如何捕获提供HTTP响应的服务器的IP地址

Question

Using Go's default HTTP client, I am unable to directly determine the IP address of the server that processed the request. For instance, when requesting example.com, what is the IP address that example.com resolved to at the time of the request?

import "net/http"

resp, err := http.Get("http://example.com/")

The resp object contains the resp.RemoteAddr property, but as detailed below is not utilized during client operations.

 // RemoteAddr allows HTTP servers and other software to record
 // the network address that sent the request, usually for
 // logging. This field is not filled in by ReadRequest and
 // has no defined format. The HTTP server in this package
 // sets RemoteAddr to an "IP:port" address before invoking a
 // handler.
 // This field is ignored by the HTTP client.
 RemoteAddr string

Is there a straightforward way to accomplish this? My initial thought would be to:

1. Initiate a DNS lookup to remote domain
2. Create new http transport using returned A/AAAA records
3. Make the request
4. Set the RemoteAddr property on the response object

Is there a better way?

Thank you.

UPDATED - to use @flimzy's suggestion. This method stores the remote IP:PORT into the request.RemoteAddr property. I've also added support for multiple redirects so that each subsequent request has its RemoteAddr populated.

request, _ := http.NewRequest("GET", "http://www.google.com", nil)
client := &http.Client{
    Transport:&http.Transport{
        DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
            conn, err := net.Dial(network, addr)
            request.RemoteAddr = conn.RemoteAddr().String()
            return conn, err
        },
    },
    CheckRedirect: func(req *http.Request, via []*http.Request) error {
        request = req
        return nil
    },
}
resp, _ := client.Do(request)

Answer 1

As far as I can tell, the only way to accomplish this with the standard library is with a custom http.Transport that records the remote IP address, for example in the DialContext function.

client := &http.Client{
    Transport: &http.Transport{
        DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
            conn, err := net.Dial(network, addr)
            fmt.Printf("Remote IP: %s
", conn.RemoteAddr())
            return conn, err
        },
    },
}
resp, _ := client.Get("http://www.google.com")

Tying the connection IP to the response is left as an exercise for the reader.