cmd/lopower: add start of DNS server

Change-Id: Ibc9b1ee914a649a4f032062c23d700cfbef84312
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/lopower/lopower.go

@@ -27,6 +27,8 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
 	qrcode "github.com/skip2/go-qrcode"
 	"github.com/tailscale/wireguard-go/conn"
 	"github.com/tailscale/wireguard-go/device"
@@ -47,6 +49,7 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/syncs"
 	"tailscale.com/tsnet"
+	"tailscale.com/types/ipproto"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/must"
@@ -76,6 +79,11 @@ type config struct {
 	V6CIDR netip.Prefix
 }
 
+// IsLocalIP reports whether ip is one of the local IPs.
+func (c *config) IsLocalIP(ip netip.Addr) bool {
+	return ip.IsValid() && (ip == c.V4 || ip == c.V6)
+}
+
 type Peer struct {
 	PrivKey key.NodePrivate // e.g. proxy client's
 	V4      netip.Addr
@@ -168,6 +176,7 @@ func newLP(ctx context.Context) *lpServer {
 		Errorf:   logf,
 	}
 	lp := &lpServer{
+		ctx:    ctx,
 		dir:    *confDir,
 		readCh: make(chan *stack.PacketBuffer, 16),
 	}
@@ -197,8 +206,9 @@ type lpServer struct {
 	tsnet  *tsnet.Server
 	d      *device.Device
 	ns     *stack.Stack
+	ctx    context.Context // canceled on shutdown
 	linkEP *channel.Endpoint
-	readCh chan *stack.PacketBuffer
+	readCh chan *stack.PacketBuffer // from gvisor/dns server => out to network
 
 	// protocolConns tracks the number of active connections for each connection.
 	// It is used to add and remove protocol addresses from netstack as needed.
@@ -553,6 +563,7 @@ func (t *nsTUN) Close() error {
 	return nil
 }
 
+// Read reads packets from gvisor (or the DNS server) to send out to the network.
 func (t *nsTUN) Read(out [][]byte, sizes []int, offset int) (int, error) {
 	select {
 	case <-t.closeCh:
@@ -578,12 +589,18 @@ func (t *nsTUN) Read(out [][]byte, sizes []int, offset int) (int, error) {
 func (t *nsTUN) Write(buffs [][]byte, offset int) (int, error) {
 	var pkt packet.Parsed
 	for _, buff := range buffs {
-		pkt.Decode(buff[offset:])
+		raw := buff[offset:]
+		pkt.Decode(raw)
 		packetBuf := stack.NewPacketBuffer(stack.PacketBufferOptions{
-			Payload: buffer.MakeWithData(slices.Clone(buff[offset:])),
+			Payload: buffer.MakeWithData(slices.Clone(raw)),
 		})
 		if *verbosePackets {
-			log.Printf("[v] nsTUN.Write (in): % 02x", buff[offset:])
+			log.Printf("[v] nsTUN.Write (in): % 02x", raw)
+		}
+		if pkt.IPProto == ipproto.UDP && pkt.Dst.Port() == 53 && t.lp.c.IsLocalIP(pkt.Dst.Addr()) {
+			// Handle DNS queries before sending to gvisor.
+			t.lp.handleDNSUDPQuery(raw)
+			continue
 		}
 		if pkt.IPVersion == 4 {
 			t.lp.linkEP.InjectInbound(ipv4.ProtocolNumber, packetBuf)
@@ -618,6 +635,119 @@ func (lp *lpServer) startTSNet(ctx context.Context) {
 	}
 }
 
+// caller owns the raw memory.
+func (lp *lpServer) handleDNSUDPQuery(raw []byte) {
+	var pkt packet.Parsed
+	pkt.Decode(raw)
+	if pkt.IPProto != ipproto.UDP || pkt.Dst.Port() != 53 || !lp.c.IsLocalIP(pkt.Dst.Addr()) {
+		panic("caller error")
+	}
+	m, ok := lp.tsnet.Sys().DNSManager.GetOK()
+	if !ok {
+		log.Printf("DNSManager.Get: not ready")
+		return
+	}
+	dnsRes, err := m.Query(context.Background(), pkt.Payload(), "udp", pkt.Src)
+	if err != nil {
+		log.Printf("DNS query error: %v", err)
+		return
+	}
+
+	ipLayer := mkIPLayer(layers.IPProtocolUDP, pkt.Dst.Addr(), pkt.Src.Addr())
+	udpLayer := &layers.UDP{
+		SrcPort: 53,
+		DstPort: layers.UDPPort(pkt.Src.Port()),
+	}
+
+	resPkt, err := mkPacket(ipLayer, udpLayer, gopacket.Payload(dnsRes))
+	if err != nil {
+		log.Printf("mkPacket: %v", err)
+		return
+	}
+	pktBuf := stack.NewPacketBuffer(stack.PacketBufferOptions{
+		Payload: buffer.MakeWithData(resPkt),
+	})
+	select {
+	case lp.readCh <- pktBuf:
+	case <-lp.ctx.Done():
+	}
+}
+
+type serializableNetworkLayer interface {
+	gopacket.SerializableLayer
+	gopacket.NetworkLayer
+}
+
+func mkIPLayer(proto layers.IPProtocol, src, dst netip.Addr) serializableNetworkLayer {
+	if src.Is4() {
+		return &layers.IPv4{
+			Protocol: proto,
+			SrcIP:    src.AsSlice(),
+			DstIP:    dst.AsSlice(),
+		}
+	}
+	if src.Is6() {
+		return &layers.IPv6{
+			NextHeader: proto,
+			SrcIP:      src.AsSlice(),
+			DstIP:      dst.AsSlice(),
+		}
+	}
+	panic("invalid src IP")
+}
+
+// mkPacket is a serializes a number of layers into a packet.
+//
+// It's a convenience wrapper around gopacket.SerializeLayers
+// that does some things automatically:
+//
+// * layers.IPv4/IPv6 Version is set to 4/6 if not already set
+// * layers.IPv4/IPv6 TTL/HopLimit is set to 64 if not already set
+// * the TCP/UDP/ICMPv6 checksum is set based on the network layer
+//
+// The provided layers in ll must be sorted from lowest (e.g. *layers.Ethernet)
+// to highest. (Depending on the need, the first layer will be either *layers.Ethernet
+// or *layers.IPv4/IPv6).
+func mkPacket(ll ...gopacket.SerializableLayer) ([]byte, error) {
+	var nl gopacket.NetworkLayer
+	for _, la := range ll {
+		switch la := la.(type) {
+		case *layers.IPv4:
+			nl = la
+			if la.Version == 0 {
+				la.Version = 4
+			}
+			if la.TTL == 0 {
+				la.TTL = 64
+			}
+		case *layers.IPv6:
+			nl = la
+			if la.Version == 0 {
+				la.Version = 6
+			}
+			if la.HopLimit == 0 {
+				la.HopLimit = 64
+			}
+		}
+	}
+	for _, la := range ll {
+		switch la := la.(type) {
+		case *layers.TCP:
+			la.SetNetworkLayerForChecksum(nl)
+		case *layers.UDP:
+			la.SetNetworkLayerForChecksum(nl)
+		case *layers.ICMPv6:
+			la.SetNetworkLayerForChecksum(nl)
+		}
+	}
+	buf := gopacket.NewSerializeBuffer()
+	opts := gopacket.SerializeOptions{FixLengths: true, ComputeChecksums: true}
+	if err := gopacket.SerializeLayers(buf, opts, ll...); err != nil {
+		return nil, fmt.Errorf("serializing packet: %v", err)
+	}
+	return buf.Bytes(), nil
+}
+
 func main() {
 	flag.Parse()
 	log.Printf("lopower starting")