cmd/cli: split-route SRV record to OS resolver

Since SRV record is mostly useful in AD environment. Even in non-AD one,
the OS resolver could still resolve the query for external services.

Users who want special treatment can still specify domain rules to
forward requests to ControlD upstreams explicitly.

cmd/cli/dns_proxy.go

@@ -445,6 +445,10 @@ func (p *prog) proxy(ctx context.Context, req *proxyRequest) *proxyResponse {
 		}
 	} else {
 		switch {
+		case isSrvLookup(req.msg):
+			upstreams = []string{upstreamOS}
+			upstreamConfigs = []*ctrld.UpstreamConfig{osUpstreamConfig}
+			ctrld.Log(ctx, mainLog.Load().Debug(), "SRV record lookup, using upstreams: %v", upstreams)
 		case isPrivatePtrLookup(req.msg):
 			isLanOrPtrQuery = true
 			if answer := p.proxyPrivatePtrLookup(ctx, req.msg); answer != nil {
@@ -1059,6 +1063,14 @@ func isLanHostnameQuery(m *dns.Msg) bool {
 		strings.HasSuffix(name, ".lan")
 }
 
+// isSrvLookup reports whether DNS message is a SRV query.
+func isSrvLookup(m *dns.Msg) bool {
+	if m == nil || len(m.Question) == 0 {
+		return false
+	}
+	return m.Question[0].Qtype == dns.TypeSRV
+}
+
 // isWanClient reports whether the input is a WAN address.
 func isWanClient(na net.Addr) bool {
 	var ip netip.Addr

cmd/cli/dns_proxy_test.go

@@ -414,6 +414,26 @@ func Test_isPrivatePtrLookup(t *testing.T) {
 	}
 }
 
+func Test_isSrvLookup(t *testing.T) {
+	tests := []struct {
+		name        string
+		msg         *dns.Msg
+		isSrvLookup bool
+	}{
+		{"SRV", newDnsMsgWithHostname("foo", dns.TypeSRV), true},
+		{"Not SRV", newDnsMsgWithHostname("foo", dns.TypeNone), false},
+	}
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			if got := isSrvLookup(tc.msg); tc.isSrvLookup != got {
+				t.Errorf("unexpected result, want: %v, got: %v", tc.isSrvLookup, got)
+			}
+		})
+	}
+}
+
 func Test_isWanClient(t *testing.T) {
 	tests := []struct {
 		name        string