net/tlsdial: add package for TLS dials, and make DERP & controlclient use it

This will do the iOS-optimized cert checking in a following change.

control/controlclient/direct.go

@@ -26,6 +26,7 @@ import (
 	"github.com/tailscale/wireguard-go/wgcfg"
 	"golang.org/x/crypto/nacl/box"
 	"golang.org/x/oauth2"
+	"tailscale.com/net/tlsdial"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 	"tailscale.com/version"
@@ -93,7 +94,6 @@ type Direct struct {
 
 type Options struct {
 	Persist         Persist           // initial persistent data
-	HTTPC           *http.Client      // HTTP client used to talk to tailcontrol
 	ServerURL       string            // URL of the tailcontrol server
 	AuthKey         string            // optional node auth key for auto registration
 	TimeNow         func() time.Time  // time.Now implementation used by Client
@@ -114,9 +114,6 @@ func NewDirect(opts Options) (*Direct, error) {
 		return nil, errors.New("controlclient.New: no server URL specified")
 	}
 	opts.ServerURL = strings.TrimRight(opts.ServerURL, "/")
-	if opts.HTTPC == nil {
-		opts.HTTPC = http.DefaultClient
-	}
 	if opts.TimeNow == nil {
 		opts.TimeNow = time.Now
 	}
@@ -125,8 +122,14 @@ func NewDirect(opts Options) (*Direct, error) {
 		// TODO(bradfitz): ... but then it shouldn't be in Options.
 		opts.Logf = log.Printf
 	}
+
+	tr := http.DefaultTransport.(*http.Transport).Clone()
+	tr.ForceAttemptHTTP2 = true
+	tr.TLSClientConfig = tlsdial.Config("", tr.TLSClientConfig)
+	httpc := &http.Client{Transport: tr}
+
 	c := &Direct{
-		httpc:           opts.HTTPC,
+		httpc:           httpc,
 		serverURL:       opts.ServerURL,
 		timeNow:         opts.TimeNow,
 		logf:            opts.Logf,

derp/derphttp/derphttp_client.go

@@ -26,6 +26,7 @@ import (
 
 	"tailscale.com/derp"
 	"tailscale.com/net/dnscache"
+	"tailscale.com/net/tlsdial"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 )
@@ -37,8 +38,8 @@ import (
 // Send/Recv will completely re-establish the connection (unless Close
 // has been called).
 type Client struct {
-	TLSConfig *tls.Config        // for sever connection, optional, nil means default
-	DNSCache  *dnscache.Resolver // optional; if nil, no caching
+	TLSConfig *tls.Config        // optional; nil means default
+	DNSCache  *dnscache.Resolver // optional; nil means no caching
 
 	privateKey key.Private
 	logf       logger.Logf
@@ -182,12 +183,7 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 
 	var httpConn net.Conn // a TCP conn or a TLS conn; what we speak HTTP to
 	if c.url.Scheme == "https" {
-		tlsConfig := &tls.Config{}
-		if c.TLSConfig != nil {
-			tlsConfig = c.TLSConfig.Clone()
-		}
-		tlsConfig.ServerName = c.url.Host
-		httpConn = tls.Client(tcpConn, tlsConfig)
+		httpConn = tls.Client(tcpConn, tlsdial.Config(c.url.Host, c.TLSConfig))
 	} else {
 		httpConn = tcpConn
 	}

net/tlsdial/tlsdial.go

@@ -0,0 +1,24 @@
+// Copyright (c) 2020 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 tlsdial sets up a tls.Config for x509 validation, using
+// a memory-optimized path for iOS.
+package tlsdial
+
+import "crypto/tls"
+
+// Config returns a tls.Config for dialing the given host.
+// If base is non-nil, it's cloned as the base config before
+// being configured and returned.
+func Config(host string, base *tls.Config) *tls.Config {
+	var conf *tls.Config
+	if base == nil {
+		conf = new(tls.Config)
+	} else {
+		conf = base.Clone()
+	}
+	conf.ServerName = host
+
+	return conf
+}