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

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问题内容

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 req.URL.Host field, but when I print it, it's empty.

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Println("uri Host: " + r.URL.Host + " Scheme: " + r.URL.Scheme)
    fmt.Println("Host: " + r.Host)
}

The documentation of http.Request gives the following comments, while net/url does not give much clue.

// For server requests Host specifies the host on which the
// URL is sought. Per RFC 2616, this is either the value of
// the "Host" header or the host name given in the URL itself.
// It may be of the form "host:port". For international domain
// names, Host may be in Punycode or Unicode form. Use
// golang.org/x/net/idna to convert it to either format if
// needed.
//
// For client requests Host optionally overrides the Host
// header to send. If empty, the Request.Write method uses
// the value of URL.Host. Host may contain an international
// domain name.
Host string

It seems to me that there are two host value in a request: uri line and Host header, like:

GET http://localhost:8080/ HTTP/1.1
Host: localhost:8080

But it does not solve many problems than it creates:

1. Why are there two different Host field in request? I mean isn't this a duplicate?
2. Can the two Host fields be different in the same request?
3. Which one should I use for what situation?

Answers with a real HTTP request example would be the best. Thanks in advance.

正确答案

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.

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