A DNS Scraper in Go

dns_scraper.go

package main

//simple usage go run dns_scraper.go -h=0.uk.pool.ntp.org -s=3

import (
	"flag"
	"fmt"
	"github.com/bogdanovich/dns_resolver"
	"io/ioutil"
	"math/rand"
	"net"
	"net/http"
	"os"
	"strings"
	"time"
)

var (
	url                = "https://public-dns.info/nameservers.txt"
	all_public_servers []string
	discovered_entries []string
)

func fetchdns() []string {

	resp, err := http.Get(url)
	if err != nil {
		fmt.Println("Error fecthing the dns list!")
		os.Exit(2)
	}
	defer resp.Body.Close()
	body, err := ioutil.ReadAll(resp.Body)
	dns_list := strings.Split(string(body), "\n")
	return dns_list

}

func getdns(dns_list []string) string {
	rand.Seed(time.Now().UnixNano())
	random_server_ip := dns_list[rand.Intn(len(dns_list))]
	random_server_name, _ := net.LookupAddr(random_server_ip)
	fmt.Printf("Using dns %s - %s\n", random_server_ip, random_server_name)
	return random_server_ip
}

func readdnsfile() []string {

	data, err := ioutil.ReadFile("./dns.list")
	if err != nil {
		fmt.Println("cannot read dns.list in the current directory")
		os.Exit(3)
	}
	dns_list := strings.Split(string(data), "\n")
	return dns_list
}

func main() {

	hostname := flag.String("h", "www.google.com", "hostname to resolve")
	servers := flag.Int("s", 3, "how many random servers query")
	fetch := flag.Bool("f", true, "fetch open DNS list from public-dns.info, if false read the file dns.list (one IP per line) on the same directory")
	flag.Parse()

	if *fetch {
		all_public_servers = fetchdns()
	} else {
		all_public_servers = readdnsfile()
	}

	for i := 0; i <= *servers; i++ {
		choosendns := getdns(all_public_servers)
		resolver := dns_resolver.New([]string{choosendns})

		// In case of i/o timeout
		resolver.RetryTimes = 5

		ip, err := resolver.LookupHost(*hostname)
		if err != nil {
			fmt.Println(err.Error())
			i--
		}
		fmt.Println(ip)
		for _, val := range ip {
			fmt.Printf("found: %s\n", val)
			discovered_entries = append(discovered_entries, val.String())
		}
	}
	fmt.Printf("\nAll entries for %s:\n  %#v\n", *hostname, discovered_entries)
}