types: leave the base domain out of search domains without MagicDNS

As SaaS. Alone, it left a client that writes resolv.conf with no nameserver.
Updates #2029

hscontrol/types/config.go

@@ -1427,7 +1427,10 @@ func dnsToTailcfgDNS(dns DNSConfig) *tailcfg.DNSConfig {
 	routes := dns.splitResolvers()
 
 	cfg.Routes = routes
-	if dns.BaseDomain != "" {
+	// Only MagicDNS answers for the base domain, and SaaS sends it only then. Sent
+	// alone, it left a client that writes resolv.conf itself with a search line
+	// and no nameserver.
+	if dns.MagicDNS && dns.BaseDomain != "" {
 		cfg.Domains = []string{dns.BaseDomain}
 	}
 

hscontrol/types/config_test.go

@@ -19,6 +19,7 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	hsdns "github.com/juanfont/headscale/hscontrol/dns"
+	"github.com/juanfont/headscale/hscontrol/types/testcapture"
 	"github.com/juanfont/headscale/hscontrol/util"
 	"github.com/libdns/cloudflare"
 	"github.com/rs/zerolog"
@@ -154,7 +155,7 @@ func TestReadConfig(t *testing.T) {
 			},
 			want: &tailcfg.DNSConfig{
 				Proxied: false,
-				Domains: []string{"example.com", "test.com", "bar.com"},
+				Domains: []string{"test.com", "bar.com"},
 				FallbackResolvers: []*dnstype.Resolver{
 					{Addr: "1.1.1.1"},
 					{Addr: "1.0.0.1"},
@@ -1827,3 +1828,24 @@ func TestLoadCLIConfigRejectsNonBoolean(t *testing.T) {
 		})
 	}
 }
+
+// TestDNSDomainsCompat: SaaS sends the tailnet domain as a search domain with
+// MagicDNS on and leaves it out with MagicDNS off.
+func TestDNSDomainsCompat(t *testing.T) {
+	for id, magicDNS := range map[string]bool{
+		"nodeattrs-nextdns-d1-profile":  false,
+		"nodeattrs-tailnet-magicdns-on": true,
+	} {
+		t.Run(id, func(t *testing.T) {
+			c, err := testcapture.Read(filepath.Join("..", "policy", "v2", "testdata", "nodeattrs_results", id+".hujson"))
+			require.NoError(t, err)
+
+			got := dnsToTailcfgDNS(DNSConfig{MagicDNS: magicDNS, BaseDomain: "example.com"})
+
+			for node, capture := range c.Captures {
+				require.NotNil(t, capture.Netmap, node)
+				assert.Equal(t, len(capture.Netmap.DNS.Domains) > 0, len(got.Domains) > 0, node)
+			}
+		})
+	}
+}