Excuse me, how to print client IP addrs on my server?

func main() {
flag.Usage = func() {
flag.PrintDefaults()
}
flag.Parse()
if serverPort == "" {
log.Fatalln("Error: please setting correct server port")
}
endProxy := goproxy.NewProxyHttpServer()
endProxy.Verbose = true
auth.ProxyBasic(endProxy, "my_realm", func(user, pwd string) bool {
return user == username && pwd == password
})
log.Println("serving proxy server port at", serverPort)
http.ListenAndServe(":"+serverPort, endProxy)
fmt.Println("The proxy server end.")
}