Fix rcode parsing, as noticed by Coverity.

Without support for OPT RR from RFC6891 (Extension mechanisms for DNS)
values of RCODE above 15 are not possible.  Remove dead code which will
never match.

src/app-layer-dns-common.h

@@ -71,6 +71,8 @@
 #define DNS_RCODE_NXRRSET       8
 #define DNS_RCODE_NOTAUTH       9
 #define DNS_RCODE_NOTZONE       10
+// Support for OPT RR from RFC6891 will be needed to
+// parse RCODE values over 15
 #define DNS_RCODE_BADVERS       16
 #define DNS_RCODE_BADSIG        16
 #define DNS_RCODE_BADKEY        17

src/app-layer-dns-tcp.c

@@ -449,7 +449,7 @@ static int DNSReponseParseData(Flow *f, DNSState *dns_state, const uint8_t *inpu
 
     /* parse rcode, e.g. "noerror" or "nxdomain" */
     uint8_t rcode = ntohs(dns_header->flags) & 0x0F;
-    if (rcode <= DNS_RCODE_NOTZONE || (rcode >= DNS_RCODE_BADSIG && rcode <= DNS_RCODE_BADTRUNC)) {
+    if (rcode <= DNS_RCODE_NOTZONE) {
         SCLogDebug("rcode %u", rcode);
         if (tx != NULL)
             tx->rcode = rcode;

src/app-layer-dns-udp.c

@@ -271,7 +271,7 @@ static int DNSUDPResponseParse(Flow *f, void *dstate,
 
     /* parse rcode, e.g. "noerror" or "nxdomain" */
     uint8_t rcode = ntohs(dns_header->flags) & 0x0F;
-    if (rcode <= DNS_RCODE_NOTZONE || (rcode >= DNS_RCODE_BADSIG && rcode <= DNS_RCODE_BADTRUNC)) {
+    if (rcode <= DNS_RCODE_NOTZONE) {
         SCLogDebug("rcode %u", rcode);
         if (tx != NULL)
             tx->rcode = rcode;