「2^64 個のグローバル IPv6 アドレスを用いたクイズを作ってみよう」のサーバーで動くプログラムのソースコード

guess-the-ipv6-address.go

package main

import (
	"encoding/binary"
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
)

const MONEY uint64 = 0x2525_beef_4649_babe

type GetResponse struct {
	Result  int    `json:"result"`
	Comment string `json:"comment"`
}

func getRequestedV6Host(r *http.Request) (net.IP, error) {
	host, _, err := net.SplitHostPort(r.Host)
	if err != nil {
		return nil, err
	}
	ip := net.ParseIP(host)
	if ip.To16() == nil {
		return nil, fmt.Errorf("invalid IPv6 address: %s", host)
	}
	return ip.To16(), nil
}

func v6OnlyMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		ip, err := getRequestedV6Host(r)

		if err != nil {
			http.Error(w, err.Error(), http.StatusInternalServerError)
			return
		}

		// If the client is accessing to this server by specifying an IPv4 address,
		// return a 400
		if ip == nil {
			http.Error(w, "IPv4 not allowed", http.StatusBadRequest)
			return
		}

		next.ServeHTTP(w, r)
	})
}

func getIndex(w http.ResponseWriter, r *http.Request) {
	const instruction = `You can make requests to: /host, /challenge
`
	io.WriteString(w, instruction)
}

func getHost(w http.ResponseWriter, r *http.Request) {
	host, err := getRequestedV6Host(r)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	io.WriteString(w, host.String())
}

func getChallenge(w http.ResponseWriter, r *http.Request) {
	var response GetResponse

	toMoney := func(r *http.Request) (uint64, error) {
		ip, err := getRequestedV6Host(r)
		if err != nil {
			return 0, err
		}
		money := binary.BigEndian.Uint64(ip[8:])
		return money, nil
	}

	money, err := toMoney(r)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}

	if money > MONEY {
		response.Result = 1
		response.Comment = "Too rich to be true (The answer is smaller than the value you specified)"
	} else if money == MONEY {
		response.Result = 0
		response.Comment = "Congratulations!!!! (The answer is the same as the value you specified)"
	} else {
		response.Result = -1
		response.Comment = "Too poor to be true (The answer is greater than the value you specified)"
	}

	json.NewEncoder(w).Encode(response)
}

func main() {
	http.Handle("/", v6OnlyMiddleware(http.HandlerFunc(getIndex)))
	http.Handle("/host", v6OnlyMiddleware(http.HandlerFunc(getHost)))
	http.Handle("/challenge", v6OnlyMiddleware(http.HandlerFunc(getChallenge)))

	log.Fatal(http.ListenAndServe(":8080", nil))
}