From 0970615b1b455a94214abd61b5e79d05d6d8f9bf Mon Sep 17 00:00:00 2001
From: Andrew Dunham <andrew@du.nham.ca>
Date: Mon, 9 Sep 2024 15:15:29 -0400
Subject: [PATCH] ipn/ipnlocal: don't program system DNS when node key is
 expired (#13370)

This mimics having Tailscale in the 'Stopped' state by programming an
empty DNS configuration when the current node key is expired.

Updates tailscale/support-escalations#55


Change-Id: I68ff4665761fb621ed57ebf879263c2f4b911610

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
---
 ipn/ipnlocal/dnsconfig_test.go | 22 +++++++++++++++++++++-
 ipn/ipnlocal/local.go          | 17 +++++++++++++++--
 ipn/ipnlocal/local_test.go     |  2 +-
 3 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/ipn/ipnlocal/dnsconfig_test.go b/ipn/ipnlocal/dnsconfig_test.go
index 02db18ffe..19d8e8b86 100644
--- a/ipn/ipnlocal/dnsconfig_test.go
+++ b/ipn/ipnlocal/dnsconfig_test.go
@@ -50,6 +50,7 @@ func TestDNSConfigForNetmap(t *testing.T) {
 	tests := []struct {
 		name    string
 		nm      *netmap.NetworkMap
+		expired bool
 		peers   []tailcfg.NodeView
 		os      string // version.OS value; empty means linux
 		cloud   cloudenv.Cloud
@@ -327,12 +328,31 @@ func TestDNSConfigForNetmap(t *testing.T) {
 				Routes: map[dnsname.FQDN][]*dnstype.Resolver{},
 			},
 		},
+		{
+			name: "self_expired",
+			nm: &netmap.NetworkMap{
+				Name: "myname.net",
+				SelfNode: (&tailcfg.Node{
+					Addresses: ipps("100.101.101.101"),
+				}).View(),
+			},
+			expired: true,
+			peers: nodeViews([]*tailcfg.Node{
+				{
+					ID:        1,
+					Name:      "peera.net",
+					Addresses: ipps("100.102.0.1", "100.102.0.2", "fe75::1001", "fe75::1002"),
+				},
+			}),
+			prefs: &ipn.Prefs{},
+			want:  &dns.Config{},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			verOS := cmp.Or(tt.os, "linux")
 			var log tstest.MemLogger
-			got := dnsConfigForNetmap(tt.nm, peersMap(tt.peers), tt.prefs.View(), log.Logf, verOS)
+			got := dnsConfigForNetmap(tt.nm, peersMap(tt.peers), tt.prefs.View(), tt.expired, log.Logf, verOS)
 			if !reflect.DeepEqual(got, tt.want) {
 				gotj, _ := json.MarshalIndent(got, "", "\t")
 				wantj, _ := json.MarshalIndent(tt.want, "", "\t")
diff --git a/ipn/ipnlocal/local.go b/ipn/ipnlocal/local.go
index 9a57776a0..fbe35b87b 100644
--- a/ipn/ipnlocal/local.go
+++ b/ipn/ipnlocal/local.go
@@ -4026,7 +4026,7 @@ func (b *LocalBackend) authReconfig() {
 	disableSubnetsIfPAC := nm.HasCap(tailcfg.NodeAttrDisableSubnetsIfPAC)
 	userDialUseRoutes := nm.HasCap(tailcfg.NodeAttrUserDialUseRoutes)
 	dohURL, dohURLOK := exitNodeCanProxyDNS(nm, b.peers, prefs.ExitNodeID())
-	dcfg := dnsConfigForNetmap(nm, b.peers, prefs, b.logf, version.OS())
+	dcfg := dnsConfigForNetmap(nm, b.peers, prefs, b.keyExpired, b.logf, version.OS())
 	// If the current node is an app connector, ensure the app connector machine is started
 	b.reconfigAppConnectorLocked(nm, prefs)
 	b.mu.Unlock()
@@ -4126,10 +4126,23 @@ func shouldUseOneCGNATRoute(logf logger.Logf, controlKnobs *controlknobs.Knobs,
 //
 // The versionOS is a Tailscale-style version ("iOS", "macOS") and not
 // a runtime.GOOS.
-func dnsConfigForNetmap(nm *netmap.NetworkMap, peers map[tailcfg.NodeID]tailcfg.NodeView, prefs ipn.PrefsView, logf logger.Logf, versionOS string) *dns.Config {
+func dnsConfigForNetmap(nm *netmap.NetworkMap, peers map[tailcfg.NodeID]tailcfg.NodeView, prefs ipn.PrefsView, selfExpired bool, logf logger.Logf, versionOS string) *dns.Config {
 	if nm == nil {
 		return nil
 	}
+
+	// If the current node's key is expired, then we don't program any DNS
+	// configuration into the operating system. This ensures that if the
+	// DNS configuration specifies a DNS server that is only reachable over
+	// Tailscale, we don't break connectivity for the user.
+	//
+	// TODO(andrew-d): this also stops returning anything from quad-100; we
+	// could do the same thing as having "CorpDNS: false" and keep that but
+	// not program the OS?
+	if selfExpired {
+		return &dns.Config{}
+	}
+
 	dcfg := &dns.Config{
 		Routes: map[dnsname.FQDN][]*dnstype.Resolver{},
 		Hosts:  map[dnsname.FQDN][]netip.Addr{},
diff --git a/ipn/ipnlocal/local_test.go b/ipn/ipnlocal/local_test.go
index bf4a28ff9..e4091ef02 100644
--- a/ipn/ipnlocal/local_test.go
+++ b/ipn/ipnlocal/local_test.go
@@ -1283,7 +1283,7 @@ type tc struct {
 			}
 
 			prefs := &ipn.Prefs{ExitNodeID: tc.exitNode, CorpDNS: true}
-			got := dnsConfigForNetmap(nm, peersMap(tc.peers), prefs.View(), t.Logf, "")
+			got := dnsConfigForNetmap(nm, peersMap(tc.peers), prefs.View(), false, t.Logf, "")
 			if !resolversEqual(t, got.DefaultResolvers, tc.wantDefaultResolvers) {
 				t.Errorf("DefaultResolvers: got %#v, want %#v", got.DefaultResolvers, tc.wantDefaultResolvers)
 			}