tailcfg, feature/relayserver, wgengine/magicsock: invert UDP relay server nodeAttrs (#16444)

Inverts the nodeAttrs related to UDP relay client/server enablement to disablement, and fixes up the corresponding logic that uses them. Also updates the doc comments on both nodeAttrs.

Fixes tailscale/corp#30024

Signed-off-by: Dylan Bargatze <dylan@tailscale.com>

feature/relayserver/relayserver.go

@@ -50,11 +50,11 @@ func newExtension(logf logger.Logf, _ ipnext.SafeBackend) (ipnext.Extension, err
 type extension struct {
 	logf logger.Logf
 
-	mu                     sync.Mutex // guards the following fields
-	shutdown               bool
-	port                   *int        // ipn.Prefs.RelayServerPort, nil if disabled
-	hasNodeAttrRelayServer bool        // tailcfg.NodeAttrRelayServer
-	server                 relayServer // lazily initialized
+	mu                            sync.Mutex // guards the following fields
+	shutdown                      bool
+	port                          *int        // ipn.Prefs.RelayServerPort, nil if disabled
+	hasNodeAttrDisableRelayServer bool        // tailcfg.NodeAttrDisableRelayServer
+	server                        relayServer // lazily initialized
 }
 
 // relayServer is the interface of [udprelay.Server].
@@ -81,8 +81,8 @@ func (e *extension) Init(host ipnext.Host) error {
 func (e *extension) selfNodeViewChanged(nodeView tailcfg.NodeView) {
 	e.mu.Lock()
 	defer e.mu.Unlock()
-	e.hasNodeAttrRelayServer = nodeView.HasCap(tailcfg.NodeAttrRelayServer)
-	if !e.hasNodeAttrRelayServer && e.server != nil {
+	e.hasNodeAttrDisableRelayServer = nodeView.HasCap(tailcfg.NodeAttrDisableRelayServer)
+	if e.hasNodeAttrDisableRelayServer && e.server != nil {
 		e.server.Close()
 		e.server = nil
 	}
@@ -130,8 +130,8 @@ func (e *extension) relayServerOrInit() (relayServer, error) {
 	if e.port == nil {
 		return nil, errors.New("relay server is not configured")
 	}
-	if !e.hasNodeAttrRelayServer {
-		return nil, errors.New("no relay:server node attribute")
+	if e.hasNodeAttrDisableRelayServer {
+		return nil, errors.New("disable-relay-server node attribute is present")
 	}
 	if !envknob.UseWIPCode() {
 		return nil, errors.New("TAILSCALE_USE_WIP_CODE envvar is not set")

tailcfg/tailcfg.go

@@ -2602,13 +2602,20 @@ const (
 	// peer node list.
 	NodeAttrNativeIPV4 NodeCapability = "native-ipv4"
 
-	// NodeAttrRelayServer permits the node to act as an underlay UDP relay
-	// server. There are no expected values for this key in NodeCapMap.
-	NodeAttrRelayServer NodeCapability = "relay:server"
+	// NodeAttrDisableRelayServer prevents the node from acting as an underlay
+	// UDP relay server. There are no expected values for this key; the key
+	// only needs to be present in [NodeCapMap] to take effect.
+	NodeAttrDisableRelayServer NodeCapability = "disable-relay-server"
 
-	// NodeAttrRelayClient permits the node to act as an underlay UDP relay
-	// client. There are no expected values for this key in NodeCapMap.
-	NodeAttrRelayClient NodeCapability = "relay:client"
+	// NodeAttrDisableRelayClient prevents the node from allocating UDP relay
+	// server endpoints itself; the node may still bind into and relay traffic
+	// using endpoints allocated by its peers. This attribute can be added to
+	// the node dynamically; if added while the node is already running, the
+	// node will be unable to allocate UDP relay server endpoints after it next
+	// updates its network map. There are no expected values for this key in
+	// [NodeCapMap]; the key only needs to be present in [NodeCapMap] to take
+	// effect.
+	NodeAttrDisableRelayClient NodeCapability = "disable-relay-client"
 
 	// NodeAttrMagicDNSPeerAAAA is a capability that tells the node's MagicDNS
 	// server to answer AAAA queries about its peers. See tailscale/tailscale#1152.

wgengine/magicsock/magicsock.go

@@ -2703,7 +2703,7 @@ func (c *Conn) onNodeViewsUpdate(update NodeViewsUpdate) {
 	peersChanged := c.updateNodes(update)
 
 	relayClientEnabled := update.SelfNode.Valid() &&
-		update.SelfNode.HasCap(tailcfg.NodeAttrRelayClient) &&
+		!update.SelfNode.HasCap(tailcfg.NodeAttrDisableRelayClient) &&
 		envknob.UseWIPCode()
 
 	c.mu.Lock()

wgengine/magicsock/magicsock_test.go

@@ -3408,9 +3408,6 @@ func Test_peerAPIIfCandidateRelayServer(t *testing.T) {
 	}
 	peerOnlyIPv4 := &tailcfg.Node{
 		Cap: math.MinInt32,
-		CapMap: map[tailcfg.NodeCapability][]tailcfg.RawMessage{
-			tailcfg.NodeAttrRelayServer: nil,
-		},
 		Addresses: []netip.Prefix{
 			netip.MustParsePrefix("2.2.2.2/32"),
 		},