[client] Distinguish between NXDOMAIN and NODATA in the dns forwarder (#4321)

Author: lixmal

client/internal/dnsfwd/forwarder.go

@@ -165,7 +165,7 @@ func (f *DNSForwarder) handleDNSQuery(w dns.ResponseWriter, query *dns.Msg) *dns
 	defer cancel()
 	ips, err := f.resolver.LookupNetIP(ctx, network, domain)
 	if err != nil {
-		f.handleDNSError(w, query, resp, domain, err)
+		f.handleDNSError(ctx, w, question, resp, domain, err)
 		return nil
 	}
 
@@ -244,20 +244,57 @@ func (f *DNSForwarder) updateFirewall(matchingEntries []*ForwarderEntry, prefixe
 	}
 }
 
+// setResponseCodeForNotFound determines and sets the appropriate response code when IsNotFound is true
+// It distinguishes between NXDOMAIN (domain doesn't exist) and NODATA (domain exists but no records of requested type)
+//
+// LIMITATION: This function only checks A and AAAA record types to determine domain existence.
+// If a domain has only other record types (MX, TXT, CNAME, etc.) but no A/AAAA records,
+// it may incorrectly return NXDOMAIN instead of NODATA. This is acceptable since the forwarder
+// only handles A/AAAA queries and returns NOTIMP for other types.
+func (f *DNSForwarder) setResponseCodeForNotFound(ctx context.Context, resp *dns.Msg, domain string, originalQtype uint16) {
+	// Try querying for a different record type to see if the domain exists
+	// If the original query was for AAAA, try A. If it was for A, try AAAA.
+	// This helps distinguish between NXDOMAIN and NODATA.
+	var alternativeNetwork string
+	switch originalQtype {
+	case dns.TypeAAAA:
+		alternativeNetwork = "ip4"
+	case dns.TypeA:
+		alternativeNetwork = "ip6"
+	default:
+		resp.Rcode = dns.RcodeNameError
+		return
+	}
+
+	if _, err := f.resolver.LookupNetIP(ctx, alternativeNetwork, domain); err != nil {
+		var dnsErr *net.DNSError
+		if errors.As(err, &dnsErr) && dnsErr.IsNotFound {
+			// Alternative query also returned not found - domain truly doesn't exist
+			resp.Rcode = dns.RcodeNameError
+			return
+		}
+		// Some other error (timeout, server failure, etc.) - can't determine, assume domain exists
+		resp.Rcode = dns.RcodeSuccess
+		return
+	}
+
+	// Alternative query succeeded - domain exists but has no records of this type
+	resp.Rcode = dns.RcodeSuccess
+}
+
 // handleDNSError processes DNS lookup errors and sends an appropriate error response
-func (f *DNSForwarder) handleDNSError(w dns.ResponseWriter, query, resp *dns.Msg, domain string, err error) {
+func (f *DNSForwarder) handleDNSError(ctx context.Context, w dns.ResponseWriter, question dns.Question, resp *dns.Msg, domain string, err error) {
 	var dnsErr *net.DNSError
 
 	switch {
 	case errors.As(err, &dnsErr):
 		resp.Rcode = dns.RcodeServerFailure
 		if dnsErr.IsNotFound {
-			// Pass through NXDOMAIN
-			resp.Rcode = dns.RcodeNameError
+			f.setResponseCodeForNotFound(ctx, resp, domain, question.Qtype)
 		}
 
 		if dnsErr.Server != "" {
-			log.Warnf("failed to resolve query for type=%s domain=%s server=%s: %v", dns.TypeToString[query.Question[0].Qtype], domain, dnsErr.Server, err)
+			log.Warnf("failed to resolve query for type=%s domain=%s server=%s: %v", dns.TypeToString[question.Qtype], domain, dnsErr.Server, err)
 		} else {
 			log.Warnf(errResolveFailed, domain, err)
 		}

client/internal/dnsfwd/forwarder_test.go

@@ -3,6 +3,7 @@ package dnsfwd
 import (
 	"context"
 	"fmt"
+	"net"
 	"net/netip"
 	"strings"
 	"testing"
@@ -16,8 +17,8 @@ import (
 	firewall "github.com/netbirdio/netbird/client/firewall/manager"
 	"github.com/netbirdio/netbird/client/internal/dns/test"
 	"github.com/netbirdio/netbird/client/internal/peer"
-	"github.com/netbirdio/netbird/shared/management/domain"
 	"github.com/netbirdio/netbird/route"
+	"github.com/netbirdio/netbird/shared/management/domain"
 )
 
 func Test_getMatchingEntries(t *testing.T) {
@@ -708,6 +709,131 @@ func TestDNSForwarder_MultipleOverlappingPatterns(t *testing.T) {
 	assert.Len(t, matches, 3, "Should match 3 patterns")
 }
 
+// TestDNSForwarder_NodataVsNxdomain tests that the forwarder correctly distinguishes
+// between NXDOMAIN (domain doesn't exist) and NODATA (domain exists but no records of that type)
+func TestDNSForwarder_NodataVsNxdomain(t *testing.T) {
+	mockFirewall := &MockFirewall{}
+	mockResolver := &MockResolver{}
+
+	forwarder := NewDNSForwarder("127.0.0.1:0", 300, mockFirewall, &peer.Status{})
+	forwarder.resolver = mockResolver
+
+	d, err := domain.FromString("example.com")
+	require.NoError(t, err)
+
+	set := firewall.NewDomainSet([]domain.Domain{d})
+	entries := []*ForwarderEntry{{Domain: d, ResID: "test-res", Set: set}}
+	forwarder.UpdateDomains(entries)
+
+	tests := []struct {
+		name           string
+		queryType      uint16
+		setupMocks     func()
+		expectedCode   int
+		expectNoAnswer bool // true if we expect NOERROR with empty answer (NODATA case)
+		description    string
+	}{
+		{
+			name:      "domain exists but no AAAA records (NODATA)",
+			queryType: dns.TypeAAAA,
+			setupMocks: func() {
+				// First query for AAAA returns not found
+				mockResolver.On("LookupNetIP", mock.Anything, "ip6", "example.com.").
+					Return([]netip.Addr{}, &net.DNSError{IsNotFound: true, Name: "example.com"}).Once()
+				// Check query for A records succeeds (domain exists)
+				mockResolver.On("LookupNetIP", mock.Anything, "ip4", "example.com.").
+					Return([]netip.Addr{netip.MustParseAddr("1.2.3.4")}, nil).Once()
+			},
+			expectedCode:   dns.RcodeSuccess,
+			expectNoAnswer: true,
+			description:    "Should return NOERROR when domain exists but has no records of requested type",
+		},
+		{
+			name:      "domain exists but no A records (NODATA)",
+			queryType: dns.TypeA,
+			setupMocks: func() {
+				// First query for A returns not found
+				mockResolver.On("LookupNetIP", mock.Anything, "ip4", "example.com.").
+					Return([]netip.Addr{}, &net.DNSError{IsNotFound: true, Name: "example.com"}).Once()
+				// Check query for AAAA records succeeds (domain exists)
+				mockResolver.On("LookupNetIP", mock.Anything, "ip6", "example.com.").
+					Return([]netip.Addr{netip.MustParseAddr("2001:db8::1")}, nil).Once()
+			},
+			expectedCode:   dns.RcodeSuccess,
+			expectNoAnswer: true,
+			description:    "Should return NOERROR when domain exists but has no A records",
+		},
+		{
+			name:      "domain doesn't exist (NXDOMAIN)",
+			queryType: dns.TypeA,
+			setupMocks: func() {
+				// First query for A returns not found
+				mockResolver.On("LookupNetIP", mock.Anything, "ip4", "example.com.").
+					Return([]netip.Addr{}, &net.DNSError{IsNotFound: true, Name: "example.com"}).Once()
+				// Check query for AAAA also returns not found (domain doesn't exist)
+				mockResolver.On("LookupNetIP", mock.Anything, "ip6", "example.com.").
+					Return([]netip.Addr{}, &net.DNSError{IsNotFound: true, Name: "example.com"}).Once()
+			},
+			expectedCode:   dns.RcodeNameError,
+			expectNoAnswer: true,
+			description:    "Should return NXDOMAIN when domain doesn't exist at all",
+		},
+		{
+			name:      "domain exists with records (normal success)",
+			queryType: dns.TypeA,
+			setupMocks: func() {
+				mockResolver.On("LookupNetIP", mock.Anything, "ip4", "example.com.").
+					Return([]netip.Addr{netip.MustParseAddr("1.2.3.4")}, nil).Once()
+				// Expect firewall update for successful resolution
+				expectedPrefix := netip.PrefixFrom(netip.MustParseAddr("1.2.3.4"), 32)
+				mockFirewall.On("UpdateSet", set, []netip.Prefix{expectedPrefix}).Return(nil).Once()
+			},
+			expectedCode:   dns.RcodeSuccess,
+			expectNoAnswer: false,
+			description:    "Should return NOERROR with answer when records exist",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Reset mock expectations
+			mockResolver.ExpectedCalls = nil
+			mockResolver.Calls = nil
+			mockFirewall.ExpectedCalls = nil
+			mockFirewall.Calls = nil
+
+			tt.setupMocks()
+
+			query := &dns.Msg{}
+			query.SetQuestion(dns.Fqdn("example.com"), tt.queryType)
+
+			var writtenResp *dns.Msg
+			mockWriter := &test.MockResponseWriter{
+				WriteMsgFunc: func(m *dns.Msg) error {
+					writtenResp = m
+					return nil
+				},
+			}
+
+			resp := forwarder.handleDNSQuery(mockWriter, query)
+
+			// If a response was returned, it means it should be written (happens in wrapper functions)
+			if resp != nil && writtenResp == nil {
+				writtenResp = resp
+			}
+
+			require.NotNil(t, writtenResp, "Expected response to be written")
+			assert.Equal(t, tt.expectedCode, writtenResp.Rcode, tt.description)
+
+			if tt.expectNoAnswer {
+				assert.Empty(t, writtenResp.Answer, "Response should have no answer records")
+			}
+
+			mockResolver.AssertExpectations(t)
+		})
+	}
+}
+
 func TestDNSForwarder_EmptyQuery(t *testing.T) {
 	// Test handling of malformed query with no questions
 	forwarder := NewDNSForwarder("127.0.0.1:0", 300, nil, &peer.Status{})