For instances that use the Custom URL feature we monitor the CNAME DNS entry for the custom url that is used to link to your instance to make sure that it is valid. If we detect an issue, a case is automatically created title "CNAME custom URL error".
Cause of This Alert
As part of configuring a Custom URL for your instance we make sure that there is a valid CNAME entry on the DNS servers for the domain you wish to use to point to the instance. An example valid CNAME entry on the DNS would look like this:
example.domain.com. 60 IN CNAME instance.service-now.com.If we detect that this CNAME entry on the DNS does not point to your instance we will create an alert to let you know as the custom URL will not point to your instance when you try to access it.
You can check the CNAME DNS entry for your domain by typing in the terminal (and replacing '<example.domain.com>' with the domain you want to check)
$ dig -t CNAME <example.domain.com> +noall +answer