net/dnsfallback: don't depend on derpmap.Prod

Move derpmap.Prod to a static JSON file (go:generate'd) instead,
to make its role explicit. And add a TODO about making dnsfallback
use an update-over-time DERP map file instead of a baked-in one.

Updates #1264

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

net/dnsfallback/update-dns-fallbacks.go

@@ -0,0 +1,47 @@
+// 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.
+
+// +build ignore
+
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"os"
+
+	"tailscale.com/tailcfg"
+)
+
+func main() {
+	res, err := http.Get("https://login.tailscale.com/derpmap/default")
+	if err != nil {
+		log.Fatal(err)
+	}
+	if res.StatusCode != 200 {
+		res.Write(os.Stderr)
+		os.Exit(1)
+	}
+	dm := new(tailcfg.DERPMap)
+	if err := json.NewDecoder(res.Body).Decode(dm); err != nil {
+		log.Fatal(err)
+	}
+	for rid, r := range dm.Regions {
+		// Names misleading to check into git, as this is a
+		// static snapshot and doesn't reflect the live DERP
+		// map.
+		r.RegionCode = fmt.Sprintf("r%d", rid)
+		r.RegionName = r.RegionCode
+	}
+	out, err := json.MarshalIndent(dm, "", "\t")
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := ioutil.WriteFile("dns-fallback-servers.json", out, 0644); err != nil {
+		log.Fatal(err)
+	}
+}