derp,cmd/derper: allow server to verify clients

This adds a flag to the DERP server which specifies to verify clients through a local
tailscaled. It is opt-in, so should not affect existing clients, and is mainly intended for
users who want to run their own DERP servers. It assumes there is a local tailscaled running and
will attempt to hit it for peer status information.

Updates #1264

Signed-off-by: julianknodt <julianknodt@gmail.com>

cmd/derper/derper.go

@@ -46,6 +46,7 @@
 	meshPSKFile   = flag.String("mesh-psk-file", defaultMeshPSKFile(), "if non-empty, path to file containing the mesh pre-shared key file. It should contain some hex string; whitespace is trimmed.")
 	meshWith      = flag.String("mesh-with", "", "optional comma-separated list of hostnames to mesh with; the server's own hostname can be in the list")
 	bootstrapDNS  = flag.String("bootstrap-dns-names", "", "optional comma-separated list of hostnames to make available at /bootstrap-dns")
+	verifyClients = flag.Bool("verify-clients", false, "verify clients to this DERP server through a local tailscaled instance.")
 )
 
 type config struct {
@@ -122,6 +123,7 @@ func main() {
 	letsEncrypt := tsweb.IsProd443(*addr)
 
 	s := derp.NewServer(key.Private(cfg.PrivateKey), log.Printf)
+	s.SetVerifyClient(*verifyClients)
 
 	if *meshPSKFile != "" {
 		b, err := ioutil.ReadFile(*meshPSKFile)

cmd/tailscale/depaware.txt

@@ -15,7 +15,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         inet.af/netaddr                                              from tailscale.com/cmd/tailscale/cli+
         rsc.io/goversion/version                                     from tailscale.com/version
         tailscale.com/atomicfile                                     from tailscale.com/ipn
-        tailscale.com/client/tailscale                               from tailscale.com/cmd/tailscale/cli
+        tailscale.com/client/tailscale                               from tailscale.com/cmd/tailscale/cli+
         tailscale.com/client/tailscale/apitype                       from tailscale.com/client/tailscale+
         tailscale.com/cmd/tailscale/cli                              from tailscale.com/cmd/tailscale
         tailscale.com/derp                                           from tailscale.com/derp/derphttp

cmd/tailscaled/depaware.txt

@@ -76,6 +76,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
    W 💣 inet.af/wf                                                   from tailscale.com/wf
         rsc.io/goversion/version                                     from tailscale.com/version
         tailscale.com/atomicfile                                     from tailscale.com/ipn+
+        tailscale.com/client/tailscale                               from tailscale.com/derp
         tailscale.com/client/tailscale/apitype                       from tailscale.com/ipn/ipnlocal+
         tailscale.com/control/controlclient                          from tailscale.com/ipn/ipnlocal+
         tailscale.com/derp                                           from tailscale.com/derp/derphttp+
@@ -117,7 +118,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/tstun                                      from tailscale.com/cmd/tailscaled+
         tailscale.com/paths                                          from tailscale.com/cmd/tailscaled+
         tailscale.com/portlist                                       from tailscale.com/ipn/ipnlocal
-        tailscale.com/safesocket                                     from tailscale.com/ipn/ipnserver
+        tailscale.com/safesocket                                     from tailscale.com/ipn/ipnserver+
         tailscale.com/smallzstd                                      from tailscale.com/ipn/ipnserver+
         tailscale.com/syncs                                          from tailscale.com/net/interfaces+
         tailscale.com/tailcfg                                        from tailscale.com/control/controlclient+

derp/derp_server.go

@@ -37,6 +37,7 @@
 	"golang.org/x/crypto/nacl/box"
 	"golang.org/x/sync/errgroup"
 	"inet.af/netaddr"
+	"tailscale.com/client/tailscale"
 	"tailscale.com/disco"
 	"tailscale.com/metrics"
 	"tailscale.com/types/key"
@@ -127,6 +128,10 @@ type Server struct {
 	removePktForwardOther    expvar.Int
 	avgQueueDuration         *uint64 // In milliseconds; accessed atomically
 
+	// verifyClients only accepts client connections to the DERP server if the clientKey is a
+	// known peer in the network, as specified by a running tailscaled's client's local api.
+	verifyClients bool
+
 	mu          sync.Mutex
 	closed      bool
 	netConns    map[Conn]chan struct{} // chan is closed when conn closes
@@ -214,6 +219,13 @@ func (s *Server) SetMeshKey(v string) {
 	s.meshKey = v
 }
 
+// SetVerifyClients sets whether this DERP server verifies clients through tailscaled.
+//
+// It must be called before serving begins.
+func (s *Server) SetVerifyClient(v bool) {
+	s.verifyClients = v
+}
+
 // HasMeshKey reports whether the server is configured with a mesh key.
 func (s *Server) HasMeshKey() bool { return s.meshKey != "" }
 
@@ -770,8 +782,17 @@ func (c *sclient) requestMeshUpdate() {
 }
 
 func (s *Server) verifyClient(clientKey key.Public, info *clientInfo) error {
-	// TODO(crawshaw): implement policy constraints on who can use the DERP server
-	// TODO(bradfitz): ... and at what rate.
+	if !s.verifyClients {
+		return nil
+	}
+	status, err := tailscale.Status(context.TODO())
+	if err != nil {
+		return fmt.Errorf("failed to query local tailscaled status: %w", err)
+	}
+	if _, exists := status.Peer[clientKey]; !exists {
+		return fmt.Errorf("client %v not in set of peers", clientKey)
+	}
+	// TODO(bradfitz): add policy for configurable bandwidth rate per client?
 	return nil
 }