Pull request: dnsforward: fix nxdomain for internal aaaa reqs

Updates #2393.

Squashed commit of the following:

commit 5ea4d3c2f217ed800da79c3d87184d8e0956e56c
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 25 17:01:08 2021 +0300

    dnsforward: fix nxdomain for internal aaaa reqs

internal/dnsforward/dns.go

@@ -192,7 +192,11 @@ func (s *Server) onDHCPLeaseChanged(flags int) {
 func (s *Server) processInternalHosts(dctx *dnsContext) (rc resultCode) {
 	req := dctx.proxyCtx.Req
 	q := req.Question[0]
-	if q.Qtype != dns.TypeA {
+
+	// Go on processing the AAAA request despite the fact that we don't
+	// support it yet.  The expected behavior here is to respond with an
+	// empty asnwer and not NXDOMAIN.
+	if q.Qtype != dns.TypeA && q.Qtype != dns.TypeAAAA {
 		return resultCodeSuccess
 	}
 

internal/dnsforward/dns_test.go

@@ -52,6 +52,14 @@ func TestServer_ProcessInternalHosts(t *testing.T) {
 		wantIP:     nil,
 		qtyp:       dns.TypeA,
 		wantRes:    resultCodeSuccess,
+	}, {
+		name:       "success_internal_aaaa",
+		host:       "example.lan",
+		suffix:     defaultAutohostSuffix,
+		wantErrMsg: "",
+		wantIP:     nil,
+		qtyp:       dns.TypeAAAA,
+		wantRes:    resultCodeSuccess,
 	}}
 
 	for _, tc := range testCases {
@@ -92,7 +100,14 @@ func TestServer_ProcessInternalHosts(t *testing.T) {
 			}
 
 			pctx := dctx.proxyCtx
-			if tc.wantIP == nil {
+			if tc.qtyp == dns.TypeAAAA {
+				// TODO(a.garipov): Remove this special handling
+				// when we fully support AAAA.
+				require.NotNil(t, pctx.Res)
+
+				ans := pctx.Res.Answer
+				require.Len(t, ans, 0)
+			} else if tc.wantIP == nil {
 				assert.Nil(t, pctx.Res)
 			} else {
 				require.NotNil(t, pctx.Res)