feature/conn25: rewrite A records for connector domains

When we are mapping a dns response, if it is a connector domain, change
the source IP addresses for our magic IP addresses. This will allow the
tailscaled to DNAT the traffic for the domain to the connector.

Updates tailscale/corp#34258
Signed-off-by: Fran Bull <fran@tailscale.com>

feature/conn25/conn25.go

@@ -17,6 +17,7 @@ import (
 	"net/http"
 	"net/netip"
 	"slices"
+	"strings"
 	"sync"
 
 	"go4.org/netipx"
@@ -55,6 +56,12 @@ func jsonDecode(target any, rc io.ReadCloser) error {
 	return err
 }
 
+func normalizeDNSName(name string) (dnsname.FQDN, error) {
+	// note that appconnector does this same thing, tsdns has its own custom lower casing
+	// it might be good to unify in a function in dnsname package.
+	return dnsname.ToFQDN(strings.ToLower(name))
+}
+
 func init() {
 	feature.Register(featureName)
 	ipnext.RegisterExtension(featureName, func(logf logger.Logf, sb ipnext.SafeBackend) (ipnext.Extension, error) {
@@ -438,7 +445,7 @@ func configFromNodeView(n tailcfg.NodeView) (config, error) {
 	for _, app := range apps {
 		selfMatchesTags := slices.ContainsFunc(app.Connectors, selfTags.Contains)
 		for _, d := range app.Domains {
-			fqdn, err := dnsname.ToFQDN(d)
+			fqdn, err := normalizeDNSName(d)
 			if err != nil {
 				return config{}, err
 			}
@@ -641,16 +648,81 @@ func (e *extension) sendAddressAssignment(ctx context.Context, as addrs) error {
 	return makePeerAPIReq(ctx, client, urlBase, as)
 }
 
+type dnsResponseRewrite struct {
+	domain dnsname.FQDN
+	dst    netip.Addr
+}
+
+func makeServFail(logf logger.Logf, h dnsmessage.Header, q dnsmessage.Question) []byte {
+	h.Response = true
+	h.Authoritative = true
+	h.RCode = dnsmessage.RCodeServerFailure
+	b := dnsmessage.NewBuilder(nil, h)
+	err := b.StartQuestions()
+	if err != nil {
+		logf("error making servfail: %v", err)
+		return []byte{}
+	}
+	err = b.Question(q)
+	if err != nil {
+		logf("error making servfail: %v", err)
+		return []byte{}
+	}
+	bs, err := b.Finish()
+	if err != nil {
+		// If there's an error here there's a bug somewhere directly above.
+		// _possibly_ some kind of question that was parseable but not encodable?,
+		// otherwise we could panic.
+		logf("error making servfail: %v", err)
+	}
+	return bs
+}
+
 func (c *client) mapDNSResponse(buf []byte) []byte {
 	var p dnsmessage.Parser
-	if _, err := p.Start(buf); err != nil {
+	hdr, err := p.Start(buf)
+	if err != nil {
 		c.logf("error parsing dns response: %v", err)
 		return buf
 	}
-	if err := p.SkipAllQuestions(); err != nil {
+	questions, err := p.AllQuestions()
+	if err != nil {
 		c.logf("error parsing dns response: %v", err)
 		return buf
 	}
+	// Any message we are interested in has one question (RFC 9619)
+	if len(questions) != 1 {
+		return buf
+	}
+	question := questions[0]
+	// The other Class types are not commonly used and supporting them hasn't been considered.
+	if question.Class != dnsmessage.ClassINET {
+		return buf
+	}
+	queriedDomain, err := normalizeDNSName(question.Name.String())
+	if err != nil {
+		return buf
+	}
+	if !c.isConnectorDomain(queriedDomain) {
+		return buf
+	}
+
+	// Now we know this is a dns response we think we should rewrite, we're going to provide our response which
+	// currently means we will:
+	//  * write the questions through as they are
+	//  * not send through the additional section
+	//  * provide our answers, or no answers if we don't handle those answers (possibly in the future we should write through answers for eg TypeTXT)
+	var answers []dnsResponseRewrite
+	if question.Type != dnsmessage.TypeA {
+		// we plan to support TypeAAAA soon (2026-03-11)
+		c.logf("mapping dns response for connector domain, unsupported type: %v", question.Type)
+		newBuf, err := c.rewriteDNSResponse(hdr, questions, answers)
+		if err != nil {
+			c.logf("error writing empty response for unsupported type: %v", err)
+			return makeServFail(c.logf, hdr, question)
+		}
+		return newBuf
+	}
 	for {
 		h, err := p.AnswerHeader()
 		if err == dnsmessage.ErrSectionDone {
@@ -658,57 +730,108 @@ func (c *client) mapDNSResponse(buf []byte) []byte {
 		}
 		if err != nil {
 			c.logf("error parsing dns response: %v", err)
-			return buf
+			return makeServFail(c.logf, hdr, question)
 		}
-
+		// other classes are unsupported, and we checked the question was for ClassINET already
 		if h.Class != dnsmessage.ClassINET {
+			c.logf("unexpected class for connector domain dns response: %v %v", queriedDomain, h.Class)
 			if err := p.SkipAnswer(); err != nil {
 				c.logf("error parsing dns response: %v", err)
-				return buf
+				return makeServFail(c.logf, hdr, question)
 			}
 			continue
 		}
-
 		switch h.Type {
+		case dnsmessage.TypeCNAME:
+			// An A record was asked for, and the answer is a CNAME, this answer will tell us which domain it's a CNAME for
+			// and a subsequent answer should tell us what the target domains address is (or possibly another CNAME). Drop
+			// this for now (2026-03-11) but in the near future we should collapse the CNAME chain and map to the ultimate
+			// destination address (see eg appc/{appconnector,observe}.go).
+			c.logf("not yet implemented CNAME answer: %v", queriedDomain)
+			if err := p.SkipAnswer(); err != nil {
+				c.logf("error parsing dns response: %v", err)
+				return makeServFail(c.logf, hdr, question)
+			}
 		case dnsmessage.TypeA:
-			domain, err := dnsname.ToFQDN(h.Name.String())
+			domain, err := normalizeDNSName(h.Name.String())
 			if err != nil {
 				c.logf("bad dnsname: %v", err)
-				return buf
+				return makeServFail(c.logf, hdr, question)
 			}
-			if !c.isConnectorDomain(domain) {
+			// answers should be for the domain that was queried
+			if domain != queriedDomain {
+				c.logf("unexpected domain for connector domain dns response: %v %v", queriedDomain, domain)
 				if err := p.SkipAnswer(); err != nil {
 					c.logf("error parsing dns response: %v", err)
-					return buf
+					return makeServFail(c.logf, hdr, question)
 				}
 				continue
 			}
 			r, err := p.AResource()
 			if err != nil {
 				c.logf("error parsing dns response: %v", err)
-				return buf
-			}
-			addrs, err := c.reserveAddresses(domain, netip.AddrFrom4(r.A))
-			if err != nil {
-				c.logf("error assigning connector addresses: %v", err)
-				return buf
-			}
-			if !addrs.isValid() {
-				c.logf("assigned connector addresses unexpectedly empty: %v", err)
-				return buf
+				return makeServFail(c.logf, hdr, question)
 			}
+			answers = append(answers, dnsResponseRewrite{domain: domain, dst: netip.AddrFrom4(r.A)})
 		default:
+			// we already checked the question was for a supported type, this answer is unexpected
+			c.logf("unexpected type for connector domain dns response: %v %v", queriedDomain, h.Type)
 			if err := p.SkipAnswer(); err != nil {
 				c.logf("error parsing dns response: %v", err)
-				return buf
+				return makeServFail(c.logf, hdr, question)
 			}
-			continue
 		}
 	}
+	newBuf, err := c.rewriteDNSResponse(hdr, questions, answers)
+	if err != nil {
+		c.logf("error rewriting dns response: %v", err)
+		return makeServFail(c.logf, hdr, question)
+	}
+	return newBuf
+}
 
-	// TODO(fran) 2026-01-21 return a dns response with addresses
-	// swapped out for the magic IPs to make conn25 work.
-	return buf
+func (c *client) rewriteDNSResponse(hdr dnsmessage.Header, questions []dnsmessage.Question, answers []dnsResponseRewrite) ([]byte, error) {
+	// We are currently (2026-03-10) only doing this for AResource records, we know that if we are here
+	// with non-empty answers, the type was AResource.
+	b := dnsmessage.NewBuilder(nil, hdr)
+	b.EnableCompression()
+	if err := b.StartQuestions(); err != nil {
+		return nil, err
+	}
+	for _, q := range questions {
+		if err := b.Question(q); err != nil {
+			return nil, err
+		}
+	}
+	if err := b.StartAnswers(); err != nil {
+		return nil, err
+	}
+	// make an answer for each rewrite
+	for _, rw := range answers {
+		as, err := c.reserveAddresses(rw.domain, rw.dst)
+		if err != nil {
+			return nil, err
+		}
+		if !as.isValid() {
+			return nil, errors.New("connector addresses empty")
+		}
+		name, err := dnsmessage.NewName(rw.domain.WithTrailingDot())
+		if err != nil {
+			return nil, err
+		}
+		// only handling TypeA right now
+		rhdr := dnsmessage.ResourceHeader{Name: name, Type: dnsmessage.TypeA, Class: dnsmessage.ClassINET, TTL: 0}
+		if err := b.AResource(rhdr, dnsmessage.AResource{A: as.magic.As4()}); err != nil {
+			return nil, err
+		}
+	}
+	// We do _not_ include the additional section in our rewrite. (We don't want to include
+	// eg DNSSEC info, or other extra info like related records).
+	out, err := b.Finish()
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
 }
 
 type connector struct {

feature/conn25/conn25_test.go

@@ -8,7 +8,6 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/netip"
-	"reflect"
 	"testing"
 	"time"
 
@@ -547,61 +546,139 @@ func rangeFrom(from, to string) netipx.IPRange {
 	)
 }
 
-func TestMapDNSResponse(t *testing.T) {
-	makeDNSResponse := func(domain string, addrs []dnsmessage.AResource) []byte {
-		b := dnsmessage.NewBuilder(nil,
-			dnsmessage.Header{
-				ID:            1,
-				Response:      true,
-				Authoritative: true,
-				RCode:         dnsmessage.RCodeSuccess,
-			})
-		b.EnableCompression()
-
-		if err := b.StartQuestions(); err != nil {
-			t.Fatal(err)
+func makeDNSResponse(t *testing.T, domain string, addrs []*dnsmessage.AResource) []byte {
+	t.Helper()
+	name := dnsmessage.MustNewName(domain)
+	questions := []dnsmessage.Question{
+		{
+			Name:  name,
+			Type:  dnsmessage.TypeA,
+			Class: dnsmessage.ClassINET,
+		},
+	}
+	var answers []dnsmessage.Resource
+	for _, addr := range addrs {
+		ans := dnsmessage.Resource{
+			Header: dnsmessage.ResourceHeader{
+				Name:  name,
+				Type:  dnsmessage.TypeA,
+				Class: dnsmessage.ClassINET,
+			},
+			Body: addr,
 		}
+		answers = append(answers, ans)
+	}
+	additional := []dnsmessage.Resource{
+		{
+			Header: dnsmessage.ResourceHeader{
+				Name:  name,
+				Type:  dnsmessage.TypeA,
+				Class: dnsmessage.ClassINET,
+			},
+			Body: &dnsmessage.AResource{A: [4]byte{9, 9, 9, 9}},
+		},
+	}
+	return makeDNSResponseForSections(t, questions, answers, additional)
+}
 
-		if err := b.Question(dnsmessage.Question{
-			Name:  dnsmessage.MustNewName(domain),
-			Type:  dnsmessage.TypeA,
+func makeV6DNSResponse(t *testing.T, domain string, addrs []*dnsmessage.AAAAResource) []byte {
+	t.Helper()
+	name := dnsmessage.MustNewName(domain)
+	questions := []dnsmessage.Question{
+		{
+			Name:  name,
+			Type:  dnsmessage.TypeAAAA,
 			Class: dnsmessage.ClassINET,
-		}); err != nil {
-			t.Fatal(err)
+		},
+	}
+	var answers []dnsmessage.Resource
+	for _, addr := range addrs {
+		ans := dnsmessage.Resource{
+			Header: dnsmessage.ResourceHeader{
+				Name:  name,
+				Type:  dnsmessage.TypeAAAA,
+				Class: dnsmessage.ClassINET,
+			},
+			Body: addr,
 		}
+		answers = append(answers, ans)
+	}
+	return makeDNSResponseForSections(t, questions, answers, nil)
+}
+
+func makeDNSResponseForSections(t *testing.T, questions []dnsmessage.Question, answers []dnsmessage.Resource, additional []dnsmessage.Resource) []byte {
+	t.Helper()
+	b := dnsmessage.NewBuilder(nil,
+		dnsmessage.Header{
+			ID:            1,
+			Response:      true,
+			Authoritative: true,
+			RCode:         dnsmessage.RCodeSuccess,
+		})
+	b.EnableCompression()
+
+	if err := b.StartQuestions(); err != nil {
+		t.Fatal(err)
+	}
 
-		if err := b.StartAnswers(); err != nil {
+	for _, q := range questions {
+		if err := b.Question(q); err != nil {
 			t.Fatal(err)
 		}
+	}
 
-		for _, addr := range addrs {
-			b.AResource(
-				dnsmessage.ResourceHeader{
-					Name:  dnsmessage.MustNewName(domain),
-					Type:  dnsmessage.TypeA,
-					Class: dnsmessage.ClassINET,
-				},
-				addr,
-			)
+	if err := b.StartAnswers(); err != nil {
+		t.Fatal(err)
+	}
+
+	for _, ans := range answers {
+		switch ans.Header.Type {
+		case dnsmessage.TypeA:
+			body, ok := (ans.Body).(*dnsmessage.AResource)
+			if !ok {
+				t.Fatalf("unexpected answer type, update test")
+			}
+			b.AResource(ans.Header, *body)
+		case dnsmessage.TypeAAAA:
+			body, ok := (ans.Body).(*dnsmessage.AAAAResource)
+			if !ok {
+				t.Fatalf("unexpected answer type, update test")
+			}
+			b.AAAAResource(ans.Header, *body)
+		default:
+			t.Fatalf("unhandled answer type, update test: %v", ans.Header.Type)
 		}
+	}
 
-		outbs, err := b.Finish()
-		if err != nil {
-			t.Fatal(err)
+	if err := b.StartAdditionals(); err != nil {
+		t.Fatal(err)
+	}
+	for _, add := range additional {
+		body, ok := (add.Body).(*dnsmessage.AResource)
+		if !ok {
+			t.Fatalf("unexpected additional type, update test")
 		}
-		return outbs
+		b.AResource(add.Header, *body)
 	}
 
+	outbs, err := b.Finish()
+	if err != nil {
+		t.Fatal(err)
+	}
+	return outbs
+}
+
+func TestMapDNSResponseAssignsAddrs(t *testing.T) {
 	for _, tt := range []struct {
 		name          string
 		domain        string
-		addrs         []dnsmessage.AResource
+		addrs         []*dnsmessage.AResource
 		wantByMagicIP map[netip.Addr]addrs
 	}{
 		{
 			name:   "one-ip-matches",
 			domain: "example.com.",
-			addrs:  []dnsmessage.AResource{{A: [4]byte{1, 0, 0, 0}}},
+			addrs:  []*dnsmessage.AResource{{A: [4]byte{1, 0, 0, 0}}},
 			// these are 'expected' because they are the beginning of the provided pools
 			wantByMagicIP: map[netip.Addr]addrs{
 				netip.MustParseAddr("100.64.0.0"): {
@@ -616,7 +693,7 @@ func TestMapDNSResponse(t *testing.T) {
 		{
 			name:   "multiple-ip-matches",
 			domain: "example.com.",
-			addrs: []dnsmessage.AResource{
+			addrs: []*dnsmessage.AResource{
 				{A: [4]byte{1, 0, 0, 0}},
 				{A: [4]byte{2, 0, 0, 0}},
 			},
@@ -640,14 +717,14 @@ func TestMapDNSResponse(t *testing.T) {
 		{
 			name:   "no-domain-match",
 			domain: "x.example.com.",
-			addrs: []dnsmessage.AResource{
+			addrs: []*dnsmessage.AResource{
 				{A: [4]byte{1, 0, 0, 0}},
 				{A: [4]byte{2, 0, 0, 0}},
 			},
 		},
 	} {
 		t.Run(tt.name, func(t *testing.T) {
-			dnsResp := makeDNSResponse(tt.domain, tt.addrs)
+			dnsResp := makeDNSResponse(t, tt.domain, tt.addrs)
 			sn := makeSelfNode(t, appctype.Conn25Attr{
 				Name:          "app1",
 				Connectors:    []string{"tag:woo"},
@@ -658,10 +735,7 @@ func TestMapDNSResponse(t *testing.T) {
 			c := newConn25(logger.Discard)
 			c.reconfig(sn)
 
-			bs := c.mapDNSResponse(dnsResp)
-			if !reflect.DeepEqual(dnsResp, bs) {
-				t.Fatal("shouldn't be changing the bytes (yet)")
-			}
+			c.mapDNSResponse(dnsResp)
 			if diff := cmp.Diff(tt.wantByMagicIP, c.client.assignments.byMagicIP, cmpopts.EquateComparable(addrs{}, netip.Addr{})); diff != "" {
 				t.Errorf("byMagicIP diff (-want, +got):\n%s", diff)
 			}
@@ -821,3 +895,291 @@ func TestEnqueueAddress(t *testing.T) {
 		t.Fatal("timed out waiting for connector to receive request")
 	}
 }
+
+func parseResponse(t *testing.T, buf []byte) ([]dnsmessage.Resource, []dnsmessage.Resource) {
+	t.Helper()
+	var p dnsmessage.Parser
+	header, err := p.Start(buf)
+	if err != nil {
+		t.Fatalf("parsing DNS response: %v", err)
+	}
+	if header.RCode != dnsmessage.RCodeSuccess {
+		t.Fatalf("RCode want: %v, got: %v", dnsmessage.RCodeSuccess, header.RCode)
+	}
+	if err := p.SkipAllQuestions(); err != nil {
+		t.Fatalf("skipping questions: %v", err)
+	}
+	answers, err := p.AllAnswers()
+	if err != nil {
+		t.Fatalf("reading answers: %v", err)
+	}
+	if err := p.SkipAllAuthorities(); err != nil {
+		t.Fatalf("skipping questions: %v", err)
+	}
+	additionals, err := p.AllAdditionals()
+	if err != nil {
+		t.Fatalf("reading additionals: %v", err)
+	}
+	return answers, additionals
+}
+
+func TestMapDNSResponseRewritesResponses(t *testing.T) {
+	configuredDomain := "example.com"
+	domainName := configuredDomain + "."
+	dnsMessageName := dnsmessage.MustNewName(domainName)
+	sn := makeSelfNode(t, appctype.Conn25Attr{
+		Name:          "app1",
+		Connectors:    []string{"tag:connector"},
+		Domains:       []string{configuredDomain},
+		MagicIPPool:   []netipx.IPRange{rangeFrom("0", "10")},
+		TransitIPPool: []netipx.IPRange{rangeFrom("40", "50")},
+	}, []string{})
+
+	compareToRecords := func(t *testing.T, resources []dnsmessage.Resource, want []netip.Addr) {
+		t.Helper()
+		var got []netip.Addr
+		for _, r := range resources {
+			if b, ok := r.Body.(*dnsmessage.AResource); ok {
+				got = append(got, netip.AddrFrom4(b.A))
+			} else if b, ok := r.Body.(*dnsmessage.AAAAResource); ok {
+				got = append(got, netip.AddrFrom16(b.AAAA))
+			}
+		}
+		if diff := cmp.Diff(want, got, cmpopts.EquateComparable(netip.Addr{})); diff != "" {
+			t.Fatalf("A/AAAA records mismatch (-want +got):\n%s", diff)
+		}
+	}
+
+	assertParsesToAnswers := func(want []netip.Addr) func(t *testing.T, bs []byte) {
+		return func(t *testing.T, bs []byte) {
+			t.Helper()
+			answers, _ := parseResponse(t, bs)
+			compareToRecords(t, answers, want)
+		}
+	}
+
+	assertParsesToAdditionals := func(want []netip.Addr) func(t *testing.T, bs []byte) {
+		return func(t *testing.T, bs []byte) {
+			t.Helper()
+			_, additionals := parseResponse(t, bs)
+			compareToRecords(t, additionals, want)
+		}
+	}
+
+	assertBytes := func(want []byte) func(t *testing.T, bs []byte) {
+		return func(t *testing.T, bs []byte) {
+			t.Helper()
+			if diff := cmp.Diff(want, bs); diff != "" {
+				t.Fatalf("bytes mismatch (-want +got):\n%s", diff)
+			}
+		}
+	}
+	assertServFail := func(t *testing.T, bs []byte) {
+		var p dnsmessage.Parser
+		header, err := p.Start(bs)
+		if err != nil {
+			t.Fatalf("parsing DNS response: %v", err)
+		}
+		if header.RCode != dnsmessage.RCodeServerFailure {
+			t.Fatalf("RCode want: %v, got: %v", dnsmessage.RCodeServerFailure, header.RCode)
+		}
+	}
+
+	ipv6ResponseUnhandledDomain := makeV6DNSResponse(t, "tailscale.com.", []*dnsmessage.AAAAResource{
+		{AAAA: netip.MustParseAddr("2606:4700::6812:1a78").As16()},
+		{AAAA: netip.MustParseAddr("2606:4700::6812:1b78").As16()},
+	})
+
+	ipv4ResponseUnhandledDomain := makeDNSResponse(t, "tailscale.com.", []*dnsmessage.AResource{
+		{A: netip.MustParseAddr("1.2.3.4").As4()},
+		{A: netip.MustParseAddr("5.6.7.8").As4()},
+	})
+
+	nonINETQuestionResp := makeDNSResponseForSections(t, []dnsmessage.Question{
+		{
+			Name:  dnsMessageName,
+			Type:  dnsmessage.TypeA,
+			Class: dnsmessage.ClassCHAOS,
+		},
+	}, nil, nil)
+
+	for _, tt := range []struct {
+		name     string
+		toMap    []byte
+		assertFx func(*testing.T, []byte)
+	}{
+		{
+			name:     "unparseable",
+			toMap:    []byte{1, 2, 3, 4},
+			assertFx: assertBytes([]byte{1, 2, 3, 4}),
+		},
+		{
+			name: "maps-multi-typea-answers",
+			toMap: makeDNSResponse(t, domainName, []*dnsmessage.AResource{
+				{A: netip.MustParseAddr("1.2.3.4").As4()},
+				{A: netip.MustParseAddr("5.6.7.8").As4()},
+			}),
+			assertFx: assertParsesToAnswers(
+				[]netip.Addr{
+					netip.MustParseAddr("100.64.0.0"),
+					netip.MustParseAddr("100.64.0.1"),
+				},
+			),
+		},
+		{
+			name: "ipv6-no-answers",
+			toMap: makeV6DNSResponse(t, domainName, []*dnsmessage.AAAAResource{
+				{AAAA: netip.MustParseAddr("2606:4700::6812:1a78").As16()},
+				{AAAA: netip.MustParseAddr("2606:4700::6812:1b78").As16()},
+			}),
+			assertFx: assertParsesToAnswers(nil),
+		},
+		{
+			name:     "not-our-domain",
+			toMap:    ipv4ResponseUnhandledDomain,
+			assertFx: assertBytes(ipv4ResponseUnhandledDomain),
+		},
+		{
+			name:     "ipv6-not-our-domain",
+			toMap:    ipv6ResponseUnhandledDomain,
+			assertFx: assertBytes(ipv6ResponseUnhandledDomain),
+		},
+		{
+			name: "case-insensitive",
+			toMap: makeDNSResponse(t, "eXample.com.", []*dnsmessage.AResource{
+				{A: netip.MustParseAddr("1.2.3.4").As4()},
+				{A: netip.MustParseAddr("5.6.7.8").As4()},
+			}),
+			assertFx: assertParsesToAnswers(
+				[]netip.Addr{
+					netip.MustParseAddr("100.64.0.0"),
+					netip.MustParseAddr("100.64.0.1"),
+				},
+			),
+		},
+		{
+			name: "unhandled-keeps-additional-section",
+			toMap: makeDNSResponse(t, "tailscale.com.", []*dnsmessage.AResource{
+				{A: netip.MustParseAddr("1.2.3.4").As4()},
+				{A: netip.MustParseAddr("5.6.7.8").As4()},
+			}),
+			assertFx: assertParsesToAdditionals(
+				// additionals are added in makeDNSResponse
+				[]netip.Addr{
+					netip.MustParseAddr("9.9.9.9"),
+				},
+			),
+		},
+		{
+			name: "handled-strips-additional-section",
+			toMap: makeDNSResponse(t, domainName, []*dnsmessage.AResource{
+				{A: netip.MustParseAddr("1.2.3.4").As4()},
+				{A: netip.MustParseAddr("5.6.7.8").As4()},
+			}),
+			assertFx: assertParsesToAdditionals(nil),
+		},
+		{
+			name: "servfail-when-we-should-handle-but-cant",
+			// produced by
+			// makeDNSResponse(t, domainName, []*dnsmessage.AResource{{A: netip.MustParseAddr("1.2.3.4").As4()}})
+			// and then taking 17 bytes off the end. So that the parsing of it breaks after we have decided we should handle it.
+			// Frozen like this so that it doesn't depend on the implementation of dnsmessage.
+			toMap:    []byte{0, 1, 132, 0, 0, 1, 0, 1, 0, 0, 0, 1, 7, 101, 120, 97, 109, 112, 108, 101, 3, 99, 111, 109, 0, 0, 1, 0, 1, 192, 12, 0, 1, 0, 1, 0, 0, 0, 0, 0, 4, 1, 2, 3},
+			assertFx: assertServFail,
+		},
+		{
+			name:     "not-inet-question",
+			toMap:    nonINETQuestionResp,
+			assertFx: assertBytes(nonINETQuestionResp),
+		},
+		{
+			name: "not-inet-answer",
+			toMap: makeDNSResponseForSections(t,
+				[]dnsmessage.Question{
+					{
+						Name:  dnsMessageName,
+						Type:  dnsmessage.TypeA,
+						Class: dnsmessage.ClassINET,
+					},
+				},
+				[]dnsmessage.Resource{
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:  dnsMessageName,
+							Type:  dnsmessage.TypeA,
+							Class: dnsmessage.ClassCHAOS,
+						},
+						Body: &dnsmessage.AResource{A: netip.MustParseAddr("1.2.3.4").As4()},
+					},
+				},
+				nil,
+			),
+			assertFx: assertParsesToAnswers(nil),
+		},
+		{
+			name: "answer-domain-mismatch",
+			toMap: makeDNSResponseForSections(t,
+				[]dnsmessage.Question{
+					{
+						Name:  dnsMessageName,
+						Type:  dnsmessage.TypeA,
+						Class: dnsmessage.ClassINET,
+					},
+				},
+				[]dnsmessage.Resource{
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:  dnsmessage.MustNewName("tailscale.com."),
+							Type:  dnsmessage.TypeA,
+							Class: dnsmessage.ClassINET,
+						},
+						Body: &dnsmessage.AResource{A: netip.MustParseAddr("1.2.3.4").As4()},
+					},
+				},
+				nil,
+			),
+			assertFx: assertParsesToAnswers(nil),
+		},
+		{
+			name: "answer-type-mismatch",
+			toMap: makeDNSResponseForSections(t,
+				[]dnsmessage.Question{
+					{
+						Name:  dnsMessageName,
+						Type:  dnsmessage.TypeA,
+						Class: dnsmessage.ClassINET,
+					},
+				},
+				[]dnsmessage.Resource{
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:  dnsMessageName,
+							Type:  dnsmessage.TypeAAAA,
+							Class: dnsmessage.ClassINET,
+						},
+						Body: &dnsmessage.AAAAResource{AAAA: netip.MustParseAddr("1.2.3.4").As16()},
+					},
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:  dnsMessageName,
+							Type:  dnsmessage.TypeA,
+							Class: dnsmessage.ClassINET,
+						},
+						Body: &dnsmessage.AResource{A: netip.MustParseAddr("5.6.7.8").As4()},
+					},
+				},
+				nil,
+			),
+			assertFx: assertParsesToAnswers([]netip.Addr{netip.MustParseAddr("100.64.0.0")}),
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			c := newConn25(logger.Discard)
+			if err := c.reconfig(sn); err != nil {
+				t.Fatal(err)
+			}
+			bs := c.mapDNSResponse(tt.toMap)
+			tt.assertFx(t, bs)
+		})
+	}
+}