Letsencrypt

It's easy in Ubuntu 20.04 using dehydrated.

Install the things

First install the nifty dehydrated utility:

apt install dehydrated

Set up the site's handshake

This script talks to the Letsencrypt CA and temporarily drops tokens in /var/lib/dehydrated/acme-challenges for your site to host for the handshake, deleting them afterwards. So make sure the HTTP side of your site can facilitate the handshake by using a known URL alias without redirecting it to HTTPS. In Apache, you need something like:

<VirtualHost *:80>
  ServerName www.myniftysite.com
  Alias "/.well-known/acme-challenge/" "/var/lib/dehydrated/acme-challenges/"
  RewriteEngine On
  RedirectMatch 301 ^(?!/\.well-known/acme-challenge/).* https://www.myniftysite.com$0
</VirtualHost>

Or the same thing in nginx:

server {
  listen 80;
  server_name www.myniftysite.com;
  location /.well-known/acme-challenge/ {
    alias /var/lib/dehydrated/acme-challenges/;
  }
  location / {
    return 301 https://$host$request_uri; 
  }
}

Fetch your certificates

Then run the following command. Once you've proven it works, bung it in /etc/cron.d/letsencrypt-renew to run every so often (Letsencrypt certificates expire in 3 months).

/usr/local/bin/dehydrated -c --domain www.myniftysite.com --challenge http-01

Set up your HTTPS site

Now you can enable the HTTPS side of your site with the shiny new certificates the Letsencrypt CA generated for you (best to refer to the Mozilla SSL config generator for the most up-to-date configuration).

<VirtualHost *:443>
  SSLEngine On
  SSLCertificateFile /var/lib/dehydrated/certs/www.myniftysite.com/fullchain.pem
  SSLCertificateKeyFile /var/lib/dehydrated/certs/www.myniftysite.com/privkey.pem
  Header always set Strict-Transport-Security "max-age=15768000"
  SSLProtocol all -SSLv3 -TLSv1 -TLSv1.1
  SSLCipherSuite ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:
                 ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:
                 ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES1
  SSLHonorCipherOrder on
  SSLCompression off

  # Ticket option and OCSP Stapling supported in Apache 2.4
  SSLSessionTickets off
  SSLUseStapling on
  SSLStaplingResponderTimeout 5
  SSLStaplingReturnResponderErrors off
  SSLStaplingCache shmcb:/var/run/ocsp(128000)
  ... 

More or less the same thing in nginx:

server {
  listen        *:443 ssl;
  server_name   cloud.jon.geek.nz;

  ssl  on;
  ssl_certificate     /var/lib/dehydrated/certs/www.myniftysite.com/fullchain.pem;
  ssl_certificate_key /var/lib/dehydrated/certs/www.myniftysite.com/privkey.pem;
  ssl_trusted_certificate /var/lib/dehydrated/certs/www.myniftysite.com/chain.pem;
  ssl_session_timeout  5m;
  ssl_protocols  TLSv1 TLSv1.1 TLSv1.2;
  ssl_ciphers EECDH+aRSA+AES256:EDH+aRSA+AES256:EECDH+aRSA+AES128:EDH+aRSA+AES128;
  ssl_session_cache shared:SSL:50m;
  ssl_prefer_server_ciphers on;
  ssl_stapling on;
  ssl_stapling_verify on;
  add_header Strict-Transport-Security max-age=63072000;

  # For this, run this command:
  #    openssl dhparam -out /etc/ssl/letsencrypt/dhparam.pem 2048
  ssl_dhparam /etc/ssl/letsencrypt/dhparam.pem;
  ...

Run a script from cron

Set up a config file:

# Stick this in /etc/dehydrated/config
CERTDIR='/var/lib/dehydrated/certs'
CHALLENGETYPE='http-01'

Then a script somewhere in /usr/local/bin to run from a cron job:

#!/bin/bash
# Renew SSL certificates with LetsEncrypt using dehydrated.
# See: https://github.com/lukas2511/dehydrated

DEHYDRATED='/usr/local/bin/dehydrated'

for domain in                   \
        my-domain-1.com         \
        my-other-domain.com     \
        foo-bar-baz.com         \
; do
    $DEHYDRATED -c --domain $domain
    sleep 2
done