ExploreDNS/internal/dns/real_exchange_test.go

package dns

import (
	"context"
	"net"
	"sync/atomic"
	"testing"
	"time"

	"github.com/miekg/dns"
)

// startTestDNSServer starts a loopback DNS server on a random port and
// returns its address. No test in this file touches the real network.
func startTestDNSServer(t *testing.T, network string, handler dns.HandlerFunc) string {
	t.Helper()

	mux := dns.NewServeMux()
	mux.HandleFunc(".", handler)

	srv := &dns.Server{Net: network, Handler: mux}
	var addr string

	switch network {
	case "udp":
		pc, err := net.ListenPacket("udp", "127.0.0.1:0")
		if err != nil {
			t.Skipf("cannot start test DNS server: %v", err)
		}
		srv.PacketConn = pc
		addr = pc.LocalAddr().String()
	case "tcp":
		l, err := net.Listen("tcp", "127.0.0.1:0")
		if err != nil {
			t.Skipf("cannot start test DNS server: %v", err)
		}
		srv.Listener = l
		addr = l.Addr().String()
	}

	started := make(chan struct{})
	srv.NotifyStartedFunc = func() { close(started) }

	go func() { _ = srv.ActivateAndServe() }()

	select {
	case <-started:
	case <-time.After(2 * time.Second):
		t.Skip("test DNS server did not start in time")
	}

	t.Cleanup(func() { _ = srv.Shutdown() })
	return addr
}

func aHandler(ip string) dns.HandlerFunc {
	return func(w dns.ResponseWriter, r *dns.Msg) {
		resp := new(dns.Msg)
		resp.SetReply(r)
		resp.Answer = append(resp.Answer, &dns.A{
			Hdr: dns.RR_Header{Name: r.Question[0].Name, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 300},
			A:   net.ParseIP(ip),
		})
		_ = w.WriteMsg(resp)
	}
}

func TestRealExchangeUDPHostPort(t *testing.T) {
	addr := startTestDNSServer(t, "udp", aHandler("1.2.3.4"))

	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
	defer cancel()

	// realExchange must honour an explicit host:port (used by root discovery
	// upstream resolvers).
	resp, err := realExchange(ctx, addr, buildQuery("example.com.", TypeA, 2048), false)
	if err != nil {
		t.Fatalf("realExchange: %v", err)
	}
	if len(resp.Answer) == 0 {
		t.Fatal("expected answers")
	}
}

func TestRealExchangeTCP(t *testing.T) {
	addr := startTestDNSServer(t, "tcp", aHandler("9.9.9.9"))

	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
	defer cancel()

	resp, err := realExchange(ctx, addr, buildQuery("example.com.", TypeA, 2048), true)
	if err != nil {
		t.Fatalf("realExchange TCP: %v", err)
	}
	if len(resp.Answer) == 0 {
		t.Fatal("expected answers")
	}
}

func TestRealExchangeUnreachable(t *testing.T) {
	ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
	defer cancel()

	_, err := realExchange(ctx, "127.0.0.1:1", buildQuery("example.com.", TypeA, 2048), false)
	if err == nil {
		t.Fatal("expected error for unreachable server")
	}
}

func TestClientAgainstLocalServer(t *testing.T) {
	// Written by the server handler goroutine, read by the test goroutine;
	// the UDP round-trip provides no happens-before edge, so use an atomic.
	var sawRD atomic.Bool
	addr := startTestDNSServer(t, "udp", func(w dns.ResponseWriter, r *dns.Msg) {
		sawRD.Store(r.RecursionDesired)
		aHandler("5.6.7.8")(w, r)
	})

	// Route the client's exchange to the test server's port while still
	// exercising realExchange.
	c := NewClient(&QueryConfig{Retries: 1, Timeout: 2 * time.Second},
		func(ctx context.Context, server string, msg *dns.Msg, useTCP bool) (*dns.Msg, error) {
			return realExchange(ctx, addr, msg, useTCP)
		})

	resp, _, err := c.Query(context.Background(), net.ParseIP("127.0.0.1"), "example.com", TypeA)
	if err != nil {
		t.Fatalf("Client.Query: %v", err)
	}
	if len(resp.Answer) == 0 {
		t.Fatal("expected answers")
	}
	if sawRD.Load() {
		t.Error("wire query must have RD=0")
	}
}