tailcfg: add a DebugFlags field for experiments and debugging.

Also replaces the IPv6Overlay bool with use of DebugFlags, since
it's currently an experimental configuration.

Signed-off-by: David Anderson <danderson@tailscale.com>

control/controlclient/direct.go

@@ -45,10 +45,6 @@
 	"tailscale.com/version"
 )
 
-var (
-	enableV6Overlay, _ = strconv.ParseBool(os.Getenv("TS_DEBUG_ENABLE_IPV6_OVERLAY"))
-)
-
 type Persist struct {
 	_ structs.Incomparable
 
@@ -111,6 +107,7 @@ type Direct struct {
 	logf            logger.Logf
 	discoPubKey     tailcfg.DiscoKey
 	machinePrivKey  wgcfg.PrivateKey
+	debugFlags      []string
 
 	mu           sync.Mutex // mutex guards the following fields
 	serverKey    wgcfg.Key
@@ -137,6 +134,7 @@ type Options struct {
 	KeepAlive         bool
 	Logf              logger.Logf
 	HTTPTestClient    *http.Client // optional HTTP client to use (for tests only)
+	DebugFlags        []string     // debug settings to send to control
 }
 
 type Decompressor interface {
@@ -189,6 +187,7 @@ func NewDirect(opts Options) (*Direct, error) {
 		persist:         opts.Persist,
 		authKey:         opts.AuthKey,
 		discoPubKey:     opts.DiscoPublicKey,
+		debugFlags:      opts.DebugFlags,
 	}
 	if opts.Hostinfo == nil {
 		c.SetHostinfo(NewHostinfo())
@@ -533,7 +532,6 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	request := tailcfg.MapRequest{
 		Version:     4,
 		IncludeIPv6: true,
-		IncludeIPv6Overlay: enableV6Overlay,
 		DeltaPeers:  true,
 		KeepAlive:   c.keepAlive,
 		NodeKey:     tailcfg.NodeKey(persist.PrivateNodeKey.Public()),
@@ -541,6 +539,7 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 		Endpoints:   ep,
 		Stream:      allowStream,
 		Hostinfo:    hostinfo,
+		DebugFlags:  c.debugFlags,
 	}
 	if c.newDecompressor != nil {
 		request.Compress = "zstd"

ipn/local.go

@@ -36,6 +36,10 @@
 	"tailscale.com/wgengine/tsdns"
 )
 
+var (
+	controlDebugFlags = strings.Split(os.Getenv("TS_DEBUG_CONTROL_FLAGS"), ",")
+)
+
 // LocalBackend is the glue between the major pieces of the Tailscale
 // network software: the cloud control plane (via controlclient), the
 // network data plane (via wgengine), and the user-facing UIs and CLIs
@@ -451,6 +455,7 @@ func (b *LocalBackend) Start(opts Options) error {
 		NewDecompressor:   b.newDecompressor,
 		HTTPTestClient:    opts.HTTPTestClient,
 		DiscoPublicKey:    discoPublic,
+		DebugFlags:        controlDebugFlags,
 	})
 	if err != nil {
 		return err

tailcfg/tailcfg.go

@@ -486,6 +486,14 @@ type MapRequest struct {
 	// being omitted in the response. (For example, a client on
 	// start up using ReadOnly to get the DERP map.)
 	OmitPeers bool `json:",omitempty"`
+
+	// DebugFlags is a list of strings specifying debugging and
+	// development features to enable in handling this map
+	// request. The values are deliberately unspecified, as they get
+	// added and removed all the time during development, and offer no
+	// compatibility promise. To roll out semantic changes, bump
+	// Version instead.
+	DebugFlags []string `json:",omitempty"`
 }
 
 // PortRange represents a range of UDP or TCP port numbers.