Retry lookups with unsigned DNAME over TCP

To prevent spoofed unsigned DNAME responses being accepted retry
response with unsigned DNAMEs over TCP if the response is not TSIG
signed or there isn't a good DNS CLIENT COOKIE.

doc/arm/reference.rst

@@ -2207,6 +2207,8 @@ Boolean Options
    autodetection of DNS COOKIE support to determine when to retry a
    request over TCP.
 
+   For DNAME lookups the default is ``yes`` and it is enforced.  Servers
+   serving DNAME must correctly support DNS over TCP.
 
    .. note::
       If a UDP response is signed using TSIG, :iscman:`named` accepts it even if

lib/dns/include/dns/message.h

@@ -261,6 +261,7 @@ struct dns_message {
 	unsigned int	     rdclass_set      : 1; /* 14 */
 	unsigned int	     fuzzing	      : 1; /* 15 */
 	unsigned int	     free_pools	      : 1; /* 16 */
+	unsigned int	     has_dname	      : 1; /* 17 */
 	unsigned int			      : 0;
 
 	unsigned int opt_reserved;
@@ -1456,3 +1457,10 @@ dns_message_createpools(isc_mem_t *mctx, isc_mempool_t **namepoolp,
 			isc_mempool_t **rdspoolp);
 void
 dns_message_destroypools(isc_mempool_t **namepoolp, isc_mempool_t **rdspoolp);
+
+bool
+dns_message_hasdname(dns_message_t *msg);
+/*%<
+ * Return whether a DNAME was detected in the ANSWER section of a QUERY
+ * message when it was parsed.
+ */

lib/dns/message.c

@@ -442,6 +442,7 @@ msginit(dns_message_t *m) {
 	m->cc_bad = 0;
 	m->tkey = 0;
 	m->rdclass_set = 0;
+	m->has_dname = 0;
 	m->querytsig = NULL;
 	m->indent.string = "\t";
 	m->indent.count = 0;
@@ -1494,6 +1495,11 @@ getsection(isc_buffer_t *source, dns_message_t *msg, dns_decompress_t dctx,
 			 */
 			msg->tsigname->attributes.nocompress = true;
 			free_name = false;
+		} else if (rdtype == dns_rdatatype_dname &&
+			   sectionid == DNS_SECTION_ANSWER &&
+			   msg->opcode == dns_opcode_query)
+		{
+			msg->has_dname = 1;
 		}
 		rdataset = NULL;
 
@@ -5083,3 +5089,9 @@ dns_message_destroypools(isc_mempool_t **namepoolp, isc_mempool_t **rdspoolp) {
 	isc_mempool_destroy(rdspoolp);
 	isc_mempool_destroy(namepoolp);
 }
+
+bool
+dns_message_hasdname(dns_message_t *msg) {
+	REQUIRE(DNS_MESSAGE_VALID(msg));
+	return msg->has_dname;
+}

lib/dns/resolver.c

@@ -854,6 +854,7 @@ typedef struct respctx {
 	bool get_nameservers; /* get a new NS rrset at
 			       * zone cut? */
 	bool resend;	      /* resend this query? */
+	bool secured;	      /* message was signed or had a valid cookie */
 	bool nextitem;	      /* invalid response; keep
 			       * listening for the correct one */
 	bool truncated;	      /* response was truncated */
@@ -7278,13 +7279,10 @@ rctx_cookiecheck(respctx_t *rctx) {
 	resquery_t *query = rctx->query;
 
 	/*
-	 * If TSIG signed, sent via TCP, or cookie present,
-	 * no need to continue.
+	 * If the message was secured or TCP is already in the
+	 * retry flags, no need to continue.
 	 */
-	if (dns_message_gettsig(query->rmessage, NULL) != NULL ||
-	    query->rmessage->cc_ok || query->rmessage->cc_bad ||
-	    (rctx->retryopts & DNS_FETCHOPT_TCP) != 0)
-	{
+	if (rctx->secured || (rctx->retryopts & DNS_FETCHOPT_TCP) != 0) {
 		return ISC_R_SUCCESS;
 	}
 
@@ -7347,6 +7345,47 @@ rctx_cookiecheck(respctx_t *rctx) {
 	return ISC_R_SUCCESS;
 }
 
+static bool
+rctx_need_tcpretry(respctx_t *rctx) {
+	resquery_t *query = rctx->query;
+	if ((rctx->retryopts & DNS_FETCHOPT_TCP) != 0) {
+		/* TCP is already in the retry flags */
+		return false;
+	}
+
+	/*
+	 * If the message was secured, no need to continue.
+	 */
+	if (rctx->secured) {
+		return false;
+	}
+
+	/*
+	 * Currently the only extra reason why we might need to
+	 * retry a UDP response over TCP is a DNAME in the message.
+	 */
+	if (dns_message_hasdname(query->rmessage)) {
+		return true;
+	}
+
+	return false;
+}
+
+static isc_result_t
+rctx_tcpretry(respctx_t *rctx) {
+	/*
+	 * Do we need to retry a UDP response over TCP?
+	 */
+	if (rctx_need_tcpretry(rctx)) {
+		rctx->retryopts |= DNS_FETCHOPT_TCP;
+		rctx->resend = true;
+		rctx_done(rctx, ISC_R_SUCCESS);
+		return ISC_R_COMPLETE;
+	}
+
+	return ISC_R_SUCCESS;
+}
+
 static void
 resquery_response_continue(void *arg, isc_result_t result) {
 	respctx_t *rctx = arg;
@@ -7366,6 +7405,17 @@ resquery_response_continue(void *arg, isc_result_t result) {
 		goto cleanup;
 	}
 
+	/*
+	 * Remember whether this message was signed or had a
+	 * valid client cookie; if not, we may need to retry over
+	 * TCP later.
+	 */
+	if (query->rmessage->cc_ok || query->rmessage->tsig != NULL ||
+	    query->rmessage->sig0 != NULL)
+	{
+		rctx->secured = true;
+	}
+
 	/*
 	 * The dispatcher should ensure we only get responses with QR
 	 * set.
@@ -7373,13 +7423,21 @@ resquery_response_continue(void *arg, isc_result_t result) {
 	INSIST((query->rmessage->flags & DNS_MESSAGEFLAG_QR) != 0);
 
 	/*
-	 * Check for cookie issues.
+	 * Check for cookie issues; if found, maybe retry over TCP.
 	 */
 	result = rctx_cookiecheck(rctx);
 	if (result == ISC_R_COMPLETE) {
 		goto cleanup;
 	}
 
+	/*
+	 * Check whether we need to retry over TCP for some other reason.
+	 */
+	result = rctx_tcpretry(rctx);
+	if (result == ISC_R_COMPLETE) {
+		goto cleanup;
+	}
+
 	/*
 	 * Check for EDNS issues.
 	 */
@@ -8092,8 +8150,8 @@ rctx_answer_positive(respctx_t *rctx) {
 	}
 
 	/*
-	 * Cache records in the authority section, if
-	 * there are any suitable for caching.
+	 * Cache records in the authority section, if there are
+	 * any suitable for caching.
 	 */
 	rctx_authority_positive(rctx);
 
@@ -8431,20 +8489,25 @@ rctx_answer_dname(respctx_t *rctx) {
 
 /*
  * rctx_authority_positive():
- * Examine the records in the authority section (if there are any) for a
- * positive answer.  We expect the names for all rdatasets in this
- * section to be subdomains of the domain being queried; any that are
- * not are skipped.  We expect to find only *one* owner name; any names
- * after the first one processed are ignored. We expect to find only
- * rdatasets of type NS; all others are ignored. Whatever remains can
- * be cached at trust level authauthority or additional
- * (depending on whether the AA bit was set on the answer).
+ * If a positive answer was received over TCP or secured with a cookie
+ * or TSIG, examine the authority section.  We expect names for all
+ * rdatasets in this section to be subdomains of the domain being queried;
+ * any that are not are skipped.  We expect to find only *one* owner name;
+ * any names after the first one processed are ignored. We expect to find
+ * only rdatasets of type NS; all others are ignored. Whatever remains can
+ * be cached at trust level authauthority or additional (depending on
+ * whether the AA bit was set on the answer).
  */
 static void
 rctx_authority_positive(respctx_t *rctx) {
 	fetchctx_t *fctx = rctx->fctx;
-
 	dns_message_t *msg = rctx->query->rmessage;
+
+	/* If it's spoofable, don't cache it. */
+	if (!rctx->secured && (rctx->query->options & DNS_FETCHOPT_TCP) == 0) {
+		return;
+	}
+
 	MSG_SECTION_FOREACH(msg, DNS_SECTION_AUTHORITY, name) {
 		if (!name_external(name, dns_rdatatype_ns, rctx) &&
 		    dns_name_issubdomain(fctx->name, name))