net/dns: set OS DNS to 100.100.100.100 for route-less ExtraRecords [cap 41]

If ExtraRecords (Hosts) are specified without a corresponding split
DNS route and global DNS is specified, then program the host OS DNS to
use 100.100.100.100 so it can blend in those ExtraRecords.

Updates #1543

Change-Id: If49014a5ecc8e38978ff26e54d1f74fe8dbbb9bc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/config.go

@@ -91,6 +91,30 @@ func (c Config) hasDefaultIPResolversOnly() bool {
 	return true
 }
 
+// hasHostsWithoutSplitDNSRoutes reports whether c contains any Host entries
+// that aren't covered by a SplitDNS route suffix.
+func (c Config) hasHostsWithoutSplitDNSRoutes() bool {
+	// TODO(bradfitz): this could be more efficient, but we imagine
+	// the number of SplitDNS routes and/or hosts will be small.
+	for host := range c.Hosts {
+		if !c.hasSplitDNSRouteForHost(host) {
+			return true
+		}
+	}
+	return false
+}
+
+// hasSplitDNSRouteForHost reports whether c contains a SplitDNS route
+// that contains hosts.
+func (c Config) hasSplitDNSRouteForHost(host dnsname.FQDN) bool {
+	for route := range c.Routes {
+		if route.Contains(host) {
+			return true
+		}
+	}
+	return false
+}
+
 func (c Config) hasDefaultResolvers() bool {
 	return len(c.DefaultResolvers) > 0
 }

net/dns/manager.go

@@ -207,9 +207,14 @@ func (m *Manager) compileConfig(cfg Config) (rcfg resolver.Config, ocfg OSConfig
 		// case where cfg is entirely zero, in which case these
 		// configs clear all Tailscale DNS settings.
 		return rcfg, ocfg, nil
-	case cfg.hasDefaultIPResolversOnly():
+	case cfg.hasDefaultIPResolversOnly() && !cfg.hasHostsWithoutSplitDNSRoutes():
-		// Trivial CorpDNS configuration, just override the OS
+		// Trivial CorpDNS configuration, just override the OS resolver.
-		// resolver.
+		//
+		// If there are hosts (ExtraRecords) that are not covered by an existing
+		// SplitDNS route, then we don't go into this path so that we fall into
+		// the next case and send the extra record hosts queries through
+		// 100.100.100.100 instead where we can answer them.
+		//
 		// TODO: for OSes that support it, pass IP:port and DoH
 		// addresses directly to OS.
 		// https://github.com/tailscale/tailscale/issues/1666

net/dns/manager_test.go

@@ -199,6 +199,71 @@ func TestManager(t *testing.T) {
 					"bradfitz.ts.com.", "2.3.4.5"),
 			},
 		},
+		{
+			// If Hosts are specified (i.e. ExtraRecords) that aren't a split
+			// DNS route and a global resolver is specified, then make
+			// everything go via 100.100.100.100.
+			name:  "hosts-with-global-dns-uses-quad100",
+			split: true,
+			in: Config{
+				DefaultResolvers: mustRes("1.1.1.1", "9.9.9.9"),
+				Hosts: hosts(
+					"foo.tld.", "1.2.3.4",
+					"bar.tld.", "2.3.4.5"),
+			},
+			os: OSConfig{
+				Nameservers: mustIPs("100.100.100.100"),
+			},
+			rs: resolver.Config{
+				Hosts: hosts(
+					"foo.tld.", "1.2.3.4",
+					"bar.tld.", "2.3.4.5"),
+				Routes: upstreams(".", "1.1.1.1", "9.9.9.9"),
+			},
+		},
+		{
+			// This is the above hosts-with-global-dns-uses-quad100 test but
+			// verifying that if global DNS servers aren't set (the 1.1.1.1 and
+			// 9.9.9.9 above), then we don't configure 100.100.100.100 as the
+			// resolver.
+			name:  "hosts-without-global-dns-not-use-quad100",
+			split: true,
+			in: Config{
+				Hosts: hosts(
+					"foo.tld.", "1.2.3.4",
+					"bar.tld.", "2.3.4.5"),
+			},
+			os: OSConfig{},
+			rs: resolver.Config{
+				Hosts: hosts(
+					"foo.tld.", "1.2.3.4",
+					"bar.tld.", "2.3.4.5"),
+			},
+		},
+		{
+			// This tests that ExtraRecords (foo.tld and bar.tld here) don't trigger forcing
+			// traffic through 100.100.100.100 if there's Split DNS support and the extra
+			// records are part of a split DNS route.
+			name:  "hosts-with-extrarecord-hosts-with-routes-no-quad100",
+			split: true,
+			in: Config{
+				Routes: upstreams(
+					"tld.", "4.4.4.4",
+				),
+				Hosts: hosts(
+					"foo.tld.", "1.2.3.4",
+					"bar.tld.", "2.3.4.5"),
+			},
+			os: OSConfig{
+				Nameservers:  mustIPs("4.4.4.4"),
+				MatchDomains: fqdns("tld."),
+			},
+			rs: resolver.Config{
+				Hosts: hosts(
+					"foo.tld.", "1.2.3.4",
+					"bar.tld.", "2.3.4.5"),
+			},
+		},
 		{
 			name: "corp",
 			in: Config{

tailcfg/tailcfg.go

@@ -77,7 +77,8 @@ type CapabilityVersion int
 //	38: 2022-08-11: added PingRequest.URLIsNoise
 //	39: 2022-08-15: clients can talk Noise over arbitrary HTTPS port
 //	40: 2022-08-22: added Node.KeySignature, PeersChangedPatch.KeySignature
-const CurrentCapabilityVersion CapabilityVersion = 40
+//	41: 2022-08-30: uses 100.100.100.100 for route-less ExtraRecords if global nameservers is set
+const CurrentCapabilityVersion CapabilityVersion = 41
 
 type StableID string