policyutil: send approved exit nodes every rule

Tailscale SaaS treats exit routes like subnet routes when reducing
filters; 0.0.0.0/0 contains every dst, so exit nodes get all rules.

Updates #3493

hscontrol/policy/policyutil/reduce.go

@@ -19,7 +19,7 @@ import (
 func ReduceFilterRules(node types.NodeView, rules []tailcfg.FilterRule) []tailcfg.FilterRule {
 	ret := []tailcfg.FilterRule{}
 	subnetRoutes := node.SubnetRoutes()
-	hasExitRoutes := node.IsExitNode()
+	exitRoutes := node.ExitRoutes()
 
 	for _, rule := range rules {
 		// Handle CapGrant rules separately — they use CapGrant[].Dsts
@@ -53,19 +53,16 @@ func ReduceFilterRules(node types.NodeView, rules []tailcfg.FilterRule) []tailcf
 			// [types.NodeView.SubnetRoutes] returns only approved,
 			// non-exit routes — matching Tailscale SaaS behavior,
 			// which does not generate filter rules for
-			// advertised-but-unapproved routes. Exit routes
-			// (0.0.0.0/0, ::/0) are excluded by
-			// [types.NodeView.SubnetRoutes] and handled separately
-			// via AllowedIPs/routing.
+			// advertised-but-unapproved routes.
 			if slices.ContainsFunc(subnetRoutes, expanded.OverlapsPrefix) {
 				dests = append(dests, dest)
 				continue
 			}
 
-			// Exit-route advertisers need rules targeting the
-			// public internet so the kernel filter accepts
-			// traffic forwarded by autogroup:internet sources.
-			if hasExitRoutes && util.IPSetSubsetOf(expanded, util.TheInternet()) {
+			// Approved exit routes count like subnet routes. They
+			// contain every destination, so Tailscale SaaS sends an
+			// exit node every rule, not only internet ones.
+			if slices.ContainsFunc(exitRoutes, expanded.OverlapsPrefix) {
 				dests = append(dests, dest)
 			}
 		}

hscontrol/policy/policyutil/reduce_test.go

@@ -886,6 +886,66 @@ func TestReduceFilterRulesPartialApproval(t *testing.T) {
 			},
 			wantCount: 0,
 		},
+		{
+			// Tailscale SaaS delivers every rule to an approved exit
+			// node: its exit routes contain every destination.
+			name: "approved-exit-route-includes-tailnet-host",
+			node: &types.Node{
+				IPv4:     ap("100.64.0.1"),
+				IPv6:     ap("fd7a:115c:a1e0::1"),
+				Hostinfo: &tailcfg.Hostinfo{RoutableIPs: tsaddr.ExitRoutes()},
+
+				ApprovedRoutes: tsaddr.ExitRoutes(),
+			},
+			rules: []tailcfg.FilterRule{
+				{
+					SrcIPs: []string{"100.64.0.17"},
+					DstPorts: []tailcfg.NetPortRange{
+						{IP: "100.64.0.16", Ports: tailcfg.PortRange{First: 53, Last: 53}},
+						{IP: "fd7a:115c:a1e0::10", Ports: tailcfg.PortRange{First: 53, Last: 53}},
+					},
+				},
+			},
+			wantCount:  1,
+			wantRoutes: []string{"100.64.0.16", "fd7a:115c:a1e0::10"},
+		},
+		{
+			name: "approved-exit-route-includes-private-subnet",
+			node: &types.Node{
+				IPv4:     ap("100.64.0.1"),
+				IPv6:     ap("fd7a:115c:a1e0::1"),
+				Hostinfo: &tailcfg.Hostinfo{RoutableIPs: tsaddr.ExitRoutes()},
+
+				ApprovedRoutes: tsaddr.ExitRoutes(),
+			},
+			rules: []tailcfg.FilterRule{
+				{
+					SrcIPs: []string{"100.64.0.17"},
+					DstPorts: []tailcfg.NetPortRange{
+						{IP: "10.33.0.0/16", Ports: tailcfg.PortRangeAny},
+					},
+				},
+			},
+			wantCount:  1,
+			wantRoutes: []string{"10.33.0.0/16"},
+		},
+		{
+			name: "unapproved-exit-route-excluded",
+			node: &types.Node{
+				IPv4:     ap("100.64.0.1"),
+				IPv6:     ap("fd7a:115c:a1e0::1"),
+				Hostinfo: &tailcfg.Hostinfo{RoutableIPs: tsaddr.ExitRoutes()},
+			},
+			rules: []tailcfg.FilterRule{
+				{
+					SrcIPs: []string{"100.64.0.17"},
+					DstPorts: []tailcfg.NetPortRange{
+						{IP: "100.64.0.16", Ports: tailcfg.PortRangeAny},
+					},
+				},
+			},
+			wantCount: 0,
+		},
 	}
 
 	for _, tt := range tests {