To verify that your DNS and CNAME records are set up correctly, go to https://mxtoolbox.com/DNSLookup.aspx.
The tool performs DNS lookup in the same way as when the certificate is being validated. So if the DNS records are not correct when looking at them in this tool, or not available at all, the same will be the case when our service attempts to validate them.
To check a DNS record, copy the 'Name' from one of the DNS records listed on the home page in Cookie Saver. Note that the records are only shown, if your setup has not been validated yet. Once you've copied the name, insert it into the 'Domain Name' field on the MXToolbox page linked above. Then press the DNS Lookup button.
If the records are set up correctly you'll see a screen similar to those shown below. The screenshots show the results for looking up the two types of DNS records for the fpc.cookiesaver.io domain.
To validate, that the records are correct, check that the 'Type' is 'CNAME', that the 'Domain Name' matches the 'Name' from the home page and that the 'Canonical Name' matches the 'Value'.
Repeat this process for each subdomain on your setup. They all have to be correct in order for the validation to succeed.
Note: Changes to your DNS setup - like creating new records - may take some time to take effect. Depending on the settings chosen when you create the records, this could be several hours. Check the TTL setting when creating the records. Until the changes take effect, they will not be detectable by the MXToolbox nor the certificate validation process in Cookie Saver.
For cookiesaver.io we are checking that two CNAMEs are set up correctly.