net/dns/resolver: factor the resolver out into a sub-package.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/resolver/tsdns_server_test.go

@@ -0,0 +1,95 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package resolver
+
+import (
+	"log"
+	"testing"
+
+	"github.com/miekg/dns"
+	"inet.af/netaddr"
+)
+
+// This file exists to isolate the test infrastructure
+// that depends on github.com/miekg/dns
+// from the rest, which only depends on dnsmessage.
+
+var dnsHandleFunc = dns.HandleFunc
+
+// resolveToIP returns a handler function which responds
+// to queries of type A it receives with an A record containing ipv4,
+// to queries of type AAAA with an AAAA record containing ipv6,
+// to queries of type NS with an NS record containg name.
+func resolveToIP(ipv4, ipv6 netaddr.IP, ns string) 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")
+		}
+		question := req.Question[0]
+
+		var ans dns.RR
+		switch question.Qtype {
+		case dns.TypeA:
+			ans = &dns.A{
+				Hdr: dns.RR_Header{
+					Name:   question.Name,
+					Rrtype: dns.TypeA,
+					Class:  dns.ClassINET,
+				},
+				A: ipv4.IPAddr().IP,
+			}
+		case dns.TypeAAAA:
+			ans = &dns.AAAA{
+				Hdr: dns.RR_Header{
+					Name:   question.Name,
+					Rrtype: dns.TypeAAAA,
+					Class:  dns.ClassINET,
+				},
+				AAAA: ipv6.IPAddr().IP,
+			}
+		case dns.TypeNS:
+			ans = &dns.NS{
+				Hdr: dns.RR_Header{
+					Name:   question.Name,
+					Rrtype: dns.TypeNS,
+					Class:  dns.ClassINET,
+				},
+				Ns: ns,
+			}
+		}
+
+		m.Answer = append(m.Answer, ans)
+		w.WriteMsg(m)
+	}
+}
+
+func resolveToNXDOMAIN(w dns.ResponseWriter, req *dns.Msg) {
+	m := new(dns.Msg)
+	m.SetRcode(req, dns.RcodeNameError)
+	w.WriteMsg(m)
+}
+
+func serveDNS(tb testing.TB, addr string) (*dns.Server, chan error) {
+	server := &dns.Server{Addr: addr, Net: "udp"}
+
+	waitch := make(chan struct{})
+	server.NotifyStartedFunc = func() { close(waitch) }
+
+	errch := make(chan error, 1)
+	go func() {
+		err := server.ListenAndServe()
+		if err != nil {
+			log.Printf("ListenAndServe(%q): %v", addr, err)
+		}
+		errch <- err
+		close(errch)
+	}()
+
+	<-waitch
+	return server, errch
+}