cmd/sniproxy: add port forwarding and prometheus metrics

1. Add TCP port forwarding.
   For example: ./sniproxy -forwards=tcp/22/github.com
   will forward SSH to github.

   % ssh -i ~/.ssh/id_ecdsa.pem -T git@github.com
   Hi GitHubUser! You've successfully authenticated, but GitHub does not
   provide shell access.

   % ssh -i ~/.ssh/id_ecdsa.pem -T git@100.65.x.y
   Hi GitHubUser! You've successfully authenticated, but GitHub does not
   provide shell access.

2. Additionally export clientmetrics as prometheus metrics for local
   scraping over the tailnet: http://sniproxy-hostname:8080/debug/varz

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

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

cmd/sniproxy/.gitignore

@@ -0,0 +1 @@
+sniproxy

cmd/sniproxy/sniproxy.go

@@ -3,15 +3,20 @@
 
 // The sniproxy is an outbound SNI proxy. It receives TLS connections over
 // Tailscale on one or more TCP ports and sends them out to the same SNI
-// hostname & port on the internet. It only does TCP.
+// hostname & port on the internet. It can optionally forward one or more
+// TCP ports to a specific destination. It only does TCP.
 package main
 
 import (
 	"context"
+	"errors"
+	"expvar"
 	"flag"
+	"fmt"
 	"log"
 	"net"
 	"net/http"
+	"strconv"
 	"strings"
 	"time"
 
@@ -19,27 +24,54 @@ import (
 	"inet.af/tcpproxy"
 	"tailscale.com/client/tailscale"
 	"tailscale.com/hostinfo"
+	"tailscale.com/metrics"
 	"tailscale.com/net/netutil"
 	"tailscale.com/tsnet"
+	"tailscale.com/tsweb"
 	"tailscale.com/types/nettype"
 	"tailscale.com/util/clientmetric"
 )
 
 var (
 	ports        = flag.String("ports", "443", "comma-separated list of ports to proxy")
+	forwards     = flag.String("forwards", "", "comma-separated list of ports to transparently forward, protocol/number/destination. For example, --forwards=tcp/22/github.com,tcp/5432/sql.example.com")
 	wgPort       = flag.Int("wg-listen-port", 0, "UDP port to listen on for WireGuard and peer-to-peer traffic; 0 means automatically select")
 	promoteHTTPS = flag.Bool("promote-https", true, "promote HTTP to HTTPS")
+	debugPort    = flag.Int("debug-port", 8080, "Listening port for debug/metrics endpoint")
 )
 
 var tsMBox = dnsmessage.MustNewName("support.tailscale.com.")
 
-var (
-	numSessions    = clientmetric.NewCounter("sniproxy_sessions")
-	numBadAddrPort = clientmetric.NewCounter("sniproxy_bad_addrport")
-	dnsResponses   = clientmetric.NewCounter("sniproxy_dns_responses")
-	dnsFailures    = clientmetric.NewCounter("sniproxy_dns_failed")
-	httpPromoted   = clientmetric.NewCounter("sniproxy_http_promoted")
-)
+// portForward is the state for a single port forwarding entry, as passed to the --forward flag.
+type portForward struct {
+	Port        int
+	Proto       string
+	Destination string
+}
+
+// parseForward takes a proto/port/destination tuple as an input, as would be passed
+// to the --forward command line flag, and returns a *portForward struct of those parameters.
+func parseForward(value string) (*portForward, error) {
+	parts := strings.Split(value, "/")
+	if len(parts) != 3 {
+		return nil, errors.New("cannot parse: " + value)
+	}
+
+	proto := parts[0]
+	if proto != "tcp" {
+		return nil, errors.New("unsupported forwarding protocol: " + proto)
+	}
+	port, err := strconv.ParseUint(parts[1], 10, 16)
+	if err != nil {
+		return nil, errors.New("bad forwarding port: " + parts[1])
+	}
+	host := parts[2]
+	if host == "" {
+		return nil, errors.New("bad destination: " + value)
+	}
+
+	return &portForward{Port: int(port), Proto: proto, Destination: host}, nil
+}
 
 func main() {
 	flag.Parse()
@@ -58,6 +90,7 @@ func main() {
 		log.Fatal(err)
 	}
 	s.lc = lc
+	s.initMetrics()
 
 	for _, portStr := range strings.Split(*ports, ",") {
 		ln, err := s.ts.Listen("tcp", ":"+portStr)
@@ -68,6 +101,34 @@ func main() {
 		go s.serve(ln)
 	}
 
+	for _, forwStr := range strings.Split(*forwards, ",") {
+		if forwStr == "" {
+			continue
+		}
+		forw, err := parseForward(forwStr)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		ln, err := s.ts.Listen("tcp", ":"+strconv.Itoa(forw.Port))
+		if err != nil {
+			log.Fatal(err)
+		}
+		log.Printf("Serving on port %d to %s...", forw.Port, forw.Destination)
+
+		// Add an entry to the expvar LabelMap for Prometheus metrics,
+		// and create a clientmetric to report that same value.
+		service := portNumberToName(forw)
+		s.numTCPsessions.SetInt64(service, 0)
+		metric := fmt.Sprintf("sniproxy_tcp_sessions_%s", service)
+		clientmetric.NewCounterFunc(metric, func() int64 {
+			return s.numTCPsessions.Get(service).Value()
+		})
+
+		go s.forward(ln, forw)
+
+	}
+
 	ln, err := s.ts.Listen("udp", ":53")
 	if err != nil {
 		log.Fatal(err)
@@ -83,12 +144,31 @@ func main() {
 		go s.promoteHTTPS(ln)
 	}
 
+	if *debugPort != 0 {
+		mux := http.NewServeMux()
+		tsweb.Debugger(mux)
+		dln, err := s.ts.Listen("tcp", fmt.Sprintf(":%d", *debugPort))
+		if err != nil {
+			log.Fatal(err)
+		}
+		go func() {
+			log.Fatal(http.Serve(dln, mux))
+		}()
+	}
+
 	select {}
 }
 
 type server struct {
 	ts tsnet.Server
 	lc *tailscale.LocalClient
+
+	numTLSsessions expvar.Int
+	numTCPsessions *metrics.LabelMap
+	numBadAddrPort expvar.Int
+	dnsResponses   expvar.Int
+	dnsFailures    expvar.Int
+	httpPromoted   expvar.Int
 }
 
 func (s *server) serve(ln net.Listener) {
@@ -101,6 +181,16 @@ func (s *server) serve(ln net.Listener) {
 	}
 }
 
+func (s *server) forward(ln net.Listener, forw *portForward) {
+	for {
+		c, err := ln.Accept()
+		if err != nil {
+			log.Fatal(err)
+		}
+		go s.forwardConn(c, forw)
+	}
+}
+
 func (s *server) serveDNS(ln net.Listener) {
 	for {
 		c, err := ln.Accept()
@@ -118,7 +208,7 @@ func (s *server) serveDNSConn(c nettype.ConnPacketConn) {
 	n, err := c.Read(buf)
 	if err != nil {
 		log.Printf("c.Read failed: %v\n ", err)
-		dnsFailures.Add(1)
+		s.dnsFailures.Add(1)
 		return
 	}
 
@@ -126,25 +216,25 @@ func (s *server) serveDNSConn(c nettype.ConnPacketConn) {
 	err = msg.Unpack(buf[:n])
 	if err != nil {
 		log.Printf("dnsmessage unpack failed: %v\n ", err)
-		dnsFailures.Add(1)
+		s.dnsFailures.Add(1)
 		return
 	}
 
 	buf, err = s.dnsResponse(&msg)
 	if err != nil {
 		log.Printf("s.dnsResponse failed: %v\n", err)
-		dnsFailures.Add(1)
+		s.dnsFailures.Add(1)
 		return
 	}
 
 	_, err = c.Write(buf)
 	if err != nil {
 		log.Printf("c.Write failed: %v\n", err)
-		dnsFailures.Add(1)
+		s.dnsFailures.Add(1)
 		return
 	}
 
-	dnsResponses.Add(1)
+	s.dnsResponses.Add(1)
 }
 
 func (s *server) serveConn(c net.Conn) {
@@ -152,7 +242,7 @@ func (s *server) serveConn(c net.Conn) {
 	_, port, err := net.SplitHostPort(addrPortStr)
 	if err != nil {
 		log.Printf("bogus addrPort %q", addrPortStr)
-		numBadAddrPort.Add(1)
+		s.numBadAddrPort.Add(1)
 		c.Close()
 		return
 	}
@@ -165,7 +255,7 @@ 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) {
-		numSessions.Add(1)
+		s.numTLSsessions.Add(1)
 		return &tcpproxy.DialProxy{
 			Addr:        net.JoinHostPort(sniName, port),
 			DialContext: dialer.DialContext,
@@ -174,6 +264,49 @@ func (s *server) serveConn(c net.Conn) {
 	p.Start()
 }
 
+// portNumberToName returns a human-readable name for several port numbers commonly forwarded,
+// and "tcp###" for everything else. It is used for metric label names.
+func portNumberToName(forw *portForward) string {
+	switch forw.Port {
+	case 22:
+		return "ssh"
+	case 1433:
+		return "sqlserver"
+	case 3306:
+		return "mysql"
+	case 3389:
+		return "rdp"
+	case 5432:
+		return "postgres"
+	default:
+		return fmt.Sprintf("%s%d", forw.Proto, forw.Port)
+	}
+}
+
+// forwardConn sets up a forwarder for a TCP connection. It does not inspect of the data
+// like the SNI forwarding does, it merely forwards all data to the destination specified
+// in the --forward=tcp/22/github.com argument.
+func (s *server) forwardConn(c net.Conn, forw *portForward) {
+	addrPortStr := c.LocalAddr().String()
+
+	var dialer net.Dialer
+	dialer.Timeout = 30 * time.Second
+
+	var p tcpproxy.Proxy
+	p.ListenFunc = func(net, laddr string) (net.Listener, error) {
+		return netutil.NewOneConnListener(c, nil), nil
+	}
+
+	dial := &tcpproxy.DialProxy{
+		Addr:        fmt.Sprintf("%s:%d", forw.Destination, forw.Port),
+		DialContext: dialer.DialContext,
+	}
+
+	p.AddRoute(addrPortStr, dial)
+	s.numTCPsessions.Add(portNumberToName(forw), 1)
+	p.Start()
+}
+
 func (s *server) dnsResponse(req *dnsmessage.Message) (buf []byte, err error) {
 	resp := dnsmessage.NewBuilder(buf,
 		dnsmessage.Header{
@@ -235,8 +368,36 @@ func (s *server) dnsResponse(req *dnsmessage.Message) (buf []byte, err error) {
 
 func (s *server) promoteHTTPS(ln net.Listener) {
 	err := http.Serve(ln, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		httpPromoted.Add(1)
+		s.httpPromoted.Add(1)
 		http.Redirect(w, r, "https://"+r.Host+r.RequestURI, http.StatusFound)
 	}))
 	log.Fatalf("promoteHTTPS http.Serve: %v", err)
 }
+
+// initMetrics sets up local prometheus metrics, and creates clientmetrics to report those
+// same counters.
+func (s *server) initMetrics() {
+	stats := new(metrics.Set)
+
+	stats.Set("tls_sessions", &s.numTLSsessions)
+	clientmetric.NewCounterFunc("sniproxy_tls_sessions", s.numTLSsessions.Value)
+
+	s.numTCPsessions = &metrics.LabelMap{Label: "proto"}
+	stats.Set("tcp_sessions", s.numTCPsessions)
+	// clientmetric doesn't have a good way to implement a Map type.
+	// We create clientmetrics dynamically when parsing the --forwards argument
+
+	stats.Set("bad_addrport", &s.numBadAddrPort)
+	clientmetric.NewCounterFunc("sniproxy_bad_addrport", s.numBadAddrPort.Value)
+
+	stats.Set("dns_responses", &s.dnsResponses)
+	clientmetric.NewCounterFunc("sniproxy_dns_responses", s.dnsResponses.Value)
+
+	stats.Set("dns_failed", &s.dnsFailures)
+	clientmetric.NewCounterFunc("sniproxy_dns_failed", s.dnsFailures.Value)
+
+	stats.Set("http_promoted", &s.httpPromoted)
+	clientmetric.NewCounterFunc("sniproxy_http_promoted", s.httpPromoted.Value)
+
+	expvar.Publish("sniproxy", stats)
+}

cmd/sniproxy/sniproxy_test.go

@@ -0,0 +1,37 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestPortForwardingArguments(t *testing.T) {
+	tests := []struct {
+		in      string
+		wanterr string
+		want    *portForward
+	}{
+		{"", "", nil},
+		{"bad port specifier", "cannot parse", nil},
+		{"tcp/xyz/example.com", "bad forwarding port", nil},
+		{"tcp//example.com", "bad forwarding port", nil},
+		{"tcp/2112/", "bad destination", nil},
+		{"udp/53/example.com", "unsupported forwarding protocol", nil},
+		{"tcp/22/github.com", "", &portForward{Proto: "tcp", Port: 22, Destination: "github.com"}},
+	}
+	for _, tt := range tests {
+		got, goterr := parseForward(tt.in)
+		if tt.wanterr != "" {
+			if !strings.Contains(goterr.Error(), tt.wanterr) {
+				t.Errorf("f(%q).err = %v; want %v", tt.in, goterr, tt.wanterr)
+			}
+		} else if diff := cmp.Diff(got, tt.want); diff != "" {
+			t.Errorf("Parsed forward (-got, +want):\n%s", diff)
+		}
+	}
+}