问题
e.g. for mail.google.com would it return google.com or mail.google.com? I can't actually test it myself
回答1:
It does include subdomain (e.g. mail.google.com)
回答2:
You can save yourself from the headache of waiting for answers by reading documentation on Msdn.
A String that contains the host name. This is usually the DNS host name or IP address of the server.
If the requested DNS record is a subdomain, that's the record it will return. Subdomains are still there own records in a zone file, so its not going to return just the root domain because that's not the same record, nor request.
I was also curious as to why you couldn't test this, but if its because of the lack of an internet connection (maybe you're posting from mobile I don't know) you can add your own records to the Windows HOSTS file and test locally.
来源：https://stackoverflow.com/questions/6593456/does-request-url-host-include-the-subdomain