refactor: embedded DERP no longer verify clients via HTTP

- register the `headscale://` protocol in `http.DefaultTransport` to intercept network requests
- update configuration to use a single boolean option `verify_clients`

config-example.yaml

@@ -87,11 +87,8 @@ derp:
     region_code: "headscale"
     region_name: "Headscale Embedded DERP"
 
-    # If non-empty, an admission controller URL for permitting client connections
-    verify_client_url: "http://127.0.0.1:8080/verify"
-
-    # Whether derp fail open if verify_client_url is unreachable
-    verify_client_url_fail_open: false
+    # Verify clients to this DERP server using the Headscale node list
+    verify_clients: true
 
     # Listens over UDP at the configured address for STUN connections - to help with NAT traversal.
     # When the embedded DERP server is enabled stun_listen_addr MUST be defined.

hscontrol/app.go

@@ -221,6 +221,7 @@ func NewHeadscale(cfg *types.Config) (*Headscale, error) {
 
 		embeddedDERPServer, err := derpServer.NewDERPServer(
 			cfg.ServerURL,
+			app.VerifyHandler,
 			key.NodePrivate(*derpServerKey),
 			&cfg.DERP,
 		)

hscontrol/derp/server/derp_server.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"net"
 	"net/http"
+	"net/http/httptest"
 	"net/netip"
 	"net/url"
 	"strconv"
@@ -39,14 +40,20 @@ type DERPServer struct {
 
 func NewDERPServer(
 	serverURL string,
+	verifyHandler func(writer http.ResponseWriter, req *http.Request),
 	derpKey key.NodePrivate,
 	cfg *types.DERPConfig,
 ) (*DERPServer, error) {
 	log.Trace().Caller().Msg("Creating new embedded DERP server")
 	server := derp.NewServer(derpKey, util.TSLogfWrapper()) // nolint // zerolinter complains
-	if cfg.ServerVerifyClientURL != "" {
-		server.SetVerifyClientURL(cfg.ServerVerifyClientURL)
-		server.SetVerifyClientURLFailOpen(cfg.ServerVerifyFailOpen)
+
+	if cfg.ServerVerifyClients {
+		t := http.DefaultTransport.(*http.Transport)
+		t.RegisterProtocol("headscale", &HeadscaleTransport{
+			verifyHandler: verifyHandler,
+		})
+		server.SetVerifyClientURL("headscale://verify")
+		server.SetVerifyClientURLFailOpen(false)
 	}
 
 	return &DERPServer{
@@ -364,3 +371,14 @@ func serverSTUNListener(ctx context.Context, packetConn *net.UDPConn) {
 		}
 	}
 }
+
+type HeadscaleTransport struct {
+	verifyHandler func(writer http.ResponseWriter, req *http.Request)
+}
+
+func (t *HeadscaleTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	recorder := httptest.NewRecorder()
+	t.verifyHandler(recorder, req)
+	resp := recorder.Result()
+	return resp, nil
+}

hscontrol/types/config.go

@@ -185,8 +185,7 @@ type DERPConfig struct {
 	ServerRegionCode                   string
 	ServerRegionName                   string
 	ServerPrivateKeyPath               string
-	ServerVerifyClientURL              string
-	ServerVerifyFailOpen               bool
+	ServerVerifyClients                bool
 	STUNAddr                           string
 	URLs                               []url.URL
 	Paths                              []string
@@ -433,8 +432,7 @@ func derpConfig() DERPConfig {
 	serverRegionID := viper.GetInt("derp.server.region_id")
 	serverRegionCode := viper.GetString("derp.server.region_code")
 	serverRegionName := viper.GetString("derp.server.region_name")
-	serverVerifyClientURL := viper.GetString("derp.server.verify_client_url")
-	serverVerifyFailOpen := viper.GetBool("derp.server.verify_client_url_fail_open")
+	serverVerifyClients := viper.GetBool("derp.server.verify_clients")
 	stunAddr := viper.GetString("derp.server.stun_listen_addr")
 	privateKeyPath := util.AbsolutePathFromConfigPath(
 		viper.GetString("derp.server.private_key_path"),
@@ -479,8 +477,7 @@ func derpConfig() DERPConfig {
 		ServerRegionID:                     serverRegionID,
 		ServerRegionCode:                   serverRegionCode,
 		ServerRegionName:                   serverRegionName,
-		ServerVerifyClientURL:              serverVerifyClientURL,
-		ServerVerifyFailOpen:               serverVerifyFailOpen,
+		ServerVerifyClients:                serverVerifyClients,
 		ServerPrivateKeyPath:               privateKeyPath,
 		STUNAddr:                           stunAddr,
 		URLs:                               urls,