Acme-challenge returns a 404

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My domain is:

I ran this command: sudo certbot certonly --standalone --debug-challenges -v --dry-run

It produced this output: Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator standalone, Installer None
Please enter the domain name(s) you would like on your certificate (comma and/or
space separated) (Enter 'c' to cancel):
Simulating a certificate request for
Performing the following challenges:
http-01 challenge for

Challenges loaded. Press continue to submit to CA.

The following URLs should be accessible from the internet and return the value

Expected value:

My web server is (include version): standalone

The operating system my web server runs on is (include version): Ubuntu 23.10

My hosting provider, if applicable, is: N/A

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

The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot): Certbot 2.7.4

You have an nginx server that responds to HTTP requests. Did you stop it before trying to use --standalone? Because standalone requires exclusive use of port 80.

Also, since you have nginx you could just use the --nginx plug-in or --webroot to satisfy the challenge.

Can you explain more what you are trying to do and why you chose --standalone? We can give better advice then.


I'm trying to set up a backup MX server. Nginx is running on my main server, not mx2. Should I stop nginx on the main before running certbot on the backup? Or maybe use the nginx plugin, even though nginx isn't on the backup server?

Your DNS for mx2 has an A record pointing to the same IP as your main nginx server. The Let's Encrypt server uses that IP to make an HTTP request and your nginx responds.

Yes, you can just setup a simple server block in nginx to handle that ACME challenge for mx2. Then copy the cert for your mail server.

You could use the --deploy-hook to copy it. See Certbot docs

Or stop nginx before using standalone but usually people want to avoid that.


So I can run certbot on the main server then just copy the certificates to mx2? Just making sure I understand.

Sure, a cert is just a couple files. Just keep them secure as you would on the main server.

Or, do you need the DNS A record for mx2 to point to your main server? Because if not you could just point that to where mx2 is and run Certbot standalone there. Still though it needs exclusive access to port 80.




I think the IP in DNS will come into play [at some point].

Even if you use the cert on MX2 only for SMTP, anyone trying to reach it would need to know it's IP.


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