cmd/sniproxy: draw the rest of the DNS owl.

Add a DNS server which always responds as its own IP addresses.

Additionally add a tsnet TailscaleIPs() function to return the
IP addresses, both IPv4 and IPv6.

Updates https://github.com/tailscale/tailscale/issues/1748

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

3 files changed, 143 insertions, 3 deletions

diff --git a/cmd/sniproxy/snipproxy.go b/cmd/sniproxy/snipproxy.go
index d465c690a..8e7b4f85d 100644
--- a/cmd/sniproxy/snipproxy.go
+++ b/cmd/sniproxy/snipproxy.go
@@ -14,6 +14,7 @@ import (
 	"strings"
 	"time"
 
+	"golang.org/x/net/dns/dnsmessage"
 	"inet.af/tcpproxy"
 	"tailscale.com/client/tailscale"
 	"tailscale.com/net/netutil"
@@ -23,6 +24,8 @@ import (
 
 var ports = flag.String("ports", "443", "comma-separated list of ports to proxy")
 
+var tsMBox = dnsmessage.MustNewName("support.tailscale.com.")
+
 func main() {
 	flag.Parse()
 	if *ports == "" {
@@ -86,8 +89,29 @@ func (s *server) serveDNSConn(c nettype.ConnPacketConn) {
 	c.SetReadDeadline(time.Now().Add(5 * time.Second))
 	buf := make([]byte, 1500)
 	n, err := c.Read(buf)
-	log.Printf("got DNS packet: %q, %v", buf[:n], err)
-	// TODO: rest of the owl
+	if err != nil {
+		log.Printf("c.Read failed: %v\n ", err)
+		return
+	}
+
+	var msg dnsmessage.Message
+	err = msg.Unpack(buf[:n])
+	if err != nil {
+		log.Printf("dnsmessage unpack failed: %v\n ", err)
+		return
+	}
+
+	buf, err = s.dnsResponse(&msg)
+	if err != nil {
+		log.Printf("s.dnsResponse failed: %v\n", err)
+		return
+	}
+
+	_, err = c.Write(buf)
+	if err != nil {
+		log.Printf("c.Write failed: %v\n", err)
+		return
+	}
 }
 
 func (s *server) serveConn(c net.Conn) {
@@ -107,7 +131,6 @@ func (s *server) serveConn(c net.Conn) {
 		return netutil.NewOneConnListener(c, nil), nil
 	}
 	p.AddSNIRouteFunc(addrPortStr, func(ctx context.Context, sniName string) (t tcpproxy.Target, ok bool) {
-		log.Printf("got req for %q from %v", sniName, c.RemoteAddr())
 		return &tcpproxy.DialProxy{
 			Addr:        net.JoinHostPort(sniName, port),
 			DialContext: dialer.DialContext,
@@ -115,3 +138,62 @@ func (s *server) serveConn(c net.Conn) {
 	})
 	p.Start()
 }
+
+func (s *server) dnsResponse(req *dnsmessage.Message) (buf []byte, err error) {
+	resp := dnsmessage.NewBuilder(buf,
+		dnsmessage.Header{
+			ID:            req.Header.ID,
+			Response:      true,
+			Authoritative: true,
+		})
+	resp.EnableCompression()
+
+	if len(req.Questions) == 0 {
+		buf, _ = resp.Finish()
+		return
+	}
+
+	q := req.Questions[0]
+	err = resp.StartQuestions()
+	if err != nil {
+		return
+	}
+	resp.Question(q)
+
+	ip4, ip6 := s.ts.TailscaleIPs()
+	err = resp.StartAnswers()
+	if err != nil {
+		return
+	}
+
+	switch q.Type {
+	case dnsmessage.TypeAAAA:
+		err = resp.AAAAResource(
+			dnsmessage.ResourceHeader{Name: q.Name, Class: q.Class, TTL: 120},
+			dnsmessage.AAAAResource{AAAA: ip6.As16()},
+		)
+
+	case dnsmessage.TypeA:
+		err = resp.AResource(
+			dnsmessage.ResourceHeader{Name: q.Name, Class: q.Class, TTL: 120},
+			dnsmessage.AResource{A: ip4.As4()},
+		)
+	case dnsmessage.TypeSOA:
+		err = resp.SOAResource(
+			dnsmessage.ResourceHeader{Name: q.Name, Class: q.Class, TTL: 120},
+			dnsmessage.SOAResource{NS: q.Name, MBox: tsMBox, Serial: 2023030600,
+				Refresh: 120, Retry: 120, Expire: 120, MinTTL: 60},
+		)
+	case dnsmessage.TypeNS:
+		err = resp.NSResource(
+			dnsmessage.ResourceHeader{Name: q.Name, Class: q.Class, TTL: 120},
+			dnsmessage.NSResource{NS: tsMBox},
+		)
+	}
+
+	if err != nil {
+		return
+	}
+
+	return resp.Finish()
+}
diff --git a/tsnet/tsnet.go b/tsnet/tsnet.go
index a348285c7..e021a55fa 100644
--- a/tsnet/tsnet.go
+++ b/tsnet/tsnet.go
@@ -351,6 +351,24 @@ func (s *Server) doInit() {
 	}
 }
 
+// TailscaleIPs returns IPv4 and IPv6 addresses for this node. If the node
+// has not yet joined a tailnet or is otherwise unaware of its own IP addresses,
+// the returned ip4, ip6 will be !netip.IsValid().
+func (s *Server) TailscaleIPs() (ip4, ip6 netip.Addr) {
+	nm := s.lb.NetMap()
+	for _, addr := range nm.Addresses {
+		ip := addr.Addr()
+		if ip.Is6() {
+			ip6 = ip
+		}
+		if ip.Is4() {
+			ip4 = ip
+		}
+	}
+
+	return ip4, ip6
+}
+
 func (s *Server) getAuthKey() string {
 	if v := s.AuthKey; v != "" {
 		return v
diff --git a/tsnet/tsnet_test.go b/tsnet/tsnet_test.go
index 488c518d4..bc5adc2e2 100644
--- a/tsnet/tsnet_test.go
+++ b/tsnet/tsnet_test.go
@@ -299,3 +299,43 @@ func TestLoopbackSOCKS5(t *testing.T) {
 		t.Errorf("got %q, want %q", got, want)
 	}
 }
+
+func TestTailscaleIPs(t *testing.T) {
+	controlURL := startControl(t)
+
+	tmp := t.TempDir()
+	tmps1 := filepath.Join(tmp, "s1")
+	os.MkdirAll(tmps1, 0755)
+	s1 := &Server{
+		Dir:        tmps1,
+		ControlURL: controlURL,
+		Hostname:   "s1",
+		Store:      new(mem.Store),
+		Ephemeral:  true,
+	}
+	defer s1.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	s1status, err := s1.Up(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var upIp4, upIp6 netip.Addr
+	for _, ip := range s1status.TailscaleIPs {
+		if ip.Is6() {
+			upIp6 = ip
+		}
+		if ip.Is4() {
+			upIp4 = ip
+		}
+	}
+
+	sIp4, sIp6 := s1.TailscaleIPs()
+	if !(upIp4 == sIp4 && upIp6 == sIp6) {
+		t.Errorf("s1.TailscaleIPs returned a different result than S1.Up, (%s, %s) != (%s, %s)",
+			sIp4, upIp4, sIp6, upIp6)
+	}
+}