tailcfg,cmd/k8s-operator: moves tailscale.com/cap/kubernetes peer cap to tailcfg (#12235)

This is done in preparation for adding kubectl
session recording rules to this capability grant that will need to
be unmarshalled by control, so will also need to be
in a shared location.

Updates tailscale/corp#19821

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

cmd/k8s-operator/proxy.go

@@ -207,8 +207,11 @@ func runAPIServerProxy(s *tsnet.Server, rt http.RoundTripper, log *zap.SugaredLo
 }
 
 const (
-	capabilityName    = "tailscale.com/cap/kubernetes"
-	oldCapabilityName = "https://" + capabilityName
+	// oldCapabilityName is a legacy form of
+	// tailfcg.PeerCapabilityKubernetes capability. The only capability rule
+	// that is respected for this form is group impersonation - for
+	// backwards compatibility reasons.
+	oldCapabilityName = "https://" + tailcfg.PeerCapabilityKubernetes
 )
 
 type capRule struct {
@@ -229,7 +232,7 @@ type impersonateRule struct {
 func addImpersonationHeaders(r *http.Request, log *zap.SugaredLogger) error {
 	log = log.With("remote", r.RemoteAddr)
 	who := whoIsKey.Value(r.Context())
-	rules, err := tailcfg.UnmarshalCapJSON[capRule](who.CapMap, capabilityName)
+	rules, err := tailcfg.UnmarshalCapJSON[capRule](who.CapMap, tailcfg.PeerCapabilityKubernetes)
 	if len(rules) == 0 && err == nil {
 		// Try the old capability name for backwards compatibility.
 		rules, err = tailcfg.UnmarshalCapJSON[capRule](who.CapMap, oldCapabilityName)

cmd/k8s-operator/proxy_test.go

@@ -49,7 +49,7 @@ func TestImpersonationHeaders(t *testing.T) {
 			name:     "user-with-cap",
 			emailish: "foo@example.com",
 			capMap: tailcfg.PeerCapMap{
-				capabilityName: {
+				tailcfg.PeerCapabilityKubernetes: {
 					tailcfg.RawMessage(`{"impersonate":{"groups":["group1","group2"]}}`),
 					tailcfg.RawMessage(`{"impersonate":{"groups":["group1","group3"]}}`), // One group is duplicated.
 					tailcfg.RawMessage(`{"impersonate":{"groups":["group4"]}}`),
@@ -71,7 +71,7 @@ func TestImpersonationHeaders(t *testing.T) {
 			emailish: "tagged-device",
 			tags:     []string{"tag:foo", "tag:bar"},
 			capMap: tailcfg.PeerCapMap{
-				capabilityName: {
+				tailcfg.PeerCapabilityKubernetes: {
 					tailcfg.RawMessage(`{"impersonate":{"groups":["group1"]}}`),
 				},
 			},
@@ -85,7 +85,7 @@ func TestImpersonationHeaders(t *testing.T) {
 			emailish: "tagged-device",
 			tags:     []string{"tag:foo", "tag:bar"},
 			capMap: tailcfg.PeerCapMap{
-				capabilityName: {
+				tailcfg.PeerCapabilityKubernetes: {
 					tailcfg.RawMessage(`[]`),
 				},
 			},

tailcfg/tailcfg.go

@@ -1375,6 +1375,12 @@ const (
 	// PeerCapabilityTaildriveSharer indicates that a peer has the ability to
 	// share folders with us.
 	PeerCapabilityTaildriveSharer PeerCapability = "tailscale.com/cap/drive-sharer"
+
+	// PeerCapabilityKubernetes grants a peer Kubernetes-specific
+	// capabilities, such as the ability to impersonate specific Tailscale
+	// user groups as Kubernetes user groups. This capability is read by
+	// peers that are Tailscale Kubernetes operator instances.
+	PeerCapabilityKubernetes PeerCapability = "tailscale.com/cap/kubernetes"
 )
 
 // NodeCapMap is a map of capabilities to their optional values. It is valid for