derp/derphttp: don't use x/net/proxy for SOCKS on iOS

We don't want those extra dependencies on iOS, at least yet.

Especially since there's no way to set the relevant environment
variables so it's just bloat with no benefits. Perhaps we'll need to
do SOCKS on iOS later, but probably differently if/when so.

Updates #227

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

derp/derphttp/derphttp_client.go

@@ -24,7 +24,6 @@ import (
 	"sync"
 	"time"
 
-	"golang.org/x/net/proxy"
 	"tailscale.com/derp"
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/tlsdial"
@@ -149,11 +148,10 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 	host := c.url.Hostname()
 	hostOrIP := host
 
-	var d dialer = new(net.Dialer)
-	var usingProxy bool
-	if cd, ok := proxy.FromEnvironmentUsing(d).(dialer); ok {
-		usingProxy = d != cd
-		d = cd
+	var stdDialer dialer = new(net.Dialer)
+	var dialer = stdDialer
+	if wrapDialer != nil {
+		dialer = wrapDialer(dialer)
 	}
 
 	if c.DNSCache != nil {
@@ -161,12 +159,14 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 		if err == nil {
 			hostOrIP = ip.String()
 		}
-		if err != nil && !usingProxy {
+		if err != nil && dialer == stdDialer {
+			// Return an error if we're not using a dial
+			// proxy that can do DNS lookups for us.
 			return nil, err
 		}
 	}
 
-	tcpConn, err = d.DialContext(ctx, "tcp", net.JoinHostPort(hostOrIP, urlPort(c.url)))
+	tcpConn, err = dialer.DialContext(ctx, "tcp", net.JoinHostPort(hostOrIP, urlPort(c.url)))
 	if err != nil {
 		return nil, fmt.Errorf("dial of %q: %v", host, err)
 	}
@@ -326,3 +326,7 @@ func (c *Client) closeForReconnect(brokenClient *derp.Client) {
 }
 
 var ErrClientClosed = errors.New("derphttp.Client closed")
+
+// wrapDialer, if non-nil, specifies a function to wrap a dialer in a
+// SOCKS-using dialer. It's set conditionally by socks.go.
+var wrapDialer func(dialer) dialer

derp/derphttp/socks.go

@@ -0,0 +1,20 @@
+// 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.
+
+// +build !ios
+
+package derphttp
+
+import "golang.org/x/net/proxy"
+
+func init() {
+	wrapDialer = wrapSocks
+}
+
+func wrapSocks(d dialer) dialer {
+	if cd, ok := proxy.FromEnvironmentUsing(d).(dialer); ok {
+		return cd
+	}
+	return d
+}