wgengine/magicsock: support multiple derp servers, and not just for handshakes

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

wgengine/magicsock/derpmap.go

@@ -0,0 +1,49 @@
+// Copyright 2019 Tailscale & 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 magicsock
+
+import (
+	"fmt"
+	"net"
+)
+
+// derpFakeIPStr is a fake WireGuard endpoint IP address that means
+// to use DERP. When used, the port number of the WireGuard endpoint
+// is the DERP server number to use.
+const derpMagicIPStr = "127.3.3.40"       // 3340 are above the keys DERP on the keyboard
+var derpMagicIP = net.IPv4(127, 3, 3, 40) // net.IP version of above
+
+var (
+	derpHostOfIndex = map[int]string{} // index (fake port number) -> hostname
+	derpIndexOfHost = map[string]int{} // derpHostOfIndex reversed
+)
+
+func init() {
+	// Just one zone for now:
+	addDerper(1, "derp.tailscale.com")
+}
+
+func addDerper(i int, host string) {
+	if other, dup := derpHostOfIndex[i]; dup {
+		panic(fmt.Sprintf("duplicate DERP index %v (host %q and %q)", i, other, host))
+	}
+	if other, dup := derpIndexOfHost[host]; dup {
+		panic(fmt.Sprintf("duplicate DERP host %q (index %v and %v)", host, other, i))
+	}
+	derpHostOfIndex[i] = host
+	derpIndexOfHost[host] = i
+}
+
+// derpHost returns the hostname of a DERP server index (a fake port
+// number used with derpMagicIP). It always returns a non-empty string.
+func derpHost(i int) string {
+	if h, ok := derpHostOfIndex[i]; ok {
+		return h
+	}
+	if 1 <= i && i <= 64<<10 {
+		return fmt.Sprintf("derp%v.tailscale.com", i)
+	}
+	return "derp.tailscale.com"
+}