What is the difference between `Host` and `URL.Host` for golang `http.Request`?

Question

When developing golang http application, I use http.Request a lot. When accessing request host address, I would use req.Host, but I find that there is

Answer 1

The r.URL field is created by parsing the HTTP request URI.

The r.Host field is the value of the Host request header. It's the same value as calling r.Header.Get("Host").

If the HTTP request on the wire is:

 GET /pub/WWW/TheProject.html HTTP/1.1
 Host: www.example.org:8080

then r.URL.Host is "" and r.Host is www.example.org:8080.

The value of r.URL.Host and r.Host are almost always different. On a proxy server, r.URL.Host is the host of the target server and r.Host is the host of the proxy server itself. When not connecting through a proxy, the client does not specify a host in the request URI. In this scenario, r.URL.Host is the empty string.

If you are not implementing a proxy, then you should use r.Host to determine the host.