First up - this walk through is for people who want to host multiple domains (with the same provider) on a Synology box AND do not have a fixed IP address. You have a vanilla Sky/TalkTalk/[enter ISP here] broadband connection and you want your domain name to resolve to a service/container on your Synology?
Synology DSM supports a number of dynamic IP service providers out of the box eg:
The problem is if you have more than 1 domain, it only provides a single entry. I have several domains hosted via google domains - so this guide shows you how to add additional "copies" of the same provider.
The parameters are held in a ddns_providers.conf file which is located in your /etc.defaults folder.
So SSH into your Synology box:
Obviously you may have a different user IP address and if you have specified a different port you will need to add it in:
ssh -p 45 firstname.lastname@example.org
Once logged in you can simply copy the file to a folder that you can access:
cp /etc.defaults/ddns_providers.conf /yourfolder
Then you can open the file in your favourite editor
copy the provider you want multiple entries for and just rename them _1 _2 etc...
Save the file and then copy it back to the original location
cp /yourfolder/ddns_providers.conf /etc.defaults
And you should now have the ability to choose more than 1 instance of the same provider.
BEWARE: sometimes DSM updates reset this file to its orginal state so you might need to re-copy your file after an update.