func (r *Resolver) Nameservers(qname string) []string {
	queryKeys := strings.Split(qname, ".")
	queryKeys = queryKeys[:len(queryKeys)-1]  
	ns := []string{}
	if v, found := r.domain_server.search(queryKeys); found {
		logger.Debug("%s be found in domain server list, upstream: %v", qname, v)
		server := v
		nameserver := net.JoinHostPort(server, "53")
		ns = append(ns, nameserver)
		return ns
	}
	for _, nameserver := range r.servers {
		ns = append(ns, nameserver)
	}
	return ns
}
Namservers return the array of nameservers, with port number appended.
'#' in the name is treated as port separator, as with dnsmasq.
package main
import (
	"bufio"
	"fmt"
	"net"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"
	"github.com/miekg/dns"
)
type ResolvError struct {
	qname, net  string
	nameservers []string
}
func (e ResolvError) Error() string {
	errmsg := fmt.Sprintf("%s resolv failed on %s (%s)", e.qname, strings.Join(e.nameservers, "; "), e.net)
	return errmsg
}
type RResp struct {
	msg        *dns.Msg
	nameserver string
	rtt        time.Duration
}
type Resolver struct {
	servers       []string
	domain_server *suffixTreeNode
	config        *ResolvSettings
}
func NewResolver(c ResolvSettings) *Resolver {
	r := &Resolver{
		servers:       []string{},
		domain_server: newSuffixTreeRoot(),
		config:        &c,
	}
	if len(c.ServerListFile) > 0 {
		r.ReadServerListFile(c.ServerListFile)
	}
	if len(c.ResolvFile) > 0 {
		clientConfig, err := dns.ClientConfigFromFile(c.ResolvFile)
		if err != nil {
			logger.Error(":%s is not a valid resolv.conf file\n", c.ResolvFile)
			logger.Error("%s", err)
			panic(err)
		}
		for _, server := range clientConfig.Servers {
			nameserver := net.JoinHostPort(server, clientConfig.Port)
			r.servers = append(r.servers, nameserver)
		}
	}
	return r
}
func (r *Resolver) parseServerListFile(buf *os.File) {
	scanner := bufio.NewScanner(buf)
	for scanner.Scan() {
		line := scanner.Text()
		line = strings.TrimSpace(line)
		if !strings.HasPrefix(line, "server") {
			continue
		}
		sli := strings.Split(line, "=")
		if len(sli) != 2 {
			continue
		}
		line = strings.TrimSpace(sli[1])
		tokens := strings.Split(line, "/")
		switch len(tokens) {
		case 3:
			domain := tokens[1]
			ip := tokens[2]
			if !isDomain(domain) || !isIP(ip) {
				continue
			}
			r.domain_server.sinsert(strings.Split(domain, "."), ip)
		case 1:
			srv_port := strings.Split(line, "#")
			if len(srv_port) > 2 {
				continue
			}
			ip := ""
			if ip = srv_port[0]; !isIP(ip) {
				continue
			}
			port := "53"
			if len(srv_port) == 2 {
				if _, err := strconv.Atoi(srv_port[1]); err != nil {
					continue
				}
				port = srv_port[1]
			}
			r.servers = append(r.servers, net.JoinHostPort(ip, port))
		}
	}
}
func (r *Resolver) ReadServerListFile(path string) {
	files := strings.Split(path, ";")
	for _, file := range files {
		buf, err := os.Open(file)
		if err != nil {
			panic("Can't open " + file)
		}
		defer buf.Close()
		r.parseServerListFile(buf)
	}
}
func (r *Resolver) Lookup(net string, req *dns.Msg) (message *dns.Msg, err error) {
	c := &dns.Client{
		Net:          net,
		ReadTimeout:  r.Timeout(),
		WriteTimeout: r.Timeout(),
	}
	if net == "udp" && settings.ResolvConfig.SetEDNS0 {
		req = req.SetEdns0(65535, true)
	}
	qname := req.Question[0].Name
	res := make(chan *RResp, 1)
	var wg sync.WaitGroup
	L := func(nameserver string) {
		defer wg.Done()
		r, rtt, err := c.Exchange(req, nameserver)
		if err != nil {
			logger.Warn("%s socket error on %s", qname, nameserver)
			logger.Warn("error:%s", err.Error())
			return
		}
		if r != nil && r.Rcode != dns.RcodeSuccess {
			logger.Warn("%s failed to get an valid answer on %s", qname, nameserver)
			if r.Rcode == dns.RcodeServerFailure {
				return
			}
		}
		re := &RResp{r, nameserver, rtt}
		select {
		case res <- re:
		default:
		}
	}
	ticker := time.NewTicker(time.Duration(settings.ResolvConfig.Interval) * time.Millisecond)
	defer ticker.Stop()
	nameservers := r.Nameservers(qname)
	for _, nameserver := range nameservers {
		wg.Add(1)
		go L(nameserver)
		select {
		case re := <-res:
			logger.Debug("%s resolv on %s rtt: %v", UnFqdn(qname), re.nameserver, re.rtt)
			return re.msg, nil
		case <-ticker.C:
			continue
		}
	}
	wg.Wait()
	select {
	case re := <-res:
		logger.Debug("%s resolv on %s rtt: %v", UnFqdn(qname), re.nameserver, re.rtt)
		return re.msg, nil
	default:
		return nil, ResolvError{qname, net, nameservers}
	}
}