control/controlhttp: package to get a controlbase.Conn over HTTP(S).

Updates #3488

Signed-off-by: David Anderson <danderson@tailscale.com>

control/controlhttp/client.go

@@ -0,0 +1,242 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package controlhttp implements the Tailscale 2021 control protocol
+// base transport over HTTP.
+//
+// This tunnels the protocol in control/controlbase over HTTP with a
+// variety of compatibility fallbacks for handling picky or deep
+// inspecting proxies.
+//
+// In the happy path, a client makes a single cleartext HTTP request
+// to the server, the server responds with 101 Switching Protocols,
+// and the control base protocol takes place over plain TCP.
+//
+// In the compatibility path, the client does the above over HTTPS,
+// resulting in double encryption (once for the control transport, and
+// once for the outer TLS layer).
+package controlhttp
+
+import (
+	"context"
+	"crypto/tls"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"net/http/httptrace"
+	"net/url"
+
+	"tailscale.com/control/controlbase"
+	"tailscale.com/net/dnscache"
+	"tailscale.com/net/dnsfallback"
+	"tailscale.com/net/netns"
+	"tailscale.com/net/tlsdial"
+	"tailscale.com/net/tshttpproxy"
+	"tailscale.com/types/key"
+)
+
+// upgradeHeader is the value of the Upgrade HTTP header used to
+// indicate the Tailscale control protocol.
+const (
+	upgradeHeaderValue  = "tailscale-control-protocol"
+	handshakeHeaderName = "X-Tailscale-Handshake"
+)
+
+// Dial connects to the HTTP server at addr, requests to switch to the
+// Tailscale control protocol, and returns an established control
+// protocol connection.
+//
+// If Dial fails to connect using addr, it also tries to tunnel over
+// TLS to <addr's host>:443 as a compatibility fallback.
+func Dial(ctx context.Context, addr string, machineKey key.MachinePrivate, controlKey key.MachinePublic) (*controlbase.Conn, error) {
+	host, port, err := net.SplitHostPort(addr)
+	if err != nil {
+		return nil, err
+	}
+	a := &dialParams{
+		ctx:        ctx,
+		host:       host,
+		httpPort:   port,
+		httpsPort:  "443",
+		machineKey: machineKey,
+		controlKey: controlKey,
+		proxyFunc:  tshttpproxy.ProxyFromEnvironment,
+	}
+	return a.dial()
+}
+
+type dialParams struct {
+	ctx        context.Context
+	host       string
+	httpPort   string
+	httpsPort  string
+	machineKey key.MachinePrivate
+	controlKey key.MachinePublic
+	proxyFunc  func(*http.Request) (*url.URL, error) // or nil
+
+	// For tests only
+	insecureTLS bool
+}
+
+func (a *dialParams) dial() (*controlbase.Conn, error) {
+	init, cont, err := controlbase.ClientDeferred(a.machineKey, a.controlKey)
+	if err != nil {
+		return nil, err
+	}
+
+	u := &url.URL{
+		Scheme: "http",
+		Host:   net.JoinHostPort(a.host, a.httpPort),
+		Path:   "/switch",
+	}
+	conn, httpErr := a.tryURL(u, init)
+	if httpErr == nil {
+		ret, err := cont(a.ctx, conn)
+		if err != nil {
+			conn.Close()
+			return nil, err
+		}
+		return ret, nil
+	}
+
+	// Connecting over plain HTTP failed, assume it's an HTTP proxy
+	// being difficult and see if we can get through over HTTPS.
+	u.Scheme = "https"
+	u.Host = net.JoinHostPort(a.host, a.httpsPort)
+	init, cont, err = controlbase.ClientDeferred(a.machineKey, a.controlKey)
+	if err != nil {
+		return nil, err
+	}
+	conn, tlsErr := a.tryURL(u, init)
+	if tlsErr == nil {
+		ret, err := cont(a.ctx, conn)
+		if err != nil {
+			conn.Close()
+			return nil, err
+		}
+		return ret, nil
+	}
+
+	return nil, fmt.Errorf("all connection attempts failed (HTTP: %v, HTTPS: %v)", httpErr, tlsErr)
+}
+
+func (a *dialParams) tryURL(u *url.URL, init []byte) (net.Conn, error) {
+	dns := &dnscache.Resolver{
+		Forward:          dnscache.Get().Forward,
+		LookupIPFallback: dnsfallback.Lookup,
+		UseLastGood:      true,
+	}
+	dialer := netns.NewDialer(log.Printf)
+	tr := http.DefaultTransport.(*http.Transport).Clone()
+	defer tr.CloseIdleConnections()
+	tr.Proxy = a.proxyFunc
+	tshttpproxy.SetTransportGetProxyConnectHeader(tr)
+	tr.DialContext = dnscache.Dialer(dialer.DialContext, dns)
+	// Disable HTTP2, since h2 can't do protocol switching.
+	tr.TLSClientConfig.NextProtos = []string{}
+	tr.TLSNextProto = map[string]func(string, *tls.Conn) http.RoundTripper{}
+	tr.TLSClientConfig = tlsdial.Config(a.host, tr.TLSClientConfig)
+	if a.insecureTLS {
+		tr.TLSClientConfig.InsecureSkipVerify = true
+		tr.TLSClientConfig.VerifyConnection = nil
+	}
+	tr.DialTLSContext = dnscache.TLSDialer(dialer.DialContext, dns, tr.TLSClientConfig)
+	tr.DisableCompression = true
+
+	// (mis)use httptrace to extract the underlying net.Conn from the
+	// transport. We make exactly 1 request using this transport, so
+	// there will be exactly 1 GotConn call. Additionally, the
+	// transport handles 101 Switching Protocols correctly, such that
+	// the Conn will not be reused or kept alive by the transport once
+	// the response has been handed back from RoundTrip.
+	//
+	// In theory, the machinery of net/http should make it such that
+	// the trace callback happens-before we get the response, but
+	// there's no promise of that. So, to make sure, we use a buffered
+	// channel as a synchronization step to avoid data races.
+	//
+	// Note that even though we're able to extract a net.Conn via this
+	// mechanism, we must still keep using the eventual resp.Body to
+	// read from, because it includes a buffer we can't get rid of. If
+	// the server never sends any data after sending the HTTP
+	// response, we could get away with it, but violating this
+	// assumption leads to very mysterious transport errors (lockups,
+	// unexpected EOFs...), and we're bound to forget someday and
+	// introduce a protocol optimization at a higher level that starts
+	// eagerly transmitting from the server.
+	connCh := make(chan net.Conn, 1)
+	trace := httptrace.ClientTrace{
+		GotConn: func(info httptrace.GotConnInfo) {
+			connCh <- info.Conn
+		},
+	}
+	ctx := httptrace.WithClientTrace(a.ctx, &trace)
+	req := &http.Request{
+		Method: "POST",
+		URL:    u,
+		Header: http.Header{
+			"Upgrade":           []string{upgradeHeaderValue},
+			"Connection":        []string{"upgrade"},
+			handshakeHeaderName: []string{base64.StdEncoding.EncodeToString(init)},
+		},
+	}
+	req = req.WithContext(ctx)
+
+	resp, err := tr.RoundTrip(req)
+	if err != nil {
+		return nil, err
+	}
+
+	if resp.StatusCode != http.StatusSwitchingProtocols {
+		return nil, fmt.Errorf("unexpected HTTP response: %s", resp.Status)
+	}
+
+	// From here on, the underlying net.Conn is ours to use, but there
+	// is still a read buffer attached to it within resp.Body. So, we
+	// must direct I/O through resp.Body, but we can still use the
+	// underlying net.Conn for stuff like deadlines.
+	var switchedConn net.Conn
+	select {
+	case switchedConn = <-connCh:
+	default:
+	}
+	if switchedConn == nil {
+		resp.Body.Close()
+		return nil, fmt.Errorf("httptrace didn't provide a connection")
+	}
+
+	if next := resp.Header.Get("Upgrade"); next != upgradeHeaderValue {
+		resp.Body.Close()
+		return nil, fmt.Errorf("server switched to unexpected protocol %q", next)
+	}
+
+	rwc, ok := resp.Body.(io.ReadWriteCloser)
+	if !ok {
+		resp.Body.Close()
+		return nil, errors.New("http Transport did not provide a writable body")
+	}
+
+	return &wrappedConn{switchedConn, rwc}, nil
+}
+
+type wrappedConn struct {
+	net.Conn
+	rwc io.ReadWriteCloser
+}
+
+func (w *wrappedConn) Read(bs []byte) (int, error) {
+	return w.rwc.Read(bs)
+}
+
+func (w *wrappedConn) Write(bs []byte) (int, error) {
+	return w.rwc.Write(bs)
+}
+
+func (w *wrappedConn) Close() error {
+	return w.rwc.Close()
+}

control/controlhttp/http_test.go

@@ -0,0 +1,398 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package controlhttp
+
+import (
+	"context"
+	"crypto/tls"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"strconv"
+	"sync"
+	"testing"
+
+	"tailscale.com/control/controlbase"
+	"tailscale.com/net/socks5"
+	"tailscale.com/types/key"
+)
+
+func TestControlHTTP(t *testing.T) {
+	tests := []struct {
+		name  string
+		proxy proxy
+	}{
+		// direct connection
+		{
+			name:  "no_proxy",
+			proxy: nil,
+		},
+		// SOCKS5
+		{
+			name:  "socks5",
+			proxy: &socksProxy{},
+		},
+		// HTTP->HTTP
+		{
+			name: "http_to_http",
+			proxy: &httpProxy{
+				useTLS:       false,
+				allowConnect: false,
+				allowHTTP:    true,
+			},
+		},
+		// HTTP->HTTPS
+		{
+			name: "http_to_https",
+			proxy: &httpProxy{
+				useTLS:       false,
+				allowConnect: true,
+				allowHTTP:    false,
+			},
+		},
+		// HTTP->any (will pick HTTP)
+		{
+			name: "http_to_any",
+			proxy: &httpProxy{
+				useTLS:       false,
+				allowConnect: true,
+				allowHTTP:    true,
+			},
+		},
+		// HTTPS->HTTP
+		{
+			name: "https_to_http",
+			proxy: &httpProxy{
+				useTLS:       true,
+				allowConnect: false,
+				allowHTTP:    true,
+			},
+		},
+		// HTTPS->HTTPS
+		{
+			name: "https_to_https",
+			proxy: &httpProxy{
+				useTLS:       true,
+				allowConnect: true,
+				allowHTTP:    false,
+			},
+		},
+		// HTTPS->any (will pick HTTP)
+		{
+			name: "https_to_any",
+			proxy: &httpProxy{
+				useTLS:       true,
+				allowConnect: true,
+				allowHTTP:    true,
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			testControlHTTP(t, test.proxy)
+		})
+	}
+}
+
+func testControlHTTP(t *testing.T, proxy proxy) {
+	client, server := key.NewMachine(), key.NewMachine()
+
+	sch := make(chan serverResult, 1)
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		conn, err := AcceptHTTP(context.Background(), w, r, server)
+		if err != nil {
+			log.Print(err)
+		}
+		res := serverResult{
+			err: err,
+		}
+		if conn != nil {
+			res.clientAddr = conn.RemoteAddr().String()
+			res.version = conn.ProtocolVersion()
+			res.peer = conn.Peer()
+			res.conn = conn
+		}
+		sch <- res
+	})
+
+	httpLn, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("HTTP listen: %v", err)
+	}
+	httpsLn, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("HTTPS listen: %v", err)
+	}
+
+	httpServer := &http.Server{Handler: handler}
+	go httpServer.Serve(httpLn)
+	defer httpServer.Close()
+
+	httpsServer := &http.Server{
+		Handler:   handler,
+		TLSConfig: tlsConfig(t),
+	}
+	go httpsServer.ServeTLS(httpsLn, "", "")
+	defer httpsServer.Close()
+
+	//ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	//defer cancel()
+
+	a := dialParams{
+		ctx:         context.Background(), //ctx,
+		host:        "localhost",
+		httpPort:    strconv.Itoa(httpLn.Addr().(*net.TCPAddr).Port),
+		httpsPort:   strconv.Itoa(httpsLn.Addr().(*net.TCPAddr).Port),
+		machineKey:  client,
+		controlKey:  server.Public(),
+		insecureTLS: true,
+	}
+
+	if proxy != nil {
+		proxyEnv := proxy.Start(t)
+		defer proxy.Close()
+		proxyURL, err := url.Parse(proxyEnv)
+		if err != nil {
+			t.Fatal(err)
+		}
+		a.proxyFunc = func(*http.Request) (*url.URL, error) {
+			return proxyURL, nil
+		}
+	} else {
+		a.proxyFunc = func(*http.Request) (*url.URL, error) {
+			return nil, nil
+		}
+	}
+
+	conn, err := a.dial()
+	if err != nil {
+		t.Fatalf("dialing controlhttp: %v", err)
+	}
+	defer conn.Close()
+	si := <-sch
+	if si.conn != nil {
+		defer si.conn.Close()
+	}
+	if si.err != nil {
+		t.Fatalf("controlhttp server got error: %v", err)
+	}
+	if clientVersion := conn.ProtocolVersion(); si.version != clientVersion {
+		t.Fatalf("client and server don't agree on protocol version: %d vs %d", clientVersion, si.version)
+	}
+	if si.peer != client.Public() {
+		t.Fatalf("server got peer pubkey %s, want %s", si.peer, client.Public())
+	}
+	if spub := conn.Peer(); spub != server.Public() {
+		t.Fatalf("client got peer pubkey %s, want %s", spub, server.Public())
+	}
+	if proxy != nil && !proxy.ConnIsFromProxy(si.clientAddr) {
+		t.Fatalf("client connected from %s, which isn't the proxy", si.clientAddr)
+	}
+}
+
+type serverResult struct {
+	err        error
+	clientAddr string
+	version    int
+	peer       key.MachinePublic
+	conn       *controlbase.Conn
+}
+
+type proxy interface {
+	Start(*testing.T) string
+	Close()
+	ConnIsFromProxy(string) bool
+}
+
+type socksProxy struct {
+	sync.Mutex
+	proxy           socks5.Server
+	ln              net.Listener
+	clientConnAddrs map[string]bool // addrs of the local end of outgoing conns from proxy
+}
+
+func (s *socksProxy) Start(t *testing.T) (url string) {
+	t.Helper()
+	s.Lock()
+	defer s.Unlock()
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listening for SOCKS server: %v", err)
+	}
+	s.ln = ln
+	s.clientConnAddrs = map[string]bool{}
+	s.proxy.Logf = t.Logf
+	s.proxy.Dialer = s.dialAndRecord
+	go s.proxy.Serve(ln)
+	return fmt.Sprintf("socks5://%s", ln.Addr().String())
+}
+
+func (s *socksProxy) Close() {
+	s.Lock()
+	defer s.Unlock()
+	s.ln.Close()
+}
+
+func (s *socksProxy) dialAndRecord(ctx context.Context, network, addr string) (net.Conn, error) {
+	var d net.Dialer
+	conn, err := d.DialContext(ctx, network, addr)
+	if err != nil {
+		return nil, err
+	}
+	s.Lock()
+	defer s.Unlock()
+	s.clientConnAddrs[conn.LocalAddr().String()] = true
+	return conn, nil
+}
+
+func (s *socksProxy) ConnIsFromProxy(addr string) bool {
+	s.Lock()
+	defer s.Unlock()
+	return s.clientConnAddrs[addr]
+}
+
+type httpProxy struct {
+	useTLS       bool // take incoming connections over TLS
+	allowConnect bool // allow CONNECT for TLS
+	allowHTTP    bool // allow plain HTTP proxying
+
+	sync.Mutex
+	ln              net.Listener
+	rp              httputil.ReverseProxy
+	s               http.Server
+	clientConnAddrs map[string]bool // addrs of the local end of outgoing conns from proxy
+}
+
+func (h *httpProxy) Start(t *testing.T) (url string) {
+	t.Helper()
+	h.Lock()
+	defer h.Unlock()
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listening for HTTP proxy: %v", err)
+	}
+	h.ln = ln
+	h.rp = httputil.ReverseProxy{
+		Director: func(*http.Request) {},
+		Transport: &http.Transport{
+			DialContext: h.dialAndRecord,
+			TLSClientConfig: &tls.Config{
+				InsecureSkipVerify: true,
+			},
+			TLSNextProto: map[string]func(string, *tls.Conn) http.RoundTripper{},
+		},
+	}
+	h.clientConnAddrs = map[string]bool{}
+	h.s.Handler = h
+	if h.useTLS {
+		h.s.TLSConfig = tlsConfig(t)
+		go h.s.ServeTLS(h.ln, "", "")
+		return fmt.Sprintf("https://%s", ln.Addr().String())
+	} else {
+		go h.s.Serve(h.ln)
+		return fmt.Sprintf("http://%s", ln.Addr().String())
+	}
+}
+
+func (h *httpProxy) Close() {
+	h.Lock()
+	defer h.Unlock()
+	h.s.Close()
+}
+
+func (h *httpProxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "CONNECT" {
+		if !h.allowHTTP {
+			http.Error(w, "http proxy not allowed", 500)
+			return
+		}
+		h.rp.ServeHTTP(w, r)
+		return
+	}
+
+	if !h.allowConnect {
+		http.Error(w, "connect not allowed", 500)
+		return
+	}
+
+	dst := r.RequestURI
+	c, err := h.dialAndRecord(context.Background(), "tcp", dst)
+	if err != nil {
+		http.Error(w, err.Error(), 500)
+		return
+	}
+	defer c.Close()
+
+	cc, ccbuf, err := w.(http.Hijacker).Hijack()
+	if err != nil {
+		http.Error(w, err.Error(), 500)
+		return
+	}
+	defer cc.Close()
+
+	io.WriteString(cc, "HTTP/1.1 200 OK\r\n\r\n")
+
+	errc := make(chan error, 1)
+	go func() {
+		_, err := io.Copy(cc, c)
+		errc <- err
+	}()
+	go func() {
+		_, err := io.Copy(c, ccbuf)
+		errc <- err
+	}()
+	<-errc
+}
+
+func (h *httpProxy) dialAndRecord(ctx context.Context, network, addr string) (net.Conn, error) {
+	var d net.Dialer
+	conn, err := d.DialContext(ctx, network, addr)
+	if err != nil {
+		return nil, err
+	}
+	h.Lock()
+	defer h.Unlock()
+	h.clientConnAddrs[conn.LocalAddr().String()] = true
+	return conn, nil
+}
+
+func (h *httpProxy) ConnIsFromProxy(addr string) bool {
+	h.Lock()
+	defer h.Unlock()
+	return h.clientConnAddrs[addr]
+}
+
+func tlsConfig(t *testing.T) *tls.Config {
+	// Cert and key taken from the example code in the crypto/tls
+	// package.
+	certPem := []byte(`-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----`)
+	keyPem := []byte(`-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIIrYSSNQFaA2Hwf1duRSxKtLYX5CB04fSeQ6tF1aY/PuoAoGCCqGSM49
+AwEHoUQDQgAEPR3tU2Fta9ktY+6P9G0cWO+0kETA6SFs38GecTyudlHz6xvCdz8q
+EKTcWGekdmdDPsHloRNtsiCa697B2O9IFA==
+-----END EC PRIVATE KEY-----`)
+	cert, err := tls.X509KeyPair(certPem, keyPem)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return &tls.Config{
+		Certificates: []tls.Certificate{cert},
+	}
+}

control/controlhttp/server.go

@@ -0,0 +1,95 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package controlhttp
+
+import (
+	"bufio"
+	"context"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+
+	"tailscale.com/control/controlbase"
+	"tailscale.com/types/key"
+)
+
+// AcceptHTTP upgrades the HTTP request given by w and r into a
+// Tailscale control protocol base transport connection.
+//
+// AcceptHTTP always writes an HTTP response to w. The caller must not
+// attempt their own response after calling AcceptHTTP.
+func AcceptHTTP(ctx context.Context, w http.ResponseWriter, r *http.Request, private key.MachinePrivate) (*controlbase.Conn, error) {
+	next := r.Header.Get("Upgrade")
+	if next == "" {
+		http.Error(w, "missing next protocol", http.StatusBadRequest)
+		return nil, errors.New("no next protocol in HTTP request")
+	}
+	if next != upgradeHeaderValue {
+		http.Error(w, "unknown next protocol", http.StatusBadRequest)
+		return nil, fmt.Errorf("client requested unhandled next protocol %q", next)
+	}
+
+	initB64 := r.Header.Get(handshakeHeaderName)
+	if initB64 == "" {
+		http.Error(w, "missing Tailscale handshake header", http.StatusBadRequest)
+		return nil, errors.New("no tailscale handshake header in HTTP request")
+	}
+	init, err := base64.StdEncoding.DecodeString(initB64)
+	if err != nil {
+		http.Error(w, "invalid tailscale handshake header", http.StatusBadRequest)
+		return nil, fmt.Errorf("decoding base64 handshake header: %v", err)
+	}
+
+	hijacker, ok := w.(http.Hijacker)
+	if !ok {
+		http.Error(w, "make request over HTTP/1", http.StatusBadRequest)
+		return nil, errors.New("can't hijack client connection")
+	}
+
+	w.Header().Set("Upgrade", upgradeHeaderValue)
+	w.Header().Set("Connection", "upgrade")
+	w.WriteHeader(http.StatusSwitchingProtocols)
+
+	conn, brw, err := hijacker.Hijack()
+	if err != nil {
+		return nil, fmt.Errorf("hijacking client connection: %w", err)
+	}
+	if err := brw.Flush(); err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("flushing hijacked HTTP buffer: %w", err)
+	}
+	if brw.Reader.Buffered() > 0 {
+		conn = &drainBufConn{conn, brw.Reader}
+	}
+
+	nc, err := controlbase.Server(ctx, conn, private, init)
+	if err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("noise handshake failed: %w", err)
+	}
+
+	return nc, nil
+}
+
+// drainBufConn is a net.Conn with an initial bunch of bytes in a
+// bufio.Reader. Read drains the bufio.Reader until empty, then passes
+// through subsequent reads to the Conn directly.
+type drainBufConn struct {
+	net.Conn
+	r *bufio.Reader
+}
+
+func (b *drainBufConn) Read(bs []byte) (int, error) {
+	if b.r == nil {
+		return b.Conn.Read(bs)
+	}
+	n, err := b.r.Read(bs)
+	if b.r.Buffered() == 0 {
+		b.r = nil
+	}
+	return n, err
+}