Let's Encrypt

/opt/letsencrypt/letsencrypt-auto certonly \
  --debug \
  --webroot \
  -w /home/user/public_html \
  -d domain.com -d www.domain.com -d domainalt.com -d www.domainalt.com \
  --config /etc/letsencrypt/config.ini \
  --agree-tos

• Certbot User Guide