feature/conn25: skip AAAA records with IPv4-in-IPv6 addresses

Skip destination IPs from AAAA records that are
in ipv4-in-ipv6 format.

Fixes https://github.com/tailscale/corp/issues/48781

Change-Id: I5eeaa9c998c79250d2e6a943fa9f26ed7feb5ec6
Signed-off-by: Mazdak Nasab <mazdak.nasab@gmail.com>

feature/conn25/conn25.go

@@ -1250,6 +1250,12 @@ var (
 	metricDNSResponseRewriteUnsupportedQuestionTypeErrorServfail = clientmetric.NewCounter(
 		"conn25_map_dns_response_rewrite_unsupported_question_type_error_servfail",
 	)
+
+	// metricDNSResponseSkippedAAAA4In6 increments when an AAAA answer for an
+	// app connector domain is dropped because it holds an IPv4-in-IPv6 address.
+	metricDNSResponseSkippedAAAA4In6 = clientmetric.NewCounter(
+		"conn25_map_dns_response_skipped_aaaa_4in6",
+	)
 )
 
 // mapDNSResponse parses and inspects the DNS response. If the domain
@@ -1418,6 +1424,12 @@ func (c *Conn25) mapDNSResponse(buf []byte) []byte {
 					return makeServFail(c.logf, hdr, question)
 				}
 				dstAddr = netip.AddrFrom16(r.AAAA)
+
+				// Skip AAAA answer with IPv4-in-IPv6 address.
+				if dstAddr.Is4In6() {
+					metricDNSResponseSkippedAAAA4In6.Add(1)
+					continue
+				}
 			}
 			answers = append(answers, dnsResponseRewrite{domain: queriedDomain, dst: dstAddr, ttlSeconds: h.TTL})
 		default:

feature/conn25/conn25_test.go

@@ -1151,6 +1151,24 @@ func TestMapDNSResponseAssignsAddrs(t *testing.T) {
 				},
 			},
 		},
+		{
+			name:       "v6-ip-4in6-skipped",
+			appDomains: []string{"example.com"},
+			domain:     "example.com.",
+			v6Addrs: []*dnsmessage.AAAAResource{
+				{AAAA: netip.MustParseAddr("::ffff:1.0.0.1").As16()},
+				{AAAA: netip.MustParseAddr("::1").As16()},
+			},
+			wantByMagicIP: map[netip.Addr]*addrs{
+				netip.MustParseAddr("fd7a:115c:a1e0:a99c::"): {
+					domain:  "example.com.",
+					dst:     netip.MustParseAddr("::1"),
+					magic:   netip.MustParseAddr("fd7a:115c:a1e0:a99c:0::"),
+					transit: netip.MustParseAddr("fd7a:115c:a1e0:a99c:40::"),
+					app:     "app1",
+				},
+			},
+		},
 		{
 			name:       "multiple-ip-matches",
 			appDomains: []string{"example.com"},
@@ -1913,6 +1931,18 @@ func TestMapDNSResponseRewritesResponses(t *testing.T) {
 				},
 			),
 		},
+		{
+			name: "ipv6-4in6-answer-dropped",
+			toMap: makeV6DNSResponse(t, domainName, []*dnsmessage.AAAAResource{
+				{AAAA: netip.MustParseAddr("::ffff:1.2.3.4").As16()},
+				{AAAA: netip.MustParseAddr("2606:4700::6812:1a78").As16()},
+			}),
+			assertFx: assertParsesToAnswers(
+				[]netip.Addr{
+					netip.MustParseAddr("2606:4700::6812:100"),
+				},
+			),
+		},
 		{
 			name:     "not-our-domain",
 			toMap:    ipv4ResponseUnhandledDomain,
@@ -2072,7 +2102,7 @@ func TestMapDNSResponseRewritesResponses(t *testing.T) {
 							Type:  dnsmessage.TypeAAAA,
 							Class: dnsmessage.ClassINET,
 						},
-						Body: &dnsmessage.AAAAResource{AAAA: netip.MustParseAddr("1.2.3.4").As16()},
+						Body: &dnsmessage.AAAAResource{AAAA: netip.MustParseAddr("2606:4700::6812:1a78").As16()},
 					},
 					{
 						Header: dnsmessage.ResourceHeader{