tstest/natlab: test home DERP is re-reported after a profile switch (#20051)

Add a vmtest that guards the fix in #20025: after an in-process control
client swap (profile switch / interactive re-login), magicsock's NetInfo
dedup cache (netInfoLast) must be cleared so the structurally-identical
post-switch NetInfo (same PreferredDERP, same NAT shape) is re-reported to
the new control session rather than suppressed as unchanged.

The test brings a node up, pins its home DERP so the reported NetInfo is
identical across the switch, records the home DERP the test control learned,
switches to a fresh login profile on the same control/network/NAT/DERP, and
asserts the control re-learns the same non-zero home DERP for the node's new
identity. Without ResetNetInfoLast the assertion times out at HomeDERP=0.

To support this, vnet now serves the test control on port 443 (TLS) in
addition to port 80: an immediate re-login makes a fresh noise dial, and
because the prior dial was recent the control client forces an HTTPS (443)
dial (controlhttp.Dialer.forceNoise443), which the harness previously did
not answer. The control endpoint gets its own self-signed cert (the existing
selfSignedDERPCert helper, renamed to the generic selfSignedCert); the cert
is not validated since control noise dials authenticate via the Noise
handshake, so it only needs a TLS peer to complete the forced 443 dial.

Add Env.ForcePreferredDERP and Env.Relogin helpers for the above.

Updates #20024

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

tstest/natlab/vnet/vnet.go

@@ -374,13 +374,22 @@ func (n *network) acceptTCP(r *tcp.ForwarderRequest) {
 		return
 	}
 
-	if destPort == 80 && fakeControl.Match(destIP) {
+	if fakeControl.Match(destIP) && (destPort == 80 || destPort == 443) {
 		r.Complete(false)
 		tc := gonet.NewTCPConn(&wq, ep)
 		context.AfterFunc(n.s.shutdownCtx, func() { tc.SetDeadline(time.Now()) })
+		// The control client's noise dialer forces an HTTPS (port 443) dial when
+		// it made a noise dial recently — e.g. an immediate re-login or profile
+		// switch; see controlhttp.Dialer.forceNoise443. Serve the test control
+		// over TLS on 443 too so that path reaches it. (The cert isn't
+		// validated: noise dials authenticate via the Noise handshake.)
+		var ln net.Listener = netutil.NewOneConnListener(tc, nil)
+		if destPort == 443 {
+			ln = netutil.NewOneConnListener(tls.Server(tc, n.s.controlTLS), nil)
+		}
 		hs := &http.Server{Handler: n.s.control}
 		n.s.wg.Go(func() {
-			hs.Serve(netutil.NewOneConnListener(tc, nil))
+			hs.Serve(ln)
 		})
 		return
 	}
@@ -776,7 +785,7 @@ type derpServer struct {
 // want to exercise sha256-raw cert pinning can read the certSHA256Hex via
 // [Server.DERPCertSHA256Hex].
 func newDERPServer(hostname string) *derpServer {
-	tlsConfig, certHex := selfSignedDERPCert(hostname)
+	tlsConfig, certHex := selfSignedCert(hostname)
 	ds := &derpServer{
 		srv:           derpserver.New(key.NewNode(), logger.Discard),
 		tlsConfig:     tlsConfig,
@@ -790,10 +799,10 @@ func newDERPServer(hostname string) *derpServer {
 	return ds
 }
 
-// selfSignedDERPCert builds a self-signed ECDSA P-256 cert valid for hostname
-// and returns a *tls.Config that serves it, along with the SHA-256 hex digest
-// of the cert's DER bytes.
-func selfSignedDERPCert(hostname string) (*tls.Config, string) {
+// selfSignedCert builds a self-signed ECDSA P-256 cert valid for hostname and
+// returns a *tls.Config that serves it, along with the SHA-256 hex digest of
+// the cert's DER bytes (used by DERP for sha256-raw cert pinning).
+func selfSignedCert(hostname string) (*tls.Config, string) {
 	key, err := ecdsa.GenerateKey(elliptic.P256(), crand.Reader)
 	if err != nil {
 		panic(fmt.Sprintf("vnet: generating DERP cert key: %v", err))
@@ -840,7 +849,12 @@ type Server struct {
 	networks     set.Set[*network]
 	networkByWAN *bart.Table[*network]
 
-	control    *testcontrol.Server
+	control *testcontrol.Server
+	// controlTLS is a self-signed cert for serving the test control over HTTPS
+	// (port 443) in addition to plaintext HTTP. The control client does not
+	// validate this cert (noise dials authenticate via the Noise handshake, not
+	// the outer TLS); it exists only so the forced-443 dial path has a TLS peer.
+	controlTLS *tls.Config
 	derps      []*derpServer
 	fakeACME   *fakeACMEServer
 	pcapWriter *pcapWriter
@@ -888,6 +902,9 @@ func (s *Server) SetDHCPCallback(fn func(MAC, int, layers.DHCPMsgType, netip.Add
 // hostname verification succeeds for tests that pin via sha256-raw.
 var derpHostnames = []string{"derp1.tailscale", "derp2.tailscale"}
 
+// controlHostname is the hostname the fake control server is reached at.
+const controlHostname = "control.tailscale"
+
 var derpMap = &tailcfg.DERPMap{
 	Regions: map[int]*tailcfg.DERPRegion{
 		1: {
@@ -933,7 +950,7 @@ func New(c *Config) (*Server, error) {
 
 		control: &testcontrol.Server{
 			DERPMap:         derpMap,
-			ExplicitBaseURL: "http://control.tailscale",
+			ExplicitBaseURL: "http://" + controlHostname,
 		},
 		fakeACME: newFakeACMEServer("http://acme.example"),
 
@@ -950,6 +967,7 @@ func New(c *Config) (*Server, error) {
 		return nil
 	}
 	s.fakeACME.lookupTXT = s.lookupTXT
+	s.controlTLS, _ = selfSignedCert(controlHostname)
 	for _, host := range derpHostnames {
 		s.derps = append(s.derps, newDERPServer(host))
 	}