cmd/k8s-nameserver: fix AAAA record query response (#12412)

Return empty response and NOERROR for AAAA record queries
for DNS names for which we have an A record.
This is to allow for callers that might be first sending an AAAA query and then,
if that does not return a response, follow with an A record query.
Previously we were returning NOTIMPL that caused some callers
to potentially not follow with an A record query or misbehave in different ways.

Also return NXDOMAIN for AAAA record queries for names
that we DO NOT have an A record for to ensure that the callers
do not follow up with an A record query.

Returning an empty response and NOERROR is the behaviour
that RFC 4074 recommends:
https://datatracker.ietf.org/doc/html/rfc4074

Updates tailscale/tailscale#12321

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

cmd/k8s-nameserver/main.go

@@ -153,11 +153,36 @@ func (n *nameserver) handleFunc() func(w dns.ResponseWriter, r *dns.Msg) {
 				m.Answer = append(m.Answer, rr)
 			}
 		case dns.TypeAAAA:
-			// TODO (irbekrm): implement IPv6 support.
-			// Kubernetes distributions that I am most familiar with
-			// default to IPv4 for Pod CIDR ranges and often many cases don't
-			// support IPv6 at all, so this should not be crucial for now.
-			fallthrough
+			// TODO (irbekrm): add IPv6 support.
+			// The nameserver currently does not support IPv6
+			// (records are not being created for IPv6 Pod addresses).
+			// However, we can expect that some callers will
+			// nevertheless send AAAA queries.
+			// We have to return NOERROR if a query is received for
+			// an AAAA record for a DNS name that we have an A
+			// record for- else the caller might not follow with an
+			// A record query.
+			// https://github.com/tailscale/tailscale/issues/12321
+			// https://datatracker.ietf.org/doc/html/rfc4074
+			q := r.Question[0].Name
+			fqdn, err := dnsname.ToFQDN(q)
+			if err != nil {
+				m = r.SetRcodeFormatError(r)
+				return
+			}
+			// The only supported use of this nameserver is as a
+			// single source of truth for MagicDNS names by
+			// non-tailnet Kubernetes workloads.
+			m.Authoritative = true
+			ips := n.lookupIP4(fqdn)
+			if len(ips) == 0 {
+				// As we are the authoritative nameserver for MagicDNS
+				// names, if we do not have a record for this MagicDNS
+				// name, it does not exist.
+				m = m.SetRcode(r, dns.RcodeNameError)
+				return
+			}
+			m.SetRcode(r, dns.RcodeSuccess)
 		default:
 			log.Printf("[unexpected] nameserver received a query for an unsupported record type: %s", r.Question[0].String())
 			m.SetRcode(r, dns.RcodeNotImplemented)

cmd/k8s-nameserver/main_test.go

@@ -79,7 +79,7 @@ func TestNameserver(t *testing.T) {
 				}},
 		},
 		{
-			name: "AAAA record query",
+			name: "AAAA record query, A record exists",
 			ip4:  map[dnsname.FQDN][]net.IP{dnsname.FQDN("foo.bar.com."): {{1, 2, 3, 4}}},
 			query: &dns.Msg{
 				Question: []dns.Question{{Name: "foo.bar.com", Qtype: dns.TypeAAAA}},
@@ -89,25 +89,27 @@ func TestNameserver(t *testing.T) {
 				Question: []dns.Question{{Name: "foo.bar.com", Qtype: dns.TypeAAAA}},
 				MsgHdr: dns.MsgHdr{
 					Id:            1,
-					Rcode:    dns.RcodeNotImplemented,
+					Rcode:         dns.RcodeSuccess,
 					Response:      true,
 					Opcode:        dns.OpcodeQuery,
+					Authoritative: true,
 				}},
 		},
 		{
-			name: "AAAA record query",
+			name: "AAAA record query, A record does not exist",
 			ip4:  map[dnsname.FQDN][]net.IP{dnsname.FQDN("foo.bar.com."): {{1, 2, 3, 4}}},
 			query: &dns.Msg{
-				Question: []dns.Question{{Name: "foo.bar.com", Qtype: dns.TypeAAAA}},
+				Question: []dns.Question{{Name: "baz.bar.com", Qtype: dns.TypeAAAA}},
 				MsgHdr:   dns.MsgHdr{Id: 1},
 			},
 			wantResp: &dns.Msg{
-				Question: []dns.Question{{Name: "foo.bar.com", Qtype: dns.TypeAAAA}},
+				Question: []dns.Question{{Name: "baz.bar.com", Qtype: dns.TypeAAAA}},
 				MsgHdr: dns.MsgHdr{
 					Id:            1,
-					Rcode:    dns.RcodeNotImplemented,
+					Rcode:         dns.RcodeNameError,
 					Response:      true,
 					Opcode:        dns.OpcodeQuery,
+					Authoritative: true,
 				}},
 		},
 		{