First of all, before providing an answer I would like to be upfront about the fact that by trusting headers you are opening the door to security vulnerabilities such as phishing. So for redirection purposes, don't use values from headers without first validating the URL is authorized.
Then, your operating system hostname might not necessarily match the DNS one. In fact, one IP might have more than one DNS name. So for HTTP purposes there is no guarantee that the hostname assigned to your machine in your operating system configuration is useable.
The best choice I can think of is to obtain your HTTP listener public IP and resolve its name via DNS. See the dns.reverse method for more info. But then, again, note that an IP might have multiple names associated with it.