To receive a certificate from Let's Encrypt certificate authority (CA), you must pass a challenge to prove you control each of the domain names that will be listed in the certificate. A challenge is one of a list of specified tasks that only someone who controls the domain should be able to accomplish, such as:
- Posting a specified file in a specified location on a web site (the HTTP-01 challenge)
- Posting a specified DNS record in the domain name system (the DNS-01 challenge)
It’s possible to complete each type of challenge automatically (Certbot directly makes the necessary changes itself, or runs another program that does so), or manually (Certbot tells you to make a certain change, and you edit a configuration file of some kind in order to accomplish it). Certbot's design favors performing challenges automatically, and this is the normal case for most users of Certbot.
Some plugins offer an authenticator, meaning that they can satisfy challenges:
- Apache plugin: (HTTP-01) Tries to edit your Apache configuration files to temporarily serve files to satisfy challenges from the certificate authority. Use the Apache plugin when you're running Certbot on a web server with Apache listening on port 80.
- Nginx plugin: (HTTP-01) Tries to edit your nginx configuration files to temporarily serve files to satisfy challenges from the certificate authority. Use the nginx plugin when you're running Certbot on a web server with nginx listening on port 80.
- Webroot plugin: (HTTP-01) Tries to place a file where it can be served over HTTP on port 80 by a web server running on your system. Use the Webroot plugin when you're running Certbot on a web server with any server application listening on port 80 serving files from a folder on disk in response.
- Standalone plugin: (HTTP-01) Tries to run a temporary web server listening on HTTP on port 80. Use the Standalone plugin if no existing program is listening to this port.
- Manual plugin: (DNS-01 or HTTP-01) Either tells you what changes to make to your configuration or updates your DNS records using an external script (for DNS-01) or your webroot (for HTTP-01). Use the Manual plugin if you have the technical knowledge to make configuration changes yourself when asked to do so, and are prepared to repeat these steps every time the certificate needs to be renewed.
Tips for Challenges
- Run Certbot on your web server, not on your laptop or another server. It’s usually the easiest way to get a certificate.
- Use a tool like the DNSchecker at dnsstuff.com to check your DNS records to make sure there are no serious errors. A DNS error can prevent a certificate authority from issuing a certificate, even if it does not prevent your site from loading in a browser.
- If you are using Apache or NGINX plugins, make sure the configuration of your Apache or NGINX server is correct.
- Make sure the domain name exists and is already pointed to the public IP address of the server where you’re requesting the certificate.
- Make sure port 80 is open, publicly reachable from the Internet, and not blocked by a router or firewall.
- When using the Webroot plugin or the manual plugin, make sure the the webroot directory exists and that you specify it properly. If you set the webroot directory for example.com to /var/www/example.com then a file placed in /var/www/example.com/.well-known/acme-challenge/testfile should appear on your web site at http://example.com/.well-known/acme-challenge/testfile (A redirection to HTTPS is OK here and should not stop the challenge from working.)
- In some web server configurations, all pages are dynamically generated by some kind of framework, usually using a database backend. In this case, there might not be a particular directory from which the web server can serve filesdirectly. Using the Webroot plugin in this case requires making a change to your web server configuration first.
- Make sure your web server serves files properly from the directory where the challenge file is placed (e. g. /.well-known/acme-challenge) to the expected location on the website without adding a header or footer.
- When using the Standalone plugin, make sure another program is not already listening to port 80 on the server.
- When using the Webroot plugin, make sure there is a web server listening on port 80.
- When using the manual plugin, make sure your DNS records are correctly updated; you must be able to make appropriate changes to your DNS zone in order to pass the challenge.