the first certificate". Only works when I disable "SSL certificate verification", which doesn't answer my question: why Postman is unable to verify my Let's Encrypt certificate?
I'm building an API that runs on the same server, using the same domain, and it's meant to be consumed by a mobile app. Currently, the Android app is throwing a "TypeError: Network request failed", which I suspect could be caused by the same thing Postman is complaining about.
When I spin the server locally and configure 1) the app to use http://localhost:8000 and 2) the server not to enforce SSL, it works in browsers, Postman and in the Android app.
I've being looking for answers in many places for days, so any clue will be very welcome.
EDIT
Interesting clue:
If I make a request to my Daphne/Django server, it servers the certificate, which is rejected. But if I first make a request to https://letsencrypt.org/ and then make a request to my server, it works!
This pattern holds true in both Postman and our Android app.
It also happens when I first make a request to https://alloy.city (instead of letsencrypt.org), which is served by a Node.js app, and uses a certificate also issued by the Let's Encrypt CA.
So maybe the question should be: how to configure my server to politely invite clients to add the CA that issued my certificate if they hadn't done it yet?.
Apparently, that's what my Node.js server does.
