These are some old scripts that gave me this idea about the complete automation. This is what people did for Synology DNS, but if you are using other DNS providers, this will not work.
#!/bin/sh
########################################################
#This script will create a new p12 certificate for Plex
#when Let's Encrypt automatically renew its one
########################################################
########################################################
#Just adapt the values below to your configuration
#Location of your script
script_folder=/volume1/scripts
#Folder and name you want for your p12 file
p12_file_path=$script_folder/syno.p12
#Add password to the p12 file (you can leave it empty)
p12cert_password=
#Synology's Default Let's encrypt folder
letsencrypt_cert_folder=/usr/syno/etc/certificate/system/default
########################################################
#Changes below are at your own own risk
########################################################
generate_p12=false
current_date=`date +"%s"`
current_certificate_date=`openssl x509 -enddate -noout -in $letsencrypt_cert_folder/cert.pem | cut -d'=' -f2`
current_certificate_timestamp=`date -d "$current_certificate_date" +"%s"`
#First of all, we check if the renew_timestamp file exists (this file keep in memory the further certificate renew date)
if [ ! -f $script_folder/renew_timestamp ]; then
echo "Generate timestamp for the current renew date... "
echo $current_certificate_timestamp > $script_folder/renew_timestamp
chmod +rw $script_folder/renew_timestamp
chown admin:users $script_folder/renew_timestamp
#We generate the first p12 file
generate_p12=true
else
renew_date=`cat $script_folder/renew_timestamp`
echo "In memory certificate expiration date is" `date -d @$renew_date` "and the current certificate expiration date is $current_certificate_date"
#Now, we check if is it necessary to renew the certificate or not
if expr "$current_certificate_timestamp" "!=" "$renew_date" > /dev/null; then
#We ask to generate a new p12 file
echo "Dates doesn't match, we have to renew the certificate..."
generate_p12=true
#We update the timestamp_date on the file
echo "Updating the new timestamp date..."
echo $current_certificate_timestamp > $script_folder/renew_timestamp
else
echo "It is not necessary to renew the certificate, abort."
exit 0
fi
fi
#We generate a new certificate file if we ask it, ans we relaunch Plex App to take effect
if expr "$generate_p12" "=" "true" > /dev/null; then
echo "Generating the p12 certificate file..."
openssl pkcs12 -export -out $p12_file_path -in $letsencrypt_cert_folder/cert.pem -inkey $letsencrypt_cert_folder/privkey.pem -certfile $letsencrypt_cert_folder/chain.pem -name "Domain" -password pass:$p12cert_password
chmod +r $p12_file_path
chown admin:users $p12_file_path
echo "Relaunching Plex App..."
sh /var/packages/Plex\ Media\ Server/scripts/start-stop-status stop
sh /var/packages/Plex\ Media\ Server/scripts/start-stop-status start
echo "Done."
Fi
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