Forcing IPv4 or IPv6 in net/http Client

In Go 1.11, the net/http package provides a DialContext function that allows you to intercept and control the outgoing network connection. This function can be used to force the client to use either IPv4 or IPv6.

IPv4-Only Connections

To restrict connections to IPv4 addresses, modify the DualStack field of the Dialer to false and define a Control function that returns an error for network type "ipv4":

type MyTransport struct {
    http.RoundTripper
}

func (t *MyTransport) RoundTrip(req *http.Request) (*http.Response, error) {
    return http.DefaultTransport.RoundTrip(req)
}

func main() {
    MyTransport := &MyTransport{
        DialContext: (&net.Dialer{
            DualStack: false,
            Control: func(network, address string, c syscall.RawConn) error {
                if network == "ipv4" {
                    return errors.New("you should not use ipv4")
                }
                return nil
            },
        }).DialContext,
    }

    client := http.Client{Transport: MyTransport}

    _, err := client.Get("http://www.github.com")
    if err != nil {
        fmt.Println(err)
    }
}

This modification intercepts network connections and returns an error if an IPv4 connection is attempted.

Note: The network value passed to the Control function can be "tcp4" for IPv4 and "tcp6" for IPv6, even if "tcp" was passed to the DialContext function.

IPv6-Only Connections

To force IPv6-only connections, set DualStack to false and return an error for network type "ipv6":

type MyTransport struct {
    http.RoundTripper
}

func (t *MyTransport) RoundTrip(req *http.Request) (*http.Response, error) {
    return http.DefaultTransport.RoundTrip(req)
}

func main() {
    MyTransport := &MyTransport{
        DialContext: (&net.Dialer{
            DualStack: false,
            Control: func(network, address string, c syscall.RawConn) error {
                if network == "ipv6" {
                    return errors.New("you should not use ipv6")
                }
                return nil
            },
        }).DialContext,
    }

    client := http.Client{Transport: MyTransport}

    _, err := client.Get("http://www.github.com")
    if err != nil {
        fmt.Println(err)
    }
}