net/dns/resolver: add Windows ExitDNS service support, using net package

Updates #1713
Updates #835

Change-Id: Ia71e96d0632c2d617b401695ad68301b07c1c2ec
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolver/tsdns_server_test.go

@@ -6,6 +6,7 @@ package resolver
 
 import (
 	"fmt"
+	"net"
 	"strings"
 	"testing"
 
@@ -179,6 +180,129 @@ var resolveToNXDOMAIN = dns.HandlerFunc(func(w dns.ResponseWriter, req *dns.Msg)
 	w.WriteMsg(m)
 })
 
+// weirdoGoCNAMEHandler returns a DNS handler that satisfies
+// Go's weird Resolver.LookupCNAME (read its godoc carefully!).
+//
+// This doesn't even return a CNAME record, because that's not
+// what Go looks for.
+func weirdoGoCNAMEHandler(target string) dns.HandlerFunc {
+	return func(w dns.ResponseWriter, req *dns.Msg) {
+		m := new(dns.Msg)
+		m.SetReply(req)
+		question := req.Question[0]
+
+		switch question.Qtype {
+		case dns.TypeA:
+			m.Answer = append(m.Answer, &dns.CNAME{
+				Hdr: dns.RR_Header{
+					Name:   target,
+					Rrtype: dns.TypeCNAME,
+					Class:  dns.ClassINET,
+					Ttl:    600,
+				},
+				Target: target,
+			})
+		case dns.TypeAAAA:
+			m.Answer = append(m.Answer, &dns.AAAA{
+				Hdr: dns.RR_Header{
+					Name:   target,
+					Rrtype: dns.TypeAAAA,
+					Class:  dns.ClassINET,
+					Ttl:    600,
+				},
+				AAAA: net.ParseIP("1::2"),
+			})
+		}
+		w.WriteMsg(m)
+	}
+}
+
+// dnsHandler returns a handler that replies with the answers/options
+// provided.
+//
+// Types supported: netaddr.IP.
+func dnsHandler(answers ...interface{}) dns.HandlerFunc {
+	return func(w dns.ResponseWriter, req *dns.Msg) {
+		m := new(dns.Msg)
+		m.SetReply(req)
+		if len(req.Question) != 1 {
+			panic("not a single-question request")
+		}
+		m.RecursionAvailable = true // to stop net package's errLameReferral on empty replies
+
+		question := req.Question[0]
+		for _, a := range answers {
+			switch a := a.(type) {
+			default:
+				panic(fmt.Sprintf("unsupported dnsHandler arg %T", a))
+			case netaddr.IP:
+				ip := a
+				if ip.Is4() {
+					m.Answer = append(m.Answer, &dns.A{
+						Hdr: dns.RR_Header{
+							Name:   question.Name,
+							Rrtype: dns.TypeA,
+							Class:  dns.ClassINET,
+						},
+						A: ip.IPAddr().IP,
+					})
+				} else if ip.Is6() {
+					m.Answer = append(m.Answer, &dns.AAAA{
+						Hdr: dns.RR_Header{
+							Name:   question.Name,
+							Rrtype: dns.TypeAAAA,
+							Class:  dns.ClassINET,
+						},
+						AAAA: ip.IPAddr().IP,
+					})
+				}
+			case dns.PTR:
+				ptr := a
+				ptr.Hdr = dns.RR_Header{
+					Name:   question.Name,
+					Rrtype: dns.TypePTR,
+					Class:  dns.ClassINET,
+				}
+				m.Answer = append(m.Answer, &ptr)
+			case dns.CNAME:
+				c := a
+				c.Hdr = dns.RR_Header{
+					Name:   question.Name,
+					Rrtype: dns.TypeCNAME,
+					Class:  dns.ClassINET,
+					Ttl:    600,
+				}
+				m.Answer = append(m.Answer, &c)
+			case dns.TXT:
+				txt := a
+				txt.Hdr = dns.RR_Header{
+					Name:   question.Name,
+					Rrtype: dns.TypeTXT,
+					Class:  dns.ClassINET,
+				}
+				m.Answer = append(m.Answer, &txt)
+			case dns.SRV:
+				srv := a
+				srv.Hdr = dns.RR_Header{
+					Name:   question.Name,
+					Rrtype: dns.TypeSRV,
+					Class:  dns.ClassINET,
+				}
+				m.Answer = append(m.Answer, &srv)
+			case dns.NS:
+				rr := a
+				rr.Hdr = dns.RR_Header{
+					Name:   question.Name,
+					Rrtype: dns.TypeNS,
+					Class:  dns.ClassINET,
+				}
+				m.Answer = append(m.Answer, &rr)
+			}
+		}
+		w.WriteMsg(m)
+	}
+}
+
 func serveDNS(tb testing.TB, addr string, records ...interface{}) *dns.Server {
 	if len(records)%2 != 0 {
 		panic("must have an even number of record values")