DNS check of LB services

dns-check.go

package main

import (
	"context"
	"fmt"
	"net"
	"sort"
	"strings"
	"time"
)

func findNS(name string) ([]string, error) {
	nameSplit := strings.Split(name, ".")
	if len(nameSplit) < 2 {
		return nil, fmt.Errorf("NS not found")
	}
	ns, err := net.LookupNS(name)
	if err != nil {
		return findNS(strings.Join(nameSplit[1:], "."))
	} else {
		hosts := []string{}
		for _, host := range ns {
			hosts = append(hosts, host.Host)
		}
		return hosts, nil
	}
}

func buildResolver(ns string) *net.Resolver {
	return &net.Resolver{
		PreferGo: true,
		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
			d := net.Dialer{
				Timeout: time.Millisecond * time.Duration(10000),
			}
			return d.DialContext(ctx, "udp", ns+":53")
		},
	}
}

func main() {
	broker := "pkc-4xwkx.eu-west-1.aws.confluent.cloud"
	elb := "ae6fe4609709447c2a66f438fc093b72-612660764.eu-west-1.elb.amazonaws.com"

	r, err := net.LookupTXT(broker)
	fmt.Println("txt: ", r, err)

	ns, err := findNS(broker)
	fmt.Println("ns: ", ns, err)
	if err != nil {
		return
	}

	resolver := buildResolver(ns[0])

	brokerHosts, err := resolver.LookupHost(context.Background(), broker)
	sort.Strings(brokerHosts)
	fmt.Println(brokerHosts, err)

	elbHosts, err := resolver.LookupHost(context.Background(), elb)
	sort.Strings(elbHosts)
	fmt.Println(elbHosts, err)
}