net/dnscache: use Go DNS resolver on Windows

Go 1.19 introduced Resolver.PreferGo which allows the Go resolver
to be used on Windows.

See golang/go#33097.

Fixes tailscale#5161

Signed-off-by: Thomas Way <thomas@6f.io>

net/dnscache/dnscache.go

@@ -31,19 +31,19 @@ import (
 var zaddr netip.Addr
 
 var single = &Resolver{
-	Forward: &net.Resolver{PreferGo: preferGoResolver()},
+	Forward: &net.Resolver{PreferGo: preferGoResolver(runtime.GOOS)},
 }
 
-func preferGoResolver() bool {
+func preferGoResolver(goos string) bool {
 	// There does not appear to be a local resolver running
 	// on iOS, and NetworkExtension is good at isolating DNS.
 	// So do not use the Go resolver on macOS/iOS.
-	if runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
+	if goos == "darwin" || goos == "ios" {
 		return false
 	}
 
 	// The local resolver is not available on Android.
-	if runtime.GOOS == "android" {
+	if goos == "android" {
 		return false
 	}
 
@@ -140,12 +140,7 @@ func (r *Resolver) dlogf(format string, args ...any) {
 // cloudHostResolver returns a Resolver for the current cloud hosting environment.
 // It currently only supports Google Cloud.
 func (r *Resolver) cloudHostResolver() (v *net.Resolver, ok bool) {
-	switch runtime.GOOS {
-	case "android", "ios", "darwin":
-		return nil, false
-	case "windows":
-		// TODO(bradfitz): remove this restriction once we're using Go 1.19
-		// which supports net.Resolver.PreferGo on Windows.
+	if !preferGoResolver(runtime.GOOS) {
 		return nil, false
 	}
 	ip := cloudenv.Get().ResolverIP()

net/dnscache/dnscache_test.go

@@ -37,6 +37,78 @@ func TestDialer(t *testing.T) {
 	c.Close()
 }
 
+func TestPreferGoResolver(t *testing.T) {
+	// List of all known GOOS values as of Go 1.20.
+	//
+	// https://github.com/golang/go/blob/go1.20.3/src/go/build/syslist.go#L14-L33
+	testCases := []struct {
+		goos      string
+		preferred bool
+	}{{
+		goos:      "aix",
+		preferred: true,
+	}, {
+		goos:      "android",
+		preferred: false,
+	}, {
+		goos:      "darwin",
+		preferred: false,
+	}, {
+		goos:      "dragonfly",
+		preferred: true,
+	}, {
+		goos:      "freebsd",
+		preferred: true,
+	}, {
+		goos:      "hurd",
+		preferred: true,
+	}, {
+		goos:      "illumos",
+		preferred: true,
+	}, {
+		goos:      "ios",
+		preferred: false,
+	}, {
+		goos:      "js",
+		preferred: true,
+	}, {
+		goos:      "linux",
+		preferred: true,
+	}, {
+		goos:      "nacl",
+		preferred: true,
+	}, {
+		goos:      "netbsd",
+		preferred: true,
+	}, {
+		goos:      "openbsd",
+		preferred: true,
+	}, {
+		goos:      "plan9",
+		preferred: true,
+	}, {
+		goos:      "solaris",
+		preferred: true,
+	}, {
+		goos:      "wasip1",
+		preferred: true,
+	}, {
+		goos:      "windows",
+		preferred: true,
+	}, {
+		goos:      "zos",
+		preferred: true,
+	}}
+	for _, tc := range testCases {
+		t.Run(tc.goos, func(t *testing.T) {
+			preferred := preferGoResolver(tc.goos)
+			if preferred != tc.preferred {
+				t.Fatalf("%s: got %t, want %t", tc.goos, preferred, tc.preferred)
+			}
+		})
+	}
+}
+
 func TestDialCall_DNSWasTrustworthy(t *testing.T) {
 	type step struct {
 		ip  netip.Addr // IP we pretended to dial