Stateless Mode

Configure your webserver to respond statelessly to challenges for a given account key. This requires nothing more than a one-time web server configuration change and no "moving parts".

First get your account key thumbprint:

root@ed:~# acme.sh --register-account
[Mon Feb  6 21:40:18 CST 2017] Registering account
[Mon Feb  6 21:40:19 CST 2017] Already registered
[Mon Feb  6 21:40:21 CST 2017] Update success.
[Mon Feb  6 21:40:21 CST 2017] ACCOUNT_THUMBPRINT='6fXAG9VyG0IahirPEU2ZerUtItW2DHzDzD9wZaEKpqd'

Remember the thumbprint in the last line: 6fXAG9VyG0IahirPEU2ZerUtItW2DHzDzD9wZaEKpqd

Configure the web server to return the account key thumbprint:

NGINX

Add something similar to your nginx.conf:

http {
...
  server {
  ...
    location ~ ^/\.well-known/acme-challenge/([-_a-zA-Z0-9]+)$ {
      default_type text/plain;
      return 200 "$1.6fXAG9VyG0IahirPEU2ZerUtItW2DHzDzD9wZaEKpqd";
    }
  ...
  }
}

CADDY

Add something similar to your Caddyfile:

example.com {
  @achallenge {
    path_regexp ch ^/\.well-known/acme-challenge/([-_a-zA-Z0-9]+)$
 }
 respond @achallenge "{re.ch.1}.6fXAG9VyG0IahirPEU2ZerUtItW2DHzDzD9wZaEKpqd"

APACHE

Add something similar to your httpd.conf:

LoadModule php7_module libexec/apache2/libphp7.so
...
<VirtualHost *:80>
    ...
    <IfModule php7_module>
        AddType application/x-httpd-php .php
        AddType application/x-httpd-php-source .phps
        <IfModule dir_module>
            DirectoryIndex index.html index.php
        </IfModule>
    </IfModule>
    ...
    <Directory "/PATH/TO/WWW/ROOT/.well-known/acme-challenge/">
        RewriteEngine On
        RewriteRule "^[-_a-zA-Z0-9]+$" "index.php"
    </Directory>
    ...
</VirtualHost>
...

Then add /PATH/TO/WWW/ROOT/.well-known/acme-challenge/index.php:

<?php
header("Content-Type: text/plain");
$token = array_pop(explode('/',$_SERVER['REQUEST_URI']));
echo "$token.6fXAG9VyG0IahirPEU2ZerUtItW2DHzDzD9wZaEKpqd";
?>

NOTE that this approach uses PHP but something similar could be done with CGI, Perl, Ruby or pretty much any other server-side language.

HAPROXY

Add the http-request return rule to your configuration:

global
    setenv ACCOUNT_THUMBPRINT '6fXAG9VyG0IahirPEU2ZerUtItW2DHzDzD9wZaEKpqd'
    log stderr local0
    stats socket /var/run/haproxy.sock level admin mode 0666

frontend web
    log global
    option httplog
    mode  http
    bind :80
    bind :443 ssl crt /etc/haproxy/certs/
    http-request return status 200 content-type text/plain lf-string "%[path,field(-1,/)].${ACCOUNT_THUMBPRINT}\n" if { path_reg '^/.well-known/acme-challenge/[-_a-zA-Z0-9]+$' }

Ok, you can issue cert now.

acme.sh --issue -d example.com  --stateless