Add -4 and -6 flags (redo)

This PR is a redo of go-acme#1802. Since that PR has been idle so long,
the branches have diverged quite a bit and it was easier to start
anew.

The work in this PR includes the work originally done by dmke in go-acme#1802.

This PR is to resolve go-acme#1801.

Author: jsumners

.golangci.yml

@@ -171,6 +171,8 @@ issues:
       text: 'dnsTimeout is a global variable'
     - path: challenge/dns01/nameserver_test.go
       text: 'findXByFqdnTestCases is a global variable'
+    - path: challenge/dns01/network.go
+      text: 'currentNetworkStack is a global variable'
     - path: challenge/http01/domain_matcher.go
       text: 'string `Host` has \d occurrences, make it a constant'
     - path: challenge/http01/domain_matcher.go

challenge/dns01/nameserver.go

@@ -265,7 +265,8 @@ func createDNSMsg(fqdn string, rtype uint16, recursive bool) *dns.Msg {
 
 func sendDNSQuery(m *dns.Msg, ns string) (*dns.Msg, error) {
 	if ok, _ := strconv.ParseBool(os.Getenv("LEGO_EXPERIMENTAL_DNS_TCP_ONLY")); ok {
-		tcp := &dns.Client{Net: "tcp", Timeout: dnsTimeout}
+		network := currentNetworkStack.Network("tcp")
+		tcp := &dns.Client{Net: network, Timeout: dnsTimeout}
 		r, _, err := tcp.Exchange(m, ns)
 		if err != nil {
 			return r, &DNSError{Message: "DNS call error", MsgIn: m, NS: ns, Err: err}
@@ -274,11 +275,16 @@ func sendDNSQuery(m *dns.Msg, ns string) (*dns.Msg, error) {
 		return r, nil
 	}
 
-	udp := &dns.Client{Net: "udp", Timeout: dnsTimeout}
+	udpNetwork := currentNetworkStack.Network("udp")
+	udp := &dns.Client{Net: udpNetwork, Timeout: dnsTimeout}
 	r, _, err := udp.Exchange(m, ns)
 
-	if r != nil && r.Truncated {
-		tcp := &dns.Client{Net: "tcp", Timeout: dnsTimeout}
+	// We can encounter a net.OpError if the nameserver is not listening
+	// on UDP at all, i.e. net.Dial could not make a connection.
+	var opErr *net.OpError
+	if (r != nil && r.Truncated) || errors.As(err, &opErr) {
+		tcpNetwork := currentNetworkStack.Network("tcp")
+		tcp := &dns.Client{Net: tcpNetwork, Timeout: dnsTimeout}
 		// If the TCP request succeeds, the "err" will reset to nil
 		r, _, err = tcp.Exchange(m, ns)
 	}

challenge/dns01/nameserver_test.go

@@ -2,14 +2,133 @@ package dns01
 
 import (
 	"errors"
+	"net"
 	"sort"
+	"sync"
 	"testing"
 
 	"github.com/miekg/dns"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
+func testDNSHandler(writer dns.ResponseWriter, reply *dns.Msg) {
+	msg := dns.Msg{}
+	msg.SetReply(reply)
+
+	if reply.Question[0].Qtype == dns.TypeA {
+		msg.Authoritative = true
+		domain := msg.Question[0].Name
+		msg.Answer = append(
+			msg.Answer,
+			&dns.A{
+				Hdr: dns.RR_Header{
+					Name:   domain,
+					Rrtype: dns.TypeA,
+					Class:  dns.ClassINET,
+					Ttl:    60,
+				},
+				A: net.IPv4(127, 0, 0, 1),
+			},
+		)
+	}
+
+	_ = writer.WriteMsg(&msg)
+}
+
+// getTestNameserver constructs a new DNS server on a local address, or set
+// of addresses, that responds to an `A` query for `example.com`.
+func getTestNameserver(t *testing.T, network string) *dns.Server {
+	t.Helper()
+	server := &dns.Server{
+		Handler: dns.HandlerFunc(testDNSHandler),
+		Net:     network,
+	}
+	switch network {
+	case "tcp", "udp":
+		server.Addr = "0.0.0.0:0"
+	case "tcp4", "udp4":
+		server.Addr = "127.0.0.1:0"
+	case "tcp6", "udp6":
+		server.Addr = "[::1]:0"
+	}
+
+	waitLock := sync.Mutex{}
+	waitLock.Lock()
+	server.NotifyStartedFunc = waitLock.Unlock
+
+	go func() { _ = server.ListenAndServe() }()
+
+	waitLock.Lock()
+	return server
+}
+
+func startTestNameserver(t *testing.T, stack networkStack, proto string) (shutdown func(), addr string) {
+	t.Helper()
+	currentNetworkStack = stack
+	srv := getTestNameserver(t, currentNetworkStack.Network(proto))
+
+	shutdown = func() { _ = srv.Shutdown() }
+	if proto == "tcp" {
+		addr = srv.Listener.Addr().String()
+	} else {
+		addr = srv.PacketConn.LocalAddr().String()
+	}
+	return
+}
+
+func TestSendDNSQuery(t *testing.T) {
+	currentNameservers := recursiveNameservers
+
+	t.Cleanup(func() {
+		recursiveNameservers = currentNameservers
+		currentNetworkStack = dualStack
+	})
+
+	t.Run("does udp4 only", func(t *testing.T) {
+		stop, addr := startTestNameserver(t, ipv4only, "udp")
+		defer stop()
+
+		recursiveNameservers = ParseNameservers([]string{addr})
+		msg := createDNSMsg("example.com.", dns.TypeA, true)
+		result, queryError := sendDNSQuery(msg, addr)
+		require.NoError(t, queryError)
+		assert.Equal(t, result.Answer[0].(*dns.A).A.String(), "127.0.0.1")
+	})
+
+	t.Run("does udp6 only", func(t *testing.T) {
+		stop, addr := startTestNameserver(t, ipv6only, "udp")
+		defer stop()
+
+		recursiveNameservers = ParseNameservers([]string{addr})
+		msg := createDNSMsg("example.com.", dns.TypeA, true)
+		result, queryError := sendDNSQuery(msg, addr)
+		require.NoError(t, queryError)
+		assert.Equal(t, result.Answer[0].(*dns.A).A.String(), "127.0.0.1")
+	})
+
+	t.Run("does tcp4 and tcp6", func(t *testing.T) {
+		stop, addr := startTestNameserver(t, dualStack, "tcp")
+		host, port, _ := net.SplitHostPort(addr)
+		defer stop()
+		t.Logf("### port: %s", port)
+
+		addr6 := net.JoinHostPort(host, port)
+		recursiveNameservers = ParseNameservers([]string{addr6})
+		msg := createDNSMsg("example.com.", dns.TypeA, true)
+		result, queryError := sendDNSQuery(msg, addr6)
+		require.NoError(t, queryError)
+		assert.Equal(t, result.Answer[0].(*dns.A).A.String(), "127.0.0.1")
+
+		addr4 := net.JoinHostPort("127.0.0.1", port)
+		recursiveNameservers = ParseNameservers([]string{addr4})
+		msg = createDNSMsg("example.com.", dns.TypeA, true)
+		result, queryError = sendDNSQuery(msg, addr4)
+		require.NoError(t, queryError)
+		assert.Equal(t, result.Answer[0].(*dns.A).A.String(), "127.0.0.1")
+	})
+}
+
 func TestLookupNameserversOK(t *testing.T) {
 	testCases := []struct {
 		fqdn string
@@ -123,8 +242,10 @@ var findXByFqdnTestCases = []struct {
 		fqdn:        "mail.google.com.",
 		zone:        "google.com.",
 		nameservers: []string{":7053", ":8053", ":9053"},
-		// use only the start of the message because the port changes with each call: 127.0.0.1:XXXXX->127.0.0.1:7053.
-		expectedError: "[fqdn=mail.google.com.] could not find the start of authority for 'mail.google.com.': DNS call error: read udp ",
+		// NOTE: On Windows, net.DialContext finds a way down to the ContectEx syscall.
+		// There a fault is marked as "connectex", not "connect", see
+		// https://cs.opensource.google/go/go/+/refs/tags/go1.19.5:src/net/fd_windows.go;l=112
+		expectedError: "could not find the start of authority for 'mail.google.com.':",
 	},
 	{
 		desc:          "no nameservers",

challenge/dns01/network.go

@@ -0,0 +1,41 @@
+package dns01
+
+// networkStack is used to indicate which IP stack should be used for DNS queries.
+type networkStack int
+
+const (
+	dualStack networkStack = iota
+	ipv4only
+	ipv6only
+)
+
+// currentNetworkStack is used to define which IP stack will be used. The default is
+// both IPv4 and IPv6. Set to IPv4Only or IPv6Only to select either version.
+var currentNetworkStack = dualStack
+
+// Network interprets the NetworkStack setting in relation to the desired
+// protocol. The proto value should be either "udp" or "tcp".
+func (s networkStack) Network(proto string) string {
+	// The DNS client passes whatever value is set in (*dns.Client).Net to
+	// the [net.Dialer](https://github.com/miekg/dns/blob/fe20d5d/client.go#L119-L141).
+	// And the net.Dialer accepts strings such as "udp4" or "tcp6"
+	// (https://cs.opensource.google/go/go/+/refs/tags/go1.18.9:src/net/dial.go;l=167-182).
+	switch s {
+	case ipv4only:
+		return proto + "4"
+	case ipv6only:
+		return proto + "6"
+	default:
+		return proto
+	}
+}
+
+// SetIPv4Only forces DNS queries to only happen over the IPv4 stack.
+func SetIPv4Only() { currentNetworkStack = ipv4only }
+
+// SetIPv6Only forces DNS queries to only happen over the IPv6 stack.
+func SetIPv6Only() { currentNetworkStack = ipv6only }
+
+// SetDualStack indicates that both IPv4 and IPv6 should be allowed.
+// This setting lets the OS determine which IP stack to use.
+func SetDualStack() { currentNetworkStack = dualStack }

challenge/http01/http_challenge_server.go

@@ -41,6 +41,28 @@ func NewUnixProviderServer(socketPath string, mode fs.FileMode) *ProviderServer
 	return &ProviderServer{network: "unix", address: socketPath, socketMode: mode, matcher: &hostMatcher{}}
 }
 
+// SetIPv4Only starts the challenge server on an IPv4 address.
+//
+// Calling this method has no effect if s was created with NewUnixProviderServer.
+func (s *ProviderServer) SetIPv4Only() { s.setTCPStack("tcp4") }
+
+// SetIPv6Only starts the challenge server on an IPv6 address.
+//
+// Calling this method has no effect if s was created with NewUnixProviderServer.
+func (s *ProviderServer) SetIPv6Only() { s.setTCPStack("tcp6") }
+
+// SetDualStack indicates that both IPv4 and IPv6 should be allowed.
+// This setting lets the OS determine which IP stack to use for the challenge server.
+//
+// Calling this method has no effect if s was created with NewUnixProviderServer.
+func (s *ProviderServer) SetDualStack() { s.setTCPStack("tcp") }
+
+func (s *ProviderServer) setTCPStack(network string) {
+	if s.network != "unix" {
+		s.network = network
+	}
+}
+
 // Present starts a web server and makes the token available at `ChallengePath(token)` for web requests.
 func (s *ProviderServer) Present(domain, token, keyAuth string) error {
 	var err error

challenge/http01/http_challenge_test.go

@@ -32,6 +32,7 @@ func TestProviderServer_GetAddress(t *testing.T) {
 	testCases := []struct {
 		desc     string
 		server   *ProviderServer
+		network  func(server *ProviderServer)
 		expected string
 	}{
 		{
@@ -49,6 +50,18 @@ func TestProviderServer_GetAddress(t *testing.T) {
 			server:   NewProviderServer("localhost", "8080"),
 			expected: "localhost:8080",
 		},
+		{
+			desc:     "TCP4 with host and port",
+			server:   NewProviderServer("localhost", "8080"),
+			network:  func(s *ProviderServer) { s.SetIPv4Only() },
+			expected: "localhost:8080",
+		},
+		{
+			desc:     "TCP6 with host and port",
+			server:   NewProviderServer("localhost", "8080"),
+			network:  func(s *ProviderServer) { s.SetIPv6Only() },
+			expected: "localhost:8080",
+		},
 		{
 			desc:     "UDS socket",
 			server:   NewUnixProviderServer(sock, fs.ModeSocket|0o666),
@@ -60,6 +73,10 @@ func TestProviderServer_GetAddress(t *testing.T) {
 		t.Run(test.desc, func(t *testing.T) {
 			t.Parallel()
 
+			if test.network != nil {
+				test.network(test.server)
+			}
+
 			address := test.server.GetAddress()
 			assert.Equal(t, test.expected, address)
 		})

challenge/tlsalpn01/tls_alpn_challenge_server.go

@@ -26,16 +26,30 @@ const (
 type ProviderServer struct {
 	iface    string
 	port     string
+	network  string
 	listener net.Listener
 }
 
 // NewProviderServer creates a new ProviderServer on the selected interface and port.
 // Setting iface and / or port to an empty string will make the server fall back to
 // the "any" interface and port 443 respectively.
 func NewProviderServer(iface, port string) *ProviderServer {
-	return &ProviderServer{iface: iface, port: port}
+	if port == "" {
+		port = defaultTLSPort
+	}
+	return &ProviderServer{iface: iface, port: port, network: "tcp"}
 }
 
+// SetIPv4Only starts the challenge server on an IPv4 address.
+func (s *ProviderServer) SetIPv4Only() { s.network = "tcp4" }
+
+// SetIPv6Only starts the challenge server on an IPv6 address.
+func (s *ProviderServer) SetIPv6Only() { s.network = "tcp6" }
+
+// SetDualStack indicates that both IPv4 and IPv6 should be allowed.
+// This setting lets the OS determine which IP stack to use for the challenge server.
+func (s *ProviderServer) SetDualStack() { s.network = "tcp" }
+
 func (s *ProviderServer) GetAddress() string {
 	return net.JoinHostPort(s.iface, s.port)
 }
@@ -65,7 +79,7 @@ func (s *ProviderServer) Present(domain, token, keyAuth string) error {
 	tlsConf.NextProtos = []string{ACMETLS1Protocol}
 
 	// Create the listener with the created tls.Config.
-	s.listener, err = tls.Listen("tcp", s.GetAddress(), tlsConf)
+	s.listener, err = tls.Listen(s.network, s.GetAddress(), tlsConf)
 	if err != nil {
 		return fmt.Errorf("could not start HTTPS server for challenge: %w", err)
 	}