a.go

package main

import (
	"bufio"
	"flag"
	"fmt"
	"log"
	"net"
	"os"
	"strings"
)

func main() {
	if err := realmain(); err != nil {
		log.Fatalf("err: %s", err)
	}
}

func realmain() error {
	var (
		filename = flag.String("filename", "", "ip.txt")
	)
	flag.Parse()
	f, err := os.Open(*filename)
	if err != nil {
		return err
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		ip := scanner.Text()
		s, err := net.LookupAddr(ip)
		if err != nil {
			return err
		}
		fmt.Printf("%s\t%s\n", ip, strings.Join(s, ","))
	}
	return scanner.Err()
}