We are hosting the page of many of our customers. We want to be able to provide our customers with a subdomain of our own domain like customerpages.ourdomain.com so they can create a CNAME to this subdomain.
www.customer1.com CNAME customerpages.ourdomain.com. {This will work just fine.}
But the situation is I don't know if all our customers will be able to place the following CNAME
customer1.com CNAME customerpages.ourdomain.com
This last CNAME looks like it's against the RFC of DNS.
Any thoughts will be appreciated.
No, you can't do that - CNAME records can only exist as single records and not combined with any other resource record (DNSSEC records excepted).
There are explicit SOA and NS records always present at the top of each domain, so that prevents the use of the CNAME at the same part of the hierarchy.
It's true, given a domain example.com, some DNS servers (tinydns for one) won't complain if you set up CNAME records for both example.com and www.example.com. But still I'd avoid it as it would break e-mail for example.com (see RFC 2821 "Address Resolution and Mail Handling").
来源：https://stackoverflow.com/questions/655235/is-root-domain-cname-to-other-domain-allowed-by-dns-rfc