Certbot failed to authenticate

[anzenctf]

My domain is:hello.anzenctf.live

I ran this command:
sudo certbot certonly --standalone

It produced this output:
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Please enter the domain name(s) you would like on your certificate (comma and/or
space separated) (Enter 'c' to cancel): hello.anzenctf.live
Requesting a certificate for hello.anzenctf.live

Certbot failed to authenticate some domains (authenticator: standalone). The Certificate Authority reported these problems:
Domain: hello.anzenctf.live
Type: unauthorized
Detail: 52.140.39.16: Invalid response from http://hello.anzenctf.live/.well-known/acme-challenge/FkHJYz6d4AgC95p-nZm9ugIqCWmfKtSi4zcdsTqaRtc: 404

Hint: The Certificate Authority failed to download the challenge files from the temporary standalone webserver started by Certbot on port 80. Ensure that the listed domains point to this machine and that it can accept inbound connections from the internet.
My web server is (include version):

The operating system my web server runs on is (include version):Ubuntu 20 Virtual machine hosted on azure

My hosting provider, if applicable, is:Azure

I can login to a root shell on my machine (yes or no, or I don't know):yes

I'm using a control panel to manage my site (no, or provide the name and version of the control panel):no
d
The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot):certbot 2.10.0

[Osiris]

There's an nginx webserver responding on the hostname hello.anzenctf.live. It's serving a webpage for "AnzenCTF", is that correct?

As a matter of fact, it's claiming to be nginx 1.26.0. But Ubuntu doesn't ship with nginx 1.26.0. Focal ships with 1.18.0 as does Ubuntu 22 (jammy). And even Ubuntu 23 (mantic) and more recent only ship 1.24.0.

So I'm curious where the Ubuntu 20 without a webserver is fitting into this picture Comparing to a (probably) non-Ubuntu server with nginx 1.26.0.

[Bruce5051]

Supplemental information: presently I see Port 80 & 443 filtered.

$ nmap -Pn -p80,443 hello.anzenctf.live
Starting Nmap 7.80 ( https://nmap.org ) at 2024-05-13 17:50 UTC
Nmap scan report for hello.anzenctf.live (52.140.39.16)
Host is up.

PORT    STATE    SERVICE
80/tcp  filtered http
443/tcp filtered https

Nmap done: 1 IP address (1 host up) scanned in 3.26 seconds

[rg305]

Hi @anzenctf, and welcome to the LE community forum

Let's verify that your DNS is pointing to the current IP, with the output of:
curl -4 ifconfig.me

[anzenctf]

Yes my ip points to the domain

[rg305]

Then you need to ensure that the HTTP requests reach the ACME client running in standalone mode.