Comment by Bender
15 hours ago
From the RFC The "_for-sale" leaf node name can be placed at any level of the DNS, except in the .arpa infrastructure TLD.
There used to be a number of sites that would let users create {username}.some.subdomain.apex.tld. Perhaps this is not a thing any more I honestly don't know. If it is still a thing and someone registers the username "_for-sale" they could cause some confusion.
Why allow any level of DNS? If you are selling a domain that should include the entire domain and thus the _for-sale should only be valid at the apex unless the goal is to cause legal problems for some big providers.
Initial underscore is a reserved namespace for this reason.
Is that universally true on all sites that allow a user to create a sub-domain?
It certainly should be. Filtering user-registrable names that will end up in host names or URL paths is table stakes. You want to block names like www, admin, root, robots.txt, .well-known, _*, and so forth.
DNS allows any character you want, including underscores. Host names in URLs cannot contain underscores. Therefore any service that create subdomains for user would reject underscores, which is why they're considered de-facto reserved for special uses in DNS. I don't think the reserved nature is codified in any RFC.
4 replies →
Country code TLDs would be one reason. Many countries reserve top-level domains names and only sell second-level domains (e.g. example.co.uk)