net/tlsdial: fix TLS cert validation of HTTPS proxies

If you had HTTPS_PROXY=https://some-valid-cert.example.com running a
CONNECT proxy, we should've been able to do a TLS CONNECT request to
e.g. controlplane.tailscale.com:443 through that, and I'm pretty sure
it used to work, but refactorings and lack of integration tests made
it regress.

It probably regressed when we added the baked-in LetsEncrypt root cert
validation fallback code, which was testing against the wrong hostname
(the ultimate one, not the one which we were being asked to validate)

Fixes #16222

Change-Id: If014e395f830e2f87f056f588edacad5c15e91bc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/proxy-test-server/proxy-test-server.go

@@ -0,0 +1,81 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// The proxy-test-server command is a simple HTTP proxy server for testing
+// Tailscale's client proxy functionality.
+package main
+
+import (
+	"crypto/tls"
+	"flag"
+	"fmt"
+	"log"
+	"net"
+	"net/http"
+	"os"
+	"strings"
+
+	"golang.org/x/crypto/acme/autocert"
+	"tailscale.com/net/connectproxy"
+	"tailscale.com/tempfork/acme"
+)
+
+var (
+	listen            = flag.String("listen", ":8080", "Address to listen on for HTTPS proxy requests")
+	hostname          = flag.String("hostname", "localhost", "Hostname for the proxy server")
+	tailscaleOnly     = flag.Bool("tailscale-only", true, "Restrict proxy to Tailscale targets only")
+	extraAllowedHosts = flag.String("allow-hosts", "", "Comma-separated list of allowed target hosts to additionally allow if --tailscale-only is true")
+)
+
+func main() {
+	flag.Parse()
+
+	am := &autocert.Manager{
+		HostPolicy: autocert.HostWhitelist(*hostname),
+		Prompt:     autocert.AcceptTOS,
+		Cache:      autocert.DirCache(os.ExpandEnv("$HOME/.cache/autocert/proxy-test-server")),
+	}
+	var allowTarget func(hostPort string) error
+	if *tailscaleOnly {
+		allowTarget = func(hostPort string) error {
+			host, port, err := net.SplitHostPort(hostPort)
+			if err != nil {
+				return fmt.Errorf("invalid target %q: %v", hostPort, err)
+			}
+			if port != "443" {
+				return fmt.Errorf("target %q must use port 443", hostPort)
+			}
+			for allowed := range strings.SplitSeq(*extraAllowedHosts, ",") {
+				if host == allowed {
+					return nil // explicitly allowed target
+				}
+			}
+			if !strings.HasSuffix(host, ".tailscale.com") {
+				return fmt.Errorf("target %q is not a Tailscale host", hostPort)
+			}
+			return nil // valid Tailscale target
+		}
+	}
+
+	go func() {
+		if err := http.ListenAndServe(":http", am.HTTPHandler(nil)); err != nil {
+			log.Fatalf("autocert HTTP server failed: %v", err)
+		}
+	}()
+	hs := &http.Server{
+		Addr: *listen,
+		Handler: &connectproxy.Handler{
+			Check: allowTarget,
+			Logf:  log.Printf,
+		},
+		TLSConfig: &tls.Config{
+			GetCertificate: am.GetCertificate,
+			NextProtos: []string{
+				"http/1.1",     // enable HTTP/2
+				acme.ALPNProto, // enable tls-alpn ACME challenges
+			},
+		},
+	}
+	log.Printf("Starting proxy-test-server on %s (hostname: %q)\n", *listen, *hostname)
+	log.Fatal(hs.ListenAndServeTLS("", "")) // cert and key are provided by autocert
+}

control/controlclient/controlclient_test.go

@@ -4,13 +4,35 @@
 package controlclient
 
 import (
+	"context"
+	"crypto/tls"
 	"errors"
+	"flag"
 	"fmt"
 	"io"
+	"net"
+	"net/http"
+	"net/netip"
+	"net/url"
 	"reflect"
 	"slices"
+	"sync/atomic"
 	"testing"
+	"time"
 
+	"tailscale.com/control/controlknobs"
+	"tailscale.com/health"
+	"tailscale.com/net/bakedroots"
+	"tailscale.com/net/connectproxy"
+	"tailscale.com/net/netmon"
+	"tailscale.com/net/tsdial"
+	"tailscale.com/tailcfg"
+	"tailscale.com/tstest"
+	"tailscale.com/tstest/integration/testcontrol"
+	"tailscale.com/tstest/tlstest"
+	"tailscale.com/tstime"
+	"tailscale.com/types/key"
+	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
 	"tailscale.com/types/persist"
 )
@@ -188,3 +210,206 @@ func isRetryableErrorForTest(err error) bool {
 	}
 	return false
 }
+
+var liveNetworkTest = flag.Bool("live-network-test", false, "run live network tests")
+
+func TestDirectProxyManual(t *testing.T) {
+	if !*liveNetworkTest {
+		t.Skip("skipping without --live-network-test")
+	}
+
+	dialer := &tsdial.Dialer{}
+	dialer.SetNetMon(netmon.NewStatic())
+
+	opts := Options{
+		Persist: persist.Persist{},
+		GetMachinePrivateKey: func() (key.MachinePrivate, error) {
+			return key.NewMachine(), nil
+		},
+		ServerURL: "https://controlplane.tailscale.com",
+		Clock:     tstime.StdClock{},
+		Hostinfo: &tailcfg.Hostinfo{
+			BackendLogID: "test-backend-log-id",
+		},
+		DiscoPublicKey: key.NewDisco().Public(),
+		Logf:           t.Logf,
+		HealthTracker:  &health.Tracker{},
+		PopBrowserURL: func(url string) {
+			t.Logf("PopBrowserURL: %q", url)
+		},
+		Dialer:       dialer,
+		ControlKnobs: &controlknobs.Knobs{},
+	}
+	d, err := NewDirect(opts)
+	if err != nil {
+		t.Fatalf("NewDirect: %v", err)
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	url, err := d.TryLogin(ctx, LoginEphemeral)
+	if err != nil {
+		t.Fatalf("TryLogin: %v", err)
+	}
+	t.Logf("URL: %q", url)
+}
+
+func TestHTTPSNoProxy(t *testing.T) { testHTTPS(t, false) }
+
+// TestTLSWithProxy verifies we can connect to the control plane via
+// an HTTPS proxy.
+func TestHTTPSWithProxy(t *testing.T) { testHTTPS(t, true) }
+
+func testHTTPS(t *testing.T, withProxy bool) {
+	bakedroots.ResetForTest(t, tlstest.TestRootCA())
+
+	controlLn, err := tls.Listen("tcp", "127.0.0.1:0", tlstest.ControlPlaneKeyPair.ServerTLSConfig())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer controlLn.Close()
+
+	proxyLn, err := tls.Listen("tcp", "127.0.0.1:0", tlstest.ProxyServerKeyPair.ServerTLSConfig())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer proxyLn.Close()
+
+	const requiredAuthKey = "hunter2"
+	const someUsername = "testuser"
+	const somePassword = "testpass"
+
+	testControl := &testcontrol.Server{
+		Logf:           tstest.WhileTestRunningLogger(t),
+		RequireAuthKey: requiredAuthKey,
+	}
+	controlSrv := &http.Server{
+		Handler:  testControl,
+		ErrorLog: logger.StdLogger(t.Logf),
+	}
+	go controlSrv.Serve(controlLn)
+
+	const fakeControlIP = "1.2.3.4"
+	const fakeProxyIP = "5.6.7.8"
+
+	dialer := &tsdial.Dialer{}
+	dialer.SetNetMon(netmon.NewStatic())
+	dialer.SetSystemDialerForTest(func(ctx context.Context, network, addr string) (net.Conn, error) {
+		host, _, err := net.SplitHostPort(addr)
+		if err != nil {
+			return nil, fmt.Errorf("SplitHostPort(%q): %v", addr, err)
+		}
+		var d net.Dialer
+		if host == fakeControlIP {
+			return d.DialContext(ctx, network, controlLn.Addr().String())
+		}
+		if host == fakeProxyIP {
+			return d.DialContext(ctx, network, proxyLn.Addr().String())
+		}
+		return nil, fmt.Errorf("unexpected dial to %q", addr)
+	})
+
+	opts := Options{
+		Persist: persist.Persist{},
+		GetMachinePrivateKey: func() (key.MachinePrivate, error) {
+			return key.NewMachine(), nil
+		},
+		AuthKey:   requiredAuthKey,
+		ServerURL: "https://controlplane.tstest",
+		Clock:     tstime.StdClock{},
+		Hostinfo: &tailcfg.Hostinfo{
+			BackendLogID: "test-backend-log-id",
+		},
+		DiscoPublicKey: key.NewDisco().Public(),
+		Logf:           t.Logf,
+		HealthTracker:  &health.Tracker{},
+		PopBrowserURL: func(url string) {
+			t.Logf("PopBrowserURL: %q", url)
+		},
+		Dialer: dialer,
+	}
+	d, err := NewDirect(opts)
+	if err != nil {
+		t.Fatalf("NewDirect: %v", err)
+	}
+
+	d.dnsCache.LookupIPForTest = func(ctx context.Context, host string) ([]netip.Addr, error) {
+		switch host {
+		case "controlplane.tstest":
+			return []netip.Addr{netip.MustParseAddr(fakeControlIP)}, nil
+		case "proxy.tstest":
+			if !withProxy {
+				t.Errorf("unexpected DNS lookup for %q with proxy disabled", host)
+				return nil, fmt.Errorf("unexpected DNS lookup for %q", host)
+			}
+			return []netip.Addr{netip.MustParseAddr(fakeProxyIP)}, nil
+		}
+		t.Errorf("unexpected DNS query for %q", host)
+		return []netip.Addr{}, nil
+	}
+
+	var proxyReqs atomic.Int64
+	if withProxy {
+		d.httpc.Transport.(*http.Transport).Proxy = func(req *http.Request) (*url.URL, error) {
+			t.Logf("using proxy for %q", req.URL)
+			u := &url.URL{
+				Scheme: "https",
+				Host:   "proxy.tstest:443",
+				User:   url.UserPassword(someUsername, somePassword),
+			}
+			return u, nil
+		}
+
+		connectProxy := &http.Server{
+			Handler: connectProxyTo(t, "controlplane.tstest:443", controlLn.Addr().String(), &proxyReqs),
+		}
+		go connectProxy.Serve(proxyLn)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	url, err := d.TryLogin(ctx, LoginEphemeral)
+	if err != nil {
+		t.Fatalf("TryLogin: %v", err)
+	}
+	if url != "" {
+		t.Errorf("got URL %q, want empty", url)
+	}
+
+	if withProxy {
+		if got, want := proxyReqs.Load(), int64(1); got != want {
+			t.Errorf("proxy CONNECT requests = %d; want %d", got, want)
+		}
+	}
+}
+
+func connectProxyTo(t testing.TB, target, backendAddrPort string, reqs *atomic.Int64) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.RequestURI != target {
+			t.Errorf("invalid CONNECT request to %q; want %q", r.RequestURI, target)
+			http.Error(w, "bad target", http.StatusBadRequest)
+			return
+		}
+
+		r.Header.Set("Authorization", r.Header.Get("Proxy-Authorization")) // for the BasicAuth method. kinda trashy.
+		user, pass, ok := r.BasicAuth()
+		if !ok || user != "testuser" || pass != "testpass" {
+			t.Errorf("invalid CONNECT auth %q:%q; want %q:%q", user, pass, "testuser", "testpass")
+			http.Error(w, "bad auth", http.StatusUnauthorized)
+			return
+		}
+
+		(&connectproxy.Handler{
+			Dial: func(ctx context.Context, network, addr string) (net.Conn, error) {
+				var d net.Dialer
+				c, err := d.DialContext(ctx, network, backendAddrPort)
+				if err == nil {
+					reqs.Add(1)
+				}
+				return c, err
+			},
+			Logf: t.Logf,
+		}).ServeHTTP(w, r)
+	})
+}

control/controlclient/direct.go

@@ -16,7 +16,6 @@ import (
 	"net"
 	"net/http"
 	"net/netip"
-	"net/url"
 	"os"
 	"reflect"
 	"runtime"
@@ -240,10 +239,6 @@ func NewDirect(opts Options) (*Direct, error) {
 		opts.ControlKnobs = &controlknobs.Knobs{}
 	}
 	opts.ServerURL = strings.TrimRight(opts.ServerURL, "/")
-	serverURL, err := url.Parse(opts.ServerURL)
-	if err != nil {
-		return nil, err
-	}
 	if opts.Clock == nil {
 		opts.Clock = tstime.StdClock{}
 	}
@@ -273,7 +268,7 @@ func NewDirect(opts Options) (*Direct, error) {
 		tr := http.DefaultTransport.(*http.Transport).Clone()
 		tr.Proxy = tshttpproxy.ProxyFromEnvironment
 		tshttpproxy.SetTransportGetProxyConnectHeader(tr)
-		tr.TLSClientConfig = tlsdial.Config(serverURL.Hostname(), opts.HealthTracker, tr.TLSClientConfig)
+		tr.TLSClientConfig = tlsdial.Config(opts.HealthTracker, tr.TLSClientConfig)
 		var dialFunc netx.DialFunc
 		dialFunc, interceptedDial = makeScreenTimeDetectingDialFunc(opts.Dialer.SystemDial)
 		tr.DialContext = dnscache.Dialer(dialFunc, dnsCache)

control/controlhttp/client.go

@@ -534,7 +534,7 @@ func (a *Dialer) tryURLUpgrade(ctx context.Context, u *url.URL, optAddr netip.Ad
 	// 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.Hostname, a.HealthTracker, tr.TLSClientConfig)
+	tr.TLSClientConfig = tlsdial.Config(a.HealthTracker, tr.TLSClientConfig)
 	if !tr.TLSClientConfig.InsecureSkipVerify {
 		panic("unexpected") // should be set by tlsdial.Config
 	}

derp/derphttp/derphttp_client.go

@@ -647,12 +647,13 @@ func (c *Client) dialRegion(ctx context.Context, reg *tailcfg.DERPRegion) (net.C
 }
 
 func (c *Client) tlsClient(nc net.Conn, node *tailcfg.DERPNode) *tls.Conn {
-	tlsConf := tlsdial.Config(c.tlsServerName(node), c.HealthTracker, c.TLSConfig)
+	tlsConf := tlsdial.Config(c.HealthTracker, c.TLSConfig)
 	if node != nil {
 		if node.InsecureForTests {
 			tlsConf.InsecureSkipVerify = true
 			tlsConf.VerifyConnection = nil
 		}
+		tlsConf.ServerName = c.tlsServerName(node)
 		if node.CertName != "" {
 			if suf, ok := strings.CutPrefix(node.CertName, "sha256-raw:"); ok {
 				tlsdial.SetConfigExpectedCertHash(tlsConf, suf)

derp/derphttp/derphttp_test.go

@@ -7,10 +7,14 @@ import (
 	"bytes"
 	"context"
 	"crypto/tls"
+	"encoding/json"
+	"flag"
 	"fmt"
+	"maps"
 	"net"
 	"net/http"
 	"net/http/httptest"
+	"slices"
 	"strings"
 	"sync"
 	"testing"
@@ -19,6 +23,7 @@ import (
 	"tailscale.com/derp"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/netx"
+	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 )
 
@@ -556,3 +561,32 @@ func TestNotifyError(t *testing.T) {
 		t.Fatalf("context done before receiving error: %v", ctx.Err())
 	}
 }
+
+var liveNetworkTest = flag.Bool("live-net-tests", false, "run live network tests")
+
+func TestManualDial(t *testing.T) {
+	if !*liveNetworkTest {
+		t.Skip("skipping live network test without --live-net-tests")
+	}
+	dm := &tailcfg.DERPMap{}
+	res, err := http.Get("https://controlplane.tailscale.com/derpmap/default")
+	if err != nil {
+		t.Fatalf("fetching DERPMap: %v", err)
+	}
+	defer res.Body.Close()
+	if err := json.NewDecoder(res.Body).Decode(dm); err != nil {
+		t.Fatalf("decoding DERPMap: %v", err)
+	}
+
+	region := slices.Sorted(maps.Keys(dm.Regions))[0]
+
+	netMon := netmon.NewStatic()
+	rc := NewRegionClient(key.NewNode(), t.Logf, netMon, func() *tailcfg.DERPRegion {
+		return dm.Regions[region]
+	})
+	defer rc.Close()
+
+	if err := rc.Connect(context.Background()); err != nil {
+		t.Fatalf("rc.Connect: %v", err)
+	}
+}

logpolicy/logpolicy.go

@@ -9,7 +9,6 @@ package logpolicy
 import (
 	"bufio"
 	"bytes"
-	"cmp"
 	"context"
 	"crypto/tls"
 	"encoding/json"
@@ -911,8 +910,7 @@ func (opts TransportOptions) New() http.RoundTripper {
 		tr.TLSNextProto = map[string]func(authority string, c *tls.Conn) http.RoundTripper{}
 	}
 
-	host := cmp.Or(opts.Host, logtail.DefaultHost)
-	tr.TLSClientConfig = tlsdial.Config(host, opts.Health, tr.TLSClientConfig)
+	tr.TLSClientConfig = tlsdial.Config(opts.Health, tr.TLSClientConfig)
 	// Force TLS 1.3 since we know log.tailscale.com supports it.
 	tr.TLSClientConfig.MinVersion = tls.VersionTLS13
 

net/bakedroots/bakedroots.go

@@ -7,6 +7,7 @@ package bakedroots
 
 import (
 	"crypto/x509"
+	"fmt"
 	"sync"
 
 	"tailscale.com/util/testenv"
@@ -14,7 +15,7 @@ import (
 
 // Get returns the baked-in roots.
 //
-// As of 2025-01-21, this includes only the LetsEncrypt ISRG Root X1 root.
+// As of 2025-01-21, this includes only the LetsEncrypt ISRG Root X1 & X2 roots.
 func Get() *x509.CertPool {
 	roots.once.Do(func() {
 		roots.parsePEM(append(
@@ -56,7 +57,7 @@ type rootsOnce struct {
 func (r *rootsOnce) parsePEM(caPEM []byte) {
 	p := x509.NewCertPool()
 	if !p.AppendCertsFromPEM(caPEM) {
-		panic("bogus PEM")
+		panic(fmt.Sprintf("bogus PEM: %q", caPEM))
 	}
 	r.p = p
 }

net/connectproxy/connectproxy.go

@@ -0,0 +1,93 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package connectproxy contains some CONNECT proxy code.
+package connectproxy
+
+import (
+	"context"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"time"
+
+	"tailscale.com/net/netx"
+	"tailscale.com/types/logger"
+)
+
+// Handler is an HTTP CONNECT proxy handler.
+type Handler struct {
+	// Dial, if non-nil, is an alternate dialer to use
+	// instead of the default dialer.
+	Dial netx.DialFunc
+
+	// Logf, if non-nil, is an alterate logger to
+	// use instead of log.Printf.
+	Logf logger.Logf
+
+	// Check, if non-nil, validates the CONNECT target.
+	Check func(hostPort string) error
+}
+
+func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	if r.Method != "CONNECT" {
+		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	dial := h.Dial
+	if dial == nil {
+		var d net.Dialer
+		dial = d.DialContext
+	}
+	logf := h.Logf
+	if logf == nil {
+		logf = log.Printf
+	}
+
+	hostPort := r.RequestURI
+	if h.Check != nil {
+		if err := h.Check(hostPort); err != nil {
+			logf("CONNECT target %q not allowed: %v", hostPort, err)
+			http.Error(w, "Invalid CONNECT target", http.StatusForbidden)
+			return
+		}
+	}
+
+	ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
+	defer cancel()
+	back, err := dial(ctx, "tcp", hostPort)
+	if err != nil {
+		logf("error CONNECT dialing %v: %v", hostPort, err)
+		http.Error(w, "Connect failure", http.StatusBadGateway)
+		return
+	}
+	defer back.Close()
+
+	hj, ok := w.(http.Hijacker)
+	if !ok {
+		http.Error(w, "CONNECT hijack unavailable", http.StatusInternalServerError)
+		return
+	}
+	c, br, err := hj.Hijack()
+	if err != nil {
+		logf("CONNECT hijack: %v", err)
+		return
+	}
+	defer c.Close()
+
+	io.WriteString(c, "HTTP/1.1 200 OK\r\n\r\n")
+
+	errc := make(chan error, 2)
+	go func() {
+		_, err := io.Copy(c, back)
+		errc <- err
+	}()
+	go func() {
+		_, err := io.Copy(back, br)
+		errc <- err
+	}()
+	<-errc
+}

net/dnscache/dnscache.go

@@ -24,6 +24,7 @@ import (
 	"tailscale.com/util/cloudenv"
 	"tailscale.com/util/singleflight"
 	"tailscale.com/util/slicesx"
+	"tailscale.com/util/testenv"
 )
 
 var zaddr netip.Addr
@@ -63,6 +64,10 @@ type Resolver struct {
 	// If nil, net.DefaultResolver is used.
 	Forward *net.Resolver
 
+	// LookupIPForTest, if non-nil and in tests, handles requests instead
+	// of the usual mechanisms.
+	LookupIPForTest func(ctx context.Context, host string) ([]netip.Addr, error)
+
 	// LookupIPFallback optionally provides a backup DNS mechanism
 	// to use if Forward returns an error or no results.
 	LookupIPFallback func(ctx context.Context, host string) ([]netip.Addr, error)
@@ -284,7 +289,13 @@ func (r *Resolver) lookupIP(ctx context.Context, host string) (ip, ip6 netip.Add
 
 	lookupCtx, lookupCancel := context.WithTimeout(ctx, r.lookupTimeoutForHost(host))
 	defer lookupCancel()
-	ips, err := r.fwd().LookupNetIP(lookupCtx, "ip", host)
+
+	var ips []netip.Addr
+	if r.LookupIPForTest != nil && testenv.InTest() {
+		ips, err = r.LookupIPForTest(ctx, host)
+	} else {
+		ips, err = r.fwd().LookupNetIP(lookupCtx, "ip", host)
+	}
 	if err != nil || len(ips) == 0 {
 		if resolver, ok := r.cloudHostResolver(); ok {
 			r.dlogf("resolving %q via cloud resolver", host)

net/dnsfallback/dnsfallback.go

@@ -286,7 +286,7 @@ func bootstrapDNSMap(ctx context.Context, serverName string, serverIP netip.Addr
 	tr.DialContext = func(ctx context.Context, netw, addr string) (net.Conn, error) {
 		return dialer.DialContext(ctx, "tcp", net.JoinHostPort(serverIP.String(), "443"))
 	}
-	tr.TLSClientConfig = tlsdial.Config(serverName, ht, tr.TLSClientConfig)
+	tr.TLSClientConfig = tlsdial.Config(ht, tr.TLSClientConfig)
 	c := &http.Client{Transport: tr}
 	req, err := http.NewRequestWithContext(ctx, "GET", "https://"+serverName+"/bootstrap-dns?q="+url.QueryEscape(queryName), nil)
 	if err != nil {

net/tlsdial/tlsdial.go

@@ -59,18 +59,26 @@ var mitmBlockWarnable = health.Register(&health.Warnable{
 	ImpactsConnectivity: true,
 })
 
-// Config returns a tls.Config for connecting to a server.
+// Config returns a tls.Config for connecting to a server that
+// uses system roots for validation but, if those fail, also tries
+// the baked-in LetsEncrypt roots as a fallback validation method.
+//
 // If base is non-nil, it's cloned as the base config before
 // being configured and returned.
 // If ht is non-nil, it's used to report health errors.
-func Config(host string, ht *health.Tracker, base *tls.Config) *tls.Config {
+func Config(ht *health.Tracker, base *tls.Config) *tls.Config {
 	var conf *tls.Config
 	if base == nil {
 		conf = new(tls.Config)
 	} else {
 		conf = base.Clone()
 	}
-	conf.ServerName = host
+
+	// Note: we do NOT set conf.ServerName here (as we accidentally did
+	// previously), as this path is also used when dialing an HTTPS proxy server
+	// (through which we'll send a CONNECT request to get a TCP connection to do
+	// the real TCP connection) because host is the ultimate hostname, but this
+	// tls.Config is used for both the proxy and the ultimate target.
 
 	if n := sslKeyLogFile; n != "" {
 		f, err := os.OpenFile(n, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0600)
@@ -93,7 +101,9 @@ func Config(host string, ht *health.Tracker, base *tls.Config) *tls.Config {
 	// (with the baked-in fallback root) in the VerifyConnection hook.
 	conf.InsecureSkipVerify = true
 	conf.VerifyConnection = func(cs tls.ConnectionState) (retErr error) {
-		if host == "log.tailscale.com" && hostinfo.IsNATLabGuestVM() {
+		dialedHost := cs.ServerName
+
+		if dialedHost == "log.tailscale.com" && hostinfo.IsNATLabGuestVM() {
 			// Allow log.tailscale.com TLS MITM for integration tests when
 			// the client's running within a NATLab VM.
 			return nil
@@ -116,7 +126,7 @@ func Config(host string, ht *health.Tracker, base *tls.Config) *tls.Config {
 					// Show a dedicated warning.
 					m, ok := blockblame.VerifyCertificate(cert)
 					if ok {
-						log.Printf("tlsdial: server cert for %q looks like %q equipment (could be blocking Tailscale)", host, m.Name)
+						log.Printf("tlsdial: server cert seen while dialing %q looks like %q equipment (could be blocking Tailscale)", dialedHost, m.Name)
 						ht.SetUnhealthy(mitmBlockWarnable, health.Args{"manufacturer": m.Name})
 					} else {
 						ht.SetHealthy(mitmBlockWarnable)
@@ -135,7 +145,7 @@ func Config(host string, ht *health.Tracker, base *tls.Config) *tls.Config {
 					ht.SetTLSConnectionError(cs.ServerName, nil)
 					if selfSignedIssuer != "" {
 						// Log the self-signed issuer, but don't treat it as an error.
-						log.Printf("tlsdial: warning: server cert for %q passed x509 validation but is self-signed by %q", host, selfSignedIssuer)
+						log.Printf("tlsdial: warning: server cert for %q passed x509 validation but is self-signed by %q", dialedHost, selfSignedIssuer)
 					}
 				}
 			}()
@@ -144,7 +154,7 @@ func Config(host string, ht *health.Tracker, base *tls.Config) *tls.Config {
 		// First try doing x509 verification with the system's
 		// root CA pool.
 		opts := x509.VerifyOptions{
-			DNSName:       cs.ServerName,
+			DNSName:       dialedHost,
 			Intermediates: x509.NewCertPool(),
 		}
 		for _, cert := range cs.PeerCertificates[1:] {
@@ -152,7 +162,7 @@ func Config(host string, ht *health.Tracker, base *tls.Config) *tls.Config {
 		}
 		_, errSys := cs.PeerCertificates[0].Verify(opts)
 		if debug() {
-			log.Printf("tlsdial(sys %q): %v", host, errSys)
+			log.Printf("tlsdial(sys %q): %v", dialedHost, errSys)
 		}
 
 		// Always verify with our baked-in Let's Encrypt certificate,
@@ -161,13 +171,11 @@ func Config(host string, ht *health.Tracker, base *tls.Config) *tls.Config {
 		opts.Roots = bakedroots.Get()
 		_, bakedErr := cs.PeerCertificates[0].Verify(opts)
 		if debug() {
-			log.Printf("tlsdial(bake %q): %v", host, bakedErr)
+			log.Printf("tlsdial(bake %q): %v", dialedHost, bakedErr)
 		} else if bakedErr != nil {
-			if _, loaded := tlsdialWarningPrinted.LoadOrStore(host, true); !loaded {
-				if errSys == nil {
-					log.Printf("tlsdial: warning: server cert for %q is not a Let's Encrypt cert", host)
-				} else {
-					log.Printf("tlsdial: error: server cert for %q failed to verify and is not a Let's Encrypt cert", host)
+			if _, loaded := tlsdialWarningPrinted.LoadOrStore(dialedHost, true); !loaded {
+				if errSys != nil {
+					log.Printf("tlsdial: error: server cert for %q failed both system roots & Let's Encrypt root validation", dialedHost)
 				}
 			}
 		}
@@ -202,9 +210,6 @@ func SetConfigExpectedCert(c *tls.Config, certDNSName string) {
 		c.ServerName = certDNSName
 		return
 	}
-	if c.VerifyPeerCertificate != nil {
-		panic("refusing to override tls.Config.VerifyPeerCertificate")
-	}
 	// Set InsecureSkipVerify to prevent crypto/tls from doing its
 	// own cert verification, but do the same work that it'd do
 	// (but using certDNSName) in the VerifyPeerCertificate hook.
@@ -257,29 +262,30 @@ func SetConfigExpectedCertHash(c *tls.Config, wantFullCertSHA256Hex string) {
 	if c.VerifyPeerCertificate != nil {
 		panic("refusing to override tls.Config.VerifyPeerCertificate")
 	}
+
 	// Set InsecureSkipVerify to prevent crypto/tls from doing its
 	// own cert verification, but do the same work that it'd do
-	// (but using certDNSName) in the VerifyPeerCertificate hook.
+	// (but using certDNSName) in the VerifyConnection hook.
 	c.InsecureSkipVerify = true
-	c.VerifyConnection = nil
-	c.VerifyPeerCertificate = func(rawCerts [][]byte, _ [][]*x509.Certificate) error {
+
+	c.VerifyConnection = func(cs tls.ConnectionState) error {
+		dialedHost := cs.ServerName
 		var sawGoodCert bool
-		for _, rawCert := range rawCerts {
-			cert, err := x509.ParseCertificate(rawCert)
-			if err != nil {
-				return fmt.Errorf("ParseCertificate: %w", err)
-			}
+
+		for _, cert := range cs.PeerCertificates {
 			if strings.HasPrefix(cert.Subject.CommonName, derpconst.MetaCertCommonNamePrefix) {
 				continue
 			}
 			if sawGoodCert {
 				return errors.New("unexpected multiple certs presented")
 			}
-			if fmt.Sprintf("%02x", sha256.Sum256(rawCert)) != wantFullCertSHA256Hex {
+			if fmt.Sprintf("%02x", sha256.Sum256(cert.Raw)) != wantFullCertSHA256Hex {
 				return fmt.Errorf("cert hash does not match expected cert hash")
 			}
-			if err := cert.VerifyHostname(c.ServerName); err != nil {
-				return fmt.Errorf("cert does not match server name %q: %w", c.ServerName, err)
+			if dialedHost != "" { // it's empty when dialing a derper by IP with no hostname
+				if err := cert.VerifyHostname(dialedHost); err != nil {
+					return fmt.Errorf("cert does not match server name %q: %w", dialedHost, err)
+				}
 			}
 			now := time.Now()
 			if now.After(cert.NotAfter) {
@@ -302,12 +308,8 @@ func SetConfigExpectedCertHash(c *tls.Config, wantFullCertSHA256Hex string) {
 func NewTransport() *http.Transport {
 	return &http.Transport{
 		DialTLSContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
-			host, _, err := net.SplitHostPort(addr)
-			if err != nil {
-				return nil, err
-			}
 			var d tls.Dialer
-			d.Config = Config(host, nil, nil)
+			d.Config = Config(nil, nil)
 			return d.DialContext(ctx, network, addr)
 		},
 	}

net/tlsdial/tlsdial_test.go

@@ -86,7 +86,7 @@ func TestFallbackRootWorks(t *testing.T) {
 		DisableKeepAlives: true, // for test cleanup ease
 	}
 	ht := new(health.Tracker)
-	tr.TLSClientConfig = Config("tlsdial.test", ht, tr.TLSClientConfig)
+	tr.TLSClientConfig = Config(ht, tr.TLSClientConfig)
 	c := &http.Client{Transport: tr}
 
 	ctr0 := atomic.LoadInt32(&counterFallbackOK)

tstest/tlstest/testdata/controlplane.tstest.key

@@ -0,0 +1,5 @@
+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIHcxOQNVyqvBSSlu7c93QW6OsyccjL+R1evW4acd32MWoAoGCCqGSM49
+AwEHoUQDQgAEIOY5/CQ8CMuKYPLf+r6OEneqfzQ5RfgPnLdkL22qhm8xb69ZCXxz
+UecawU0KEDfHLYbUYXSuhAFxxuPh9I3x5Q==
+-----END EC PRIVATE KEY-----

tstest/tlstest/testdata/proxy.tstest.key

@@ -0,0 +1,5 @@
+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEING1XBDWFXQjqBmLjhp20hXOf2rk/I0N6W7muv9RVvk3oAoGCCqGSM49
+AwEHoUQDQgAE8lxnEEeLqYikwmXbXSsIQSw20R0oLA831s960KQZEgt0P9SbWcJc
+QTk98rdfYT/QDdHn157Oh4FPcDtxmdQ4vw==
+-----END EC PRIVATE KEY-----

tstest/tlstest/testdata/root-ca.key

@@ -0,0 +1,5 @@
+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIMl3xjqt1dnXBpYJSEqevirAcnSJ79I2tucdRazlrDG9oAoGCCqGSM49
+AwEHoUQDQgAEQ/+Jme+16hgO7TtPSIFHVV0Yt969ltVlARVcNUZmWc0upQaq7uiJ
+Aur5KtzwxU3YI4bhNK0593OK2TLvEEWIdw==
+-----END EC PRIVATE KEY-----

tstest/tlstest/tlstest.go

@@ -0,0 +1,167 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package tlstest contains code to help test Tailscale's client proxy support.
+package tlstest
+
+import (
+	"bytes"
+	"crypto/ecdsa"
+	"crypto/rand"
+	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	_ "embed"
+	"encoding/pem"
+	"fmt"
+	"math/big"
+	"sync"
+	"time"
+)
+
+// Some baked-in ECDSA keys to speed up tests, not having to burn CPU to
+// generate them each time. We only make the certs (which have expiry times)
+// at runtime.
+//
+// They were made with:
+//
+//	openssl ecparam -name prime256v1 -genkey -noout -out root-ca.key
+var (
+	//go:embed testdata/root-ca.key
+	rootCAKeyPEM []byte
+
+	// TestProxyServerKey is the PEM private key for [TestProxyServerCert].
+	//
+	//go:embed testdata/proxy.tstest.key
+	TestProxyServerKey []byte
+
+	// TestControlPlaneKey is the PEM private key for [TestControlPlaneCert].
+	//
+	//go:embed testdata/controlplane.tstest.key
+	TestControlPlaneKey []byte
+)
+
+// TestRootCA returns a self-signed ECDSA root CA certificate (as PEM) for
+// testing purposes.
+func TestRootCA() []byte {
+	return bytes.Clone(testRootCAOncer())
+}
+
+var testRootCAOncer = sync.OnceValue(func() []byte {
+	key := rootCAKey()
+	now := time.Now().Add(-time.Hour)
+	tpl := &x509.Certificate{
+		SerialNumber: big.NewInt(1),
+		Subject: pkix.Name{
+			CommonName:   "Tailscale Unit Test ECDSA Root",
+			Organization: []string{"Tailscale Test Org"},
+		},
+		NotBefore: now,
+		NotAfter:  now.AddDate(5, 0, 0),
+
+		IsCA:                  true,
+		BasicConstraintsValid: true,
+		KeyUsage:              x509.KeyUsageCertSign | x509.KeyUsageCRLSign,
+		SubjectKeyId:          mustSKID(&key.PublicKey),
+	}
+
+	der, err := x509.CreateCertificate(rand.Reader, tpl, tpl, &key.PublicKey, key)
+	if err != nil {
+		panic(err)
+	}
+	return pemCert(der)
+})
+
+func pemCert(der []byte) []byte {
+	var buf bytes.Buffer
+	if err := pem.Encode(&buf, &pem.Block{Type: "CERTIFICATE", Bytes: der}); err != nil {
+		panic(fmt.Sprintf("failed to encode PEM: %v", err))
+	}
+	return buf.Bytes()
+}
+
+var rootCAKey = sync.OnceValue(func() *ecdsa.PrivateKey {
+	return mustParsePEM(rootCAKeyPEM, x509.ParseECPrivateKey)
+})
+
+func mustParsePEM[T any](pemBytes []byte, parse func([]byte) (T, error)) T {
+	block, rest := pem.Decode(pemBytes)
+	if block == nil || len(rest) > 0 {
+		panic("invalid PEM")
+	}
+	v, err := parse(block.Bytes)
+	if err != nil {
+		panic(fmt.Sprintf("invalid PEM: %v", err))
+	}
+	return v
+}
+
+// KeyPair is a simple struct to hold a certificate and its private key.
+type KeyPair struct {
+	Domain string
+	KeyPEM []byte // PEM-encoded private key
+}
+
+// ServerTLSConfig returns a TLS configuration suitable for a server
+// using the KeyPair's certificate and private key.
+func (p KeyPair) ServerTLSConfig() *tls.Config {
+	cert, err := tls.X509KeyPair(p.CertPEM(), p.KeyPEM)
+	if err != nil {
+		panic("invalid TLS key pair: " + err.Error())
+	}
+	return &tls.Config{
+		Certificates: []tls.Certificate{cert},
+	}
+}
+
+// ProxyServerKeyPair is a KeyPair for a test control plane server
+// with domain name "proxy.tstest".
+var ProxyServerKeyPair = KeyPair{
+	Domain: "proxy.tstest",
+	KeyPEM: TestProxyServerKey,
+}
+
+// ControlPlaneKeyPair is a KeyPair for a test control plane server
+// with domain name "controlplane.tstest".
+var ControlPlaneKeyPair = KeyPair{
+	Domain: "controlplane.tstest",
+	KeyPEM: TestControlPlaneKey,
+}
+
+func (p KeyPair) CertPEM() []byte {
+	caCert := mustParsePEM(TestRootCA(), x509.ParseCertificate)
+	caPriv := mustParsePEM(rootCAKeyPEM, x509.ParseECPrivateKey)
+	leafKey := mustParsePEM(p.KeyPEM, x509.ParseECPrivateKey)
+
+	serial, err := rand.Int(rand.Reader, big.NewInt(0).Lsh(big.NewInt(1), 128))
+	if err != nil {
+		panic(err)
+	}
+
+	now := time.Now().Add(-time.Hour)
+	tpl := &x509.Certificate{
+		SerialNumber: serial,
+		Subject:      pkix.Name{CommonName: p.Domain},
+		NotBefore:    now,
+		NotAfter:     now.AddDate(2, 0, 0),
+
+		KeyUsage:              x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
+		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
+		BasicConstraintsValid: true,
+		DNSNames:              []string{p.Domain},
+	}
+
+	der, err := x509.CreateCertificate(rand.Reader, tpl, caCert, &leafKey.PublicKey, caPriv)
+	if err != nil {
+		panic(err)
+	}
+	return pemCert(der)
+}
+
+func mustSKID(pub *ecdsa.PublicKey) []byte {
+	skid, err := x509.MarshalPKIXPublicKey(pub)
+	if err != nil {
+		panic(err)
+	}
+	return skid[:20] // same as x509 library
+}