find-my-ip.go

package main

import (
	"fmt"
	"net"
	"net/http"
	"strings"
)

type mux struct {
}

func main() {
	http.ListenAndServe(":9000", mux{})
}

const (
	headerContentType = "Content-Type"
	headerVersion     = "Version"
)

func (mux) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	if req.URL.Path == "/favicon.ico" {
		w.WriteHeader(http.StatusNotFound)
	}
	ip := req.Header.Get("X-Forwarded-For")
	if ip != "" {
		if index := strings.IndexByte(ip, ','); index != -1 {
			ip = ip[:index]
		}
	} else {
		ip, _, _ = net.SplitHostPort(req.RemoteAddr)
	}
	if ip != "" {
		addr := net.ParseIP(ip)
		if addr == nil {
			w.WriteHeader(http.StatusBadRequest)
			return
		}
		ipVersion := version(4)
		if strings.IndexByte(ip, ':') != -1 {
			ipVersion = 6
		} else {
			addr = addr.To4() // convert to 4 bytes IPv4
		}
		format := strings.ToLower(req.URL.Query().Get("format"))
		switch format {
		case "json":
			w.Header().Add(headerContentType, "application/json")
			fmt.Fprintf(w, "{\"version\":%s,\"address\":\"%s\"}", ipVersion.String(), addr.String())
		case "", "plain":
			w.Header().Add(headerContentType, "text/plain")
			w.Header().Add(headerVersion, ipVersion.String())
			w.Write([]byte(addr.String()))
		case "binary":
			w.Header().Add(headerContentType, "application/octet-stream")
			w.Header().Add(headerVersion, ipVersion.String())
			w.Write(addr)
		default:
			w.WriteHeader(http.StatusBadRequest)
			w.Write([]byte("invalid format"))
		}
	}
}

type version byte

func (v version) String() string {
	return string([]byte{'0' + byte(v)})
}