ssh/tailssh: add nodeCap SSH principal

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

ssh/tailssh/tailssh.go

@@ -1201,6 +1201,9 @@ func (c *conn) principalMatchesTailscaleIdentity(p *tailcfg.SSHPrincipal) bool {
 			return true
 		}
 	}
+	if p.NodeCap != "" && ci.node.Valid() && ci.node.HasCap(p.NodeCap) {
+		return true
+	}
 	if p.UserLogin != "" && ci.uprof.LoginName == p.UserLogin {
 		return true
 	}

ssh/tailssh/tailssh_test.go

@@ -202,6 +202,16 @@ func TestMatchRule(t *testing.T) {
 			ci:       &sshConnInfo{node: (&tailcfg.Node{StableID: "some-node-ID"}).View()},
 			wantUser: "ubuntu",
 		},
+		{
+			name: "match-principal-node-cap",
+			rule: &tailcfg.SSHRule{
+				Action:     someAction,
+				Principals: []*tailcfg.SSHPrincipal{{NodeCap: "some-node-cap"}},
+				SSHUsers:   map[string]string{"*": "ubuntu"},
+			},
+			ci:       &sshConnInfo{node: (&tailcfg.Node{CapMap: tailcfg.NodeCapMap{"some-node-cap": nil}}).View()},
+			wantUser: "ubuntu",
+		},
 		{
 			name: "match-principal-userlogin",
 			rule: &tailcfg.SSHRule{

tailcfg/tailcfg.go

@@ -2841,13 +2841,14 @@ type SSHRule struct {
 
 // SSHPrincipal is either a particular node or a user on any node.
 type SSHPrincipal struct {
-	// Matching any one of the following four field causes a match.
+	// Matching any one of the following five field causes a match.
 	// It must also match Certs, if non-empty.
 
-	Node      StableNodeID `json:"node,omitempty"`
-	NodeIP    string       `json:"nodeIP,omitempty"`
-	UserLogin string       `json:"userLogin,omitempty"` // email-ish: foo@example.com, bar@github
-	Any       bool         `json:"any,omitempty"`       // if true, match any connection
+	Node      StableNodeID   `json:"node,omitempty"`
+	NodeIP    string         `json:"nodeIP,omitempty"`
+	NodeCap   NodeCapability `json:"nodeCap,omitempty"`
+	UserLogin string         `json:"userLogin,omitempty"` // email-ish: foo@example.com, bar@github
+	Any       bool           `json:"any,omitempty"`       // if true, match any connection
 	// TODO(bradfitz): add StableUserID, once that exists
 
 	// UnusedPubKeys was public key support. It never became an official product

tailcfg/tailcfg_clone.go

@@ -569,6 +569,7 @@ func (src *SSHPrincipal) Clone() *SSHPrincipal {
 var _SSHPrincipalCloneNeedsRegeneration = SSHPrincipal(struct {
 	Node          StableNodeID
 	NodeIP        string
+	NodeCap       NodeCapability
 	UserLogin     string
 	Any           bool
 	UnusedPubKeys []string

tailcfg/tailcfg_view.go

@@ -2220,8 +2220,9 @@ func (v *SSHPrincipalView) UnmarshalJSONFrom(dec *jsontext.Decoder) error {
 	return nil
 }
 
-func (v SSHPrincipalView) Node() StableNodeID { return v.ж.Node }
-func (v SSHPrincipalView) NodeIP() string     { return v.ж.NodeIP }
+func (v SSHPrincipalView) Node() StableNodeID      { return v.ж.Node }
+func (v SSHPrincipalView) NodeIP() string          { return v.ж.NodeIP }
+func (v SSHPrincipalView) NodeCap() NodeCapability { return v.ж.NodeCap }
 
 // email-ish: foo@example.com, bar@github
 func (v SSHPrincipalView) UserLogin() string { return v.ж.UserLogin }
@@ -2243,6 +2244,7 @@ func (v SSHPrincipalView) UnusedPubKeys() views.Slice[string] {
 var _SSHPrincipalViewNeedsRegeneration = SSHPrincipal(struct {
 	Node          StableNodeID
 	NodeIP        string
+	NodeCap       NodeCapability
 	UserLogin     string
 	Any           bool
 	UnusedPubKeys []string