derp, wgengine: let clients advertise an opaque app name to DERP servers

Add an AppName field to the DERP ClientInfo so DERP servers can
attribute connections to the application making them, primarily for
best effort stats purposes. The value is plumbed per engine instance
rather than via a process global, so a process hosting multiple stacks
can attribute each one's DERP connections separately:
wgengine.Config.DERPAppName flows through magicsock.Options and
derphttp.Client into the naclbox-sealed ClientInfo JSON. Old servers
ignore the unknown field.

There are no callers in the tree yet setting the name.

Updates tailscale/corp#24454

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ia7d3e9c2b6f8140e5a9d7c3b2e6f1a8d4c0b5e9f

derp/derp_client.go

@@ -33,6 +33,7 @@ type Client struct {
 	meshKey     key.DERPMesh
 	canAckPings bool
 	isProber    bool
+	appName     string
 
 	wmu  sync.Mutex // hold while writing to bw
 	bw   *bufio.Writer
@@ -60,6 +61,7 @@ type clientOpt struct {
 	ServerPub   key.NodePublic
 	CanAckPings bool
 	IsProber    bool
+	AppName     string
 }
 
 // MeshKey returns a ClientOpt to pass to the DERP server during connect to get
@@ -84,6 +86,13 @@ func CanAckPings(v bool) ClientOpt {
 	return clientOptFunc(func(o *clientOpt) { o.CanAckPings = v })
 }
 
+// AppName returns a ClientOpt to set an opaque app name string to
+// advertise to the DERP server for stats purposes. It is sent to the
+// server in the ClientInfo.
+func AppName(name string) ClientOpt {
+	return clientOptFunc(func(o *clientOpt) { o.AppName = name })
+}
+
 func NewClient(privateKey key.NodePrivate, nc Conn, brw *bufio.ReadWriter, logf logger.Logf, opts ...ClientOpt) (*Client, error) {
 	var opt clientOpt
 	for _, o := range opts {
@@ -106,6 +115,7 @@ func newClient(privateKey key.NodePrivate, nc Conn, brw *bufio.ReadWriter, logf
 		meshKey:     opt.MeshKey,
 		canAckPings: opt.CanAckPings,
 		isProber:    opt.IsProber,
+		appName:     opt.AppName,
 		clock:       tstime.StdClock{},
 	}
 	if opt.ServerPub.IsZero() {
@@ -179,6 +189,10 @@ type ClientInfo struct {
 
 	// IsProber is whether this client is a prober.
 	IsProber bool `json:",omitempty"`
+
+	// AppName is an optional opaque app name string the client
+	// advertises to the server for stats purposes.
+	AppName string `json:",omitempty"`
 }
 
 // Equal reports if two clientInfo values are equal.
@@ -186,7 +200,7 @@ func (c *ClientInfo) Equal(other *ClientInfo) bool {
 	if c == nil || other == nil {
 		return c == other
 	}
-	if c.Version != other.Version || c.CanAckPings != other.CanAckPings || c.IsProber != other.IsProber {
+	if c.Version != other.Version || c.CanAckPings != other.CanAckPings || c.IsProber != other.IsProber || c.AppName != other.AppName {
 		return false
 	}
 	return c.MeshKey.Equal(other.MeshKey)
@@ -198,6 +212,7 @@ func (c *Client) sendClientKey() error {
 		MeshKey:     c.meshKey,
 		CanAckPings: c.canAckPings,
 		IsProber:    c.isProber,
+		AppName:     c.appName,
 	})
 	if err != nil {
 		return err

derp/derphttp/derphttp_client.go

@@ -60,6 +60,7 @@ type Client struct {
 	DNSCache      *dnscache.Resolver // optional; nil means no caching
 	MeshKey       key.DERPMesh       // optional; for trusted clients
 	IsProber      bool               // optional; for probers to optional declare themselves as such
+	AppName       string             // optional; opaque app name to advertise to the server for stats
 
 	// WatchConnectionChanges is whether the client wishes to subscribe to
 	// notifications about clients connecting & disconnecting.
@@ -413,6 +414,7 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 			derp.MeshKey(c.MeshKey),
 			derp.CanAckPings(c.canAckPings),
 			derp.IsProber(c.IsProber),
+			derp.AppName(c.AppName),
 		)
 		if err != nil {
 			return nil, 0, err
@@ -558,6 +560,7 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 		derp.ServerPublicKey(serverPub),
 		derp.CanAckPings(c.canAckPings),
 		derp.IsProber(c.IsProber),
+		derp.AppName(c.AppName),
 	)
 	if err != nil {
 		return nil, 0, err

derp/derpserver/derpserver.go

@@ -141,6 +141,12 @@ type Server struct {
 	debug       bool
 	localClient local.Client
 
+	// onClientInfoForTest, if non-nil, is called with each connecting
+	// client's key and ClientInfo. It is set (before the server accepts
+	// any connections) via forTest.SetOnClientInfo and is nil outside
+	// of tests.
+	onClientInfoForTest func(key.NodePublic, derp.ClientInfo)
+
 	// Counters:
 	packetsSent, bytesSent     expvar.Int
 	packetsRecv, bytesRecv     expvar.Int
@@ -1061,6 +1067,9 @@ func (s *Server) accept(ctx context.Context, nc derp.Conn, brw *bufio.ReadWriter
 	if s.debug {
 		c.debug = true
 	}
+	if f := s.onClientInfoForTest; f != nil {
+		f(clientKey, c.info)
+	}
 
 	s.registerClient(c)
 	defer s.unregisterClient(c)

derp/derpserver/fortest.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package derpserver
+
+import (
+	"tailscale.com/derp"
+	"tailscale.com/types/key"
+	"tailscale.com/util/testenv"
+)
+
+// forTest is an unexported type to hide the test-only methods on
+// [Server] from godoc.
+type forTest struct{ s *Server }
+
+// ForTest returns a handle to test-only methods on s. The resulting
+// type is unexported to make it very obvious in godoc that this is
+// not stable API. This method panics if called outside of tests,
+// which also centralizes all must-be-in-tests validation.
+func (s *Server) ForTest() forTest {
+	testenv.AssertInTest()
+	return forTest{s}
+}
+
+// SetOnClientInfo sets a func to be called with each connecting
+// client's key and the ClientInfo it sent. It must be called before
+// the server accepts any connections.
+func (f forTest) SetOnClientInfo(fn func(key.NodePublic, derp.ClientInfo)) {
+	f.s.onClientInfoForTest = fn
+}

wgengine/magicsock/derp.go

@@ -414,6 +414,7 @@ func (c *Conn) derpWriteChanForRegion(regionID int, peer key.NodePublic) chan de
 		return derpMap.Regions[regionID]
 	})
 	dc.HealthTracker = c.health
+	dc.AppName = c.derpAppName
 	if c.extraRootCAs != nil {
 		dc.TLSConfig = &tls.Config{RootCAs: c.extraRootCAs}
 	}

wgengine/magicsock/magicsock.go

@@ -170,6 +170,7 @@ type Conn struct {
 	health                 *health.Tracker                        // or nil
 	extraRootCAs           *x509.CertPool                         // additional trusted root CAs; or nil
 	controlKnobs           *controlknobs.Knobs                    // or nil
+	derpAppName            string                                 // or empty, see Options.DERPAppName
 
 	// ================================================================
 	// No locking required to access these fields, either because
@@ -489,6 +490,10 @@ type Options struct {
 	// for TLS connections to DERP servers.
 	ExtraRootCAs *x509.CertPool
 
+	// DERPAppName, if non-empty, is an opaque app name string to
+	// advertise to DERP servers for stats purposes.
+	DERPAppName string
+
 	// Metrics specifies the metrics registry to record metrics to.
 	Metrics *usermetric.Registry
 
@@ -705,6 +710,7 @@ func NewConn(opts Options) (*Conn, error) {
 	c.netMon = opts.NetMon
 	c.health = opts.HealthTracker
 	c.extraRootCAs = opts.ExtraRootCAs
+	c.derpAppName = opts.DERPAppName
 	c.getPeerByKey = opts.PeerByKeyFunc
 
 	if err := c.rebind(keepCurrentPort); err != nil {

wgengine/userspace.go

@@ -210,6 +210,11 @@ type Config struct {
 	// connections (e.g. DERP). Passed through to magicsock.
 	ExtraRootCAs *x509.CertPool
 
+	// DERPAppName, if non-empty, is an opaque app name string to
+	// advertise to DERP servers for stats purposes. It is passed
+	// through to magicsock.
+	DERPAppName string
+
 	// ControlKnobs is the set of control plane-provied knobs
 	// to use.
 	// If nil, defaults are used.
@@ -425,6 +430,7 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 		NetMon:         e.netMon,
 		HealthTracker:  e.health,
 		ExtraRootCAs:   conf.ExtraRootCAs,
+		DERPAppName:    conf.DERPAppName,
 		Metrics:        conf.Metrics,
 		ControlKnobs:   conf.ControlKnobs,
 		PeerByKeyFunc:  e.PeerByKey,

wgengine/userspace_test.go

@@ -4,9 +4,13 @@
 package wgengine
 
 import (
+	"crypto/tls"
 	"errors"
 	"fmt"
 	"math/rand"
+	"net"
+	"net/http"
+	"net/http/httptest"
 	"net/netip"
 	"os"
 	"runtime"
@@ -19,12 +23,15 @@ import (
 	"go4.org/mem"
 	"tailscale.com/cmd/testwrapper/flakytest"
 	"tailscale.com/control/controlknobs"
+	"tailscale.com/derp"
+	"tailscale.com/derp/derpserver"
 	"tailscale.com/envknob"
 	"tailscale.com/health"
 	"tailscale.com/net/dns"
 	"tailscale.com/net/dns/resolver"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/netmon"
+	"tailscale.com/net/stun/stuntest"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/dnstype"
 	"tailscale.com/types/key"
@@ -553,3 +560,86 @@ func TestCloseWaitsForLinkChange(t *testing.T) {
 		t.Fatal("Close returned with link change work still in flight")
 	}
 }
+
+// TestDERPAppNamePlumbing tests that Config.DERPAppName makes it all
+// the way from the engine config to the ClientInfo received by an
+// in-process DERP server.
+func TestDERPAppNamePlumbing(t *testing.T) {
+	const appName = "app-name-plumbing-test"
+
+	priv := key.NewNode()
+	infoCh := make(chan derp.ClientInfo, 1)
+
+	derpSrv := derpserver.New(key.NewNode(), t.Logf)
+	derpSrv.ForTest().SetOnClientInfo(func(k key.NodePublic, info derp.ClientInfo) {
+		if k != priv.Public() {
+			return
+		}
+		select {
+		case infoCh <- info:
+		default:
+		}
+	})
+	httpsrv := httptest.NewUnstartedServer(derpserver.Handler(derpSrv))
+	httpsrv.Config.ErrorLog = logger.StdLogger(t.Logf)
+	httpsrv.Config.TLSNextProto = make(map[string]func(*http.Server, *tls.Conn, http.Handler))
+	httpsrv.StartTLS()
+	t.Cleanup(func() {
+		httpsrv.CloseClientConnections()
+		httpsrv.Close()
+		derpSrv.Close()
+	})
+
+	stunAddr, stunCleanup := stuntest.Serve(t)
+	t.Cleanup(stunCleanup)
+
+	derpMap := &tailcfg.DERPMap{
+		Regions: map[int]*tailcfg.DERPRegion{
+			1: {
+				RegionID:   1,
+				RegionCode: "test",
+				Nodes: []*tailcfg.DERPNode{{
+					Name:             "t1",
+					RegionID:         1,
+					HostName:         "test-node.unused",
+					IPv4:             "127.0.0.1",
+					IPv6:             "none",
+					STUNPort:         stunAddr.Port,
+					DERPPort:         httpsrv.Listener.Addr().(*net.TCPAddr).Port,
+					InsecureForTests: true,
+				}},
+			},
+		},
+	}
+
+	bus := eventbustest.NewBus(t)
+	noopDNS, err := dns.NewNoopManager()
+	if err != nil {
+		t.Fatal(err)
+	}
+	e, err := NewUserspaceEngine(t.Logf, Config{
+		HealthTracker: health.NewTracker(bus),
+		Metrics:       new(usermetric.Registry),
+		EventBus:      bus,
+		DNS:           noopDNS,
+		DERPAppName:   appName,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(e.Close)
+
+	if err := e.Reconfig(&wgcfg.Config{PrivateKey: priv}, &router.Config{}, &dns.Config{}); err != nil {
+		t.Fatalf("Reconfig: %v", err)
+	}
+	e.(*userspaceEngine).magicConn.SetDERPMap(derpMap)
+
+	select {
+	case info := <-infoCh:
+		if info.AppName != appName {
+			t.Fatalf("ClientInfo.AppName = %q; want %q", info.AppName, appName)
+		}
+	case <-time.After(10 * time.Second):
+		t.Fatal("timeout waiting for engine to connect to the test DERP server")
+	}
+}