yourip/main.go

package main

import (
	"encoding/json"
	"flag"
	"log"
	"net/http"
	"strings"
)

type jsonOutput struct {
	Ip string `json:"ip"`
}

func ipJsonOutput(w http.ResponseWriter, r *http.Request) {
	var ip jsonOutput
	ip.Ip = remoteAddr(r)
	output, err := json.Marshal(ip)
	if err != nil {
		w.Write([]byte(err.Error()))
	}
	w.Write([]byte(output))
}

func ipOutput(w http.ResponseWriter, r *http.Request) {
	ip := remoteAddr(r)
	w.Write([]byte(ip))
}

func remoteAddr(r *http.Request) string {
	var ip string
	if r.RemoteAddr[0] == '[' {
		ip = strings.Split(r.RemoteAddr, "]")[0][1:]
	} else {
		ip = strings.Split(r.RemoteAddr, ":")[0]
	}
	return ip
}

func main() {
	addr := flag.String("addr", ":8080", "local ip:port to bind")
	flag.Parse()
	http.HandleFunc("/", ipOutput)
	http.HandleFunc("/json", ipJsonOutput)
	log.Fatal(http.ListenAndServe(*addr, nil))
}