all: disable TCP keep-alives on iOS/Android

Updates #2442
Updates tailscale/corp#2750

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

net/netknob/netknob.go

@@ -0,0 +1,30 @@
+// 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 netknob has Tailscale network knobs.
+package netknob
+
+import (
+	"runtime"
+	"time"
+)
+
+// PlatformTCPKeepAlive returns the default net.Dialer.KeepAlive
+// value for the current runtime.GOOS.
+func PlatformTCPKeepAlive() time.Duration {
+	switch runtime.GOOS {
+	case "ios", "android":
+		// Disable TCP keep-alives on mobile platforms.
+		// See https://github.com/golang/go/issues/48622.
+		//
+		// TODO(bradfitz): in 1.17.x, try disabling TCP
+		// keep-alives on for all platforms.
+		return -1
+	}
+
+	// Otherwise, default to 30 seconds, which is mostly what we
+	// used to do. In some places we used the zero value, which Go
+	// defaults to 15 seconds. But 30 seconds is fine.
+	return 30 * time.Second
+}