切换为默认的dns解析;增加resolver=system的支持

resolver/exchanger/system.go

@@ -0,0 +1,108 @@
+package exchanger
+
+import (
+	"context"
+	"errors"
+	"net"
+	"strings"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+type systemExchanger struct {
+	rawAddr  string
+	timeout  time.Duration
+	lookupIP func(ctx context.Context, network, host string) ([]net.IP, error)
+}
+
+func newSystemExchanger(rawAddr string, opts Options) Exchanger {
+	if opts.timeout <= 0 {
+		opts.timeout = 5 * time.Second
+	}
+	ex := &systemExchanger{
+		rawAddr: rawAddr,
+		timeout: opts.timeout,
+	}
+	ex.lookupIP = net.DefaultResolver.LookupIP
+	return ex
+}
+
+func (ex *systemExchanger) Exchange(ctx context.Context, msg []byte) ([]byte, error) {
+	mq := new(dns.Msg)
+	if err := mq.Unpack(msg); err != nil {
+		return nil, err
+	}
+
+	mr := new(dns.Msg)
+	mr.SetReply(mq)
+	mr.RecursionAvailable = true
+
+	if len(mq.Question) == 0 {
+		return mr.Pack()
+	}
+
+	q := mq.Question[0]
+	network := "ip"
+	switch q.Qtype {
+	case dns.TypeA:
+		network = "ip4"
+	case dns.TypeAAAA:
+		network = "ip6"
+	default:
+		return mr.Pack()
+	}
+
+	if ex.timeout > 0 {
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithTimeout(ctx, ex.timeout)
+		defer cancel()
+	}
+
+	host := strings.TrimSuffix(q.Name, ".")
+	ips, err := ex.lookupIP(ctx, network, host)
+	if err != nil {
+		var dnsErr *net.DNSError
+		if !errors.As(err, &dnsErr) || !dnsErr.IsNotFound {
+			return nil, err
+		}
+	}
+
+	for _, ip := range ips {
+		switch q.Qtype {
+		case dns.TypeA:
+			if ip4 := ip.To4(); ip4 != nil {
+				mr.Answer = append(mr.Answer, &dns.A{
+					Hdr: dns.RR_Header{
+						Name:   q.Name,
+						Rrtype: dns.TypeA,
+						Class:  q.Qclass,
+					},
+					A: ip4,
+				})
+			}
+		case dns.TypeAAAA:
+			if ip.To4() == nil {
+				if ip16 := ip.To16(); ip16 != nil {
+					mr.Answer = append(mr.Answer, &dns.AAAA{
+						Hdr: dns.RR_Header{
+							Name:   q.Name,
+							Rrtype: dns.TypeAAAA,
+							Class:  q.Qclass,
+						},
+						AAAA: ip16,
+					})
+				}
+			}
+		}
+	}
+
+	return mr.Pack()
+}
+
+func (ex *systemExchanger) String() string {
+	if ex.rawAddr == "" {
+		return "system"
+	}
+	return ex.rawAddr
+}