How to force client to use http/2? (instead of falling back to http 1.1)

Question

How can I force a simple Go client to use HTTP/2 and prevent it from falling back to HTTP 1.1 ?

I have a simple HTTP/2 server running on "localhost" and it returns details of the request in its reply. Here is the output using Google Chrome for this URL: https://localhost:40443/bananas

I like bananas!
Method       = GET
URL          = /bananas
Proto        = HTTP/2.0
Host         = localhost:40443
RequestURI   = /bananas

But here is what I get for my Go client code. You can see it falls back to HTTP 1.1

I like monkeys!
Method       = GET
URL          = /monkeys
Proto        = HTTP/1.1
Host         = localhost:40443
RequestURI   = /monkeys

Below is the source code of my best attempt to contact the same server using HTTP/2, but it always falls back to HTTP 1.1

// simple http/2 client

package main

import (
    "crypto/tls"
    "crypto/x509"
    "fmt"
    "io/ioutil"
    "log"
    "net/http"
)

const (
    certFile = "client-cert.pem"
    keyFile  = "client-key.pem"
    caFile   = "server-cert.pem"
)

func main() {
    // Load client certificate
    cert, err := tls.LoadX509KeyPair(certFile, keyFile)
    if err != nil {
        log.Fatal(err)
    }

    // Load CA cert
    caCert, err := ioutil.ReadFile(caFile)
    if err != nil {
        log.Fatal(err)
    }
    caCertPool := x509.NewCertPool()
    caCertPool.AppendCertsFromPEM(caCert)

    // Setup HTTPS client
    tlsConfig := &tls.Config{
        Certificates: []tls.Certificate{cert},
        RootCAs:      caCertPool,
    }
    tlsConfig.BuildNameToCertificate()
    transport := &http.Transport{TLSClientConfig: tlsConfig}
    client := &http.Client{Transport: transport}

    response, err := client.Get("https://localhost:40443/monkeys")
    if err != nil {
        log.Fatal(err)
    }
    defer response.Body.Close()

    // dump response
    text, err := ioutil.ReadAll(response.Body)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Body:\n%s", text)
}

Any suggestions would be appreciated, including pointers to other working examples that illustrate how to make HTTP/2 client requests in Go.

Answer 1

First import "golang.org/x/net/http2" package. And then change

transport := &http.Transport{TLSClientConfig: tlsConfig}

to

transport := &http2.Transport{TLSClientConfig: tlsConfig}