Add an embedded DERP server to Headscale

This series of commit will be adding an embedded DERP server (and STUN) to Headscale,
thus making it completely self-contained and not dependant in other infrastructure.

derp_embedded.go

@@ -0,0 +1,178 @@
+package headscale
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net"
+	"net/http"
+	"strings"
+	"sync/atomic"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/rs/zerolog/log"
+	"tailscale.com/derp"
+	"tailscale.com/net/stun"
+	"tailscale.com/types/key"
+)
+
+// fastStartHeader is the header (with value "1") that signals to the HTTP
+// server that the DERP HTTP client does not want the HTTP 101 response
+// headers and it will begin writing & reading the DERP protocol immediately
+// following its HTTP request.
+const fastStartHeader = "Derp-Fast-Start"
+
+var (
+	dnsCache     atomic.Value // of []byte
+	bootstrapDNS = "derp.tailscale.com"
+)
+
+type EmbeddedDerpServer struct {
+	tailscaleDerp *derp.Server
+}
+
+func (h *Headscale) NewEmbeddedDerpServer() (*EmbeddedDerpServer, error) {
+	s := derp.NewServer(key.NodePrivate(*h.privateKey), log.Info().Msgf)
+	return &EmbeddedDerpServer{s}, nil
+
+}
+
+func (h *Headscale) EmbeddedDerpHandler(ctx *gin.Context) {
+	up := strings.ToLower(ctx.Request.Header.Get("Upgrade"))
+	if up != "websocket" && up != "derp" {
+		if up != "" {
+			log.Warn().Caller().Msgf("Weird websockets connection upgrade: %q", up)
+		}
+		ctx.String(http.StatusUpgradeRequired, "DERP requires connection upgrade")
+		return
+	}
+
+	fastStart := ctx.Request.Header.Get(fastStartHeader) == "1"
+
+	hijacker, ok := ctx.Writer.(http.Hijacker)
+	if !ok {
+		log.Error().Caller().Msg("DERP requires Hijacker interface from Gin")
+		ctx.String(http.StatusInternalServerError, "HTTP does not support general TCP support")
+		return
+	}
+
+	netConn, conn, err := hijacker.Hijack()
+	if err != nil {
+		log.Error().Caller().Err(err).Msgf("Hijack failed")
+		ctx.String(http.StatusInternalServerError, "HTTP does not support general TCP support")
+		return
+	}
+
+	if !fastStart {
+		pubKey := h.privateKey.Public()
+		fmt.Fprintf(conn, "HTTP/1.1 101 Switching Protocols\r\n"+
+			"Upgrade: DERP\r\n"+
+			"Connection: Upgrade\r\n"+
+			"Derp-Version: %v\r\n"+
+			"Derp-Public-Key: %s\r\n\r\n",
+			derp.ProtocolVersion,
+			pubKey.UntypedHexString())
+	}
+
+	h.EmbeddedDerpServer.tailscaleDerp.Accept(netConn, conn, netConn.RemoteAddr().String())
+}
+
+// EmbeddedDerpProbeHandler is the endpoint that js/wasm clients hit to measure
+// DERP latency, since they can't do UDP STUN queries.
+func (h *Headscale) EmbeddedDerpProbeHandler(ctx *gin.Context) {
+	switch ctx.Request.Method {
+	case "HEAD", "GET":
+		ctx.Writer.Header().Set("Access-Control-Allow-Origin", "*")
+	default:
+		ctx.String(http.StatusMethodNotAllowed, "bogus probe method")
+	}
+}
+
+func (h *Headscale) EmbeddedDerpBootstrapDNSHandler(ctx *gin.Context) {
+	ctx.Header("Content-Type", "application/json")
+	j, _ := dnsCache.Load().([]byte)
+	// Bootstrap DNS requests occur cross-regions,
+	// and are randomized per request,
+	// so keeping a connection open is pointlessly expensive.
+	ctx.Header("Connection", "close")
+	ctx.Writer.Write(j)
+}
+
+// ServeSTUN starts a STUN server on udp/3478
+func (h *Headscale) ServeSTUN() {
+	pc, err := net.ListenPacket("udp", "0.0.0.0:3478")
+	if err != nil {
+		log.Fatal().Msgf("failed to open STUN listener: %v", err)
+	}
+	log.Printf("running STUN server on %v", pc.LocalAddr())
+	serverSTUNListener(context.Background(), pc.(*net.UDPConn))
+}
+
+func serverSTUNListener(ctx context.Context, pc *net.UDPConn) {
+	var buf [64 << 10]byte
+	var (
+		n   int
+		ua  *net.UDPAddr
+		err error
+	)
+	for {
+		n, ua, err = pc.ReadFromUDP(buf[:])
+		if err != nil {
+			if ctx.Err() != nil {
+				return
+			}
+			log.Printf("STUN ReadFrom: %v", err)
+			time.Sleep(time.Second)
+			continue
+		}
+		pkt := buf[:n]
+		if !stun.Is(pkt) {
+			continue
+		}
+		txid, err := stun.ParseBindingRequest(pkt)
+		if err != nil {
+			continue
+		}
+
+		res := stun.Response(txid, ua.IP, uint16(ua.Port))
+		pc.WriteTo(res, ua)
+	}
+}
+
+// Shamelessly taken from
+// https://github.com/tailscale/tailscale/blob/main/cmd/derper/bootstrap_dns.go
+func refreshBootstrapDNSLoop() {
+	if bootstrapDNS == "" {
+		return
+	}
+	for {
+		refreshBootstrapDNS()
+		time.Sleep(10 * time.Minute)
+	}
+}
+
+func refreshBootstrapDNS() {
+	if bootstrapDNS == "" {
+		return
+	}
+	dnsEntries := make(map[string][]net.IP)
+	ctx, cancel := context.WithTimeout(context.Background(), time.Minute)
+	defer cancel()
+	names := strings.Split(bootstrapDNS, ",")
+	var r net.Resolver
+	for _, name := range names {
+		addrs, err := r.LookupIP(ctx, "ip", name)
+		if err != nil {
+			log.Printf("bootstrap DNS lookup %q: %v", name, err)
+			continue
+		}
+		dnsEntries[name] = addrs
+	}
+	j, err := json.MarshalIndent(dnsEntries, "", "\t")
+	if err != nil {
+		// leave the old values in place
+		return
+	}
+	dnsCache.Store(j)
+}