dns: add support for Legacy Unicast Responses

As per RFC 6762 Section 6.7 (Legacy Unicast Responses)

"If the source UDP port in a received Multicast DNS query is not port
5353, this indicates that the querier originating the query is a
simple resolver such as described in Section 5.1, "One-Shot Multicast
DNS Queries", which does not fully implement all of Multicast DNS.
In this case, the Multicast DNS responder MUST send a UDP response
directly back to the querier, via unicast, to the query packet's
source IP address and port.  This unicast response MUST be a
conventional unicast response as would be generated by a conventional
Unicast DNS server; for example, it MUST repeat the query ID and the
question given in the query message."

Therefore, umdns should not ignore DNS questions coming from non-
multicast sources, and also provide question section which is not
usually provided in mDNS responses.

Signed-off-by: Mohd Husaam Mehdi <[email protected]>

dns.c

@@ -393,10 +393,17 @@ parse_question(struct interface *iface, struct sockaddr *from, char *name, struc
 	char *host;
 
 	/* TODO: Multicast if more than one quarter of TTL has passed */
-	if (is_unicast) {
+	if (is_unicast || port != MCAST_PORT) {
 		to = from;
 		if (interface_multicast(iface))
 			iface = interface_get(iface->name, iface->type | SOCKTYPE_BIT_UNICAST);
+	} else {
+		/* if the query is from multicast port, no need for original buffer
+		 * while responding as per rfc 6762, section 6:
+		 * Multicast DNS responses MUST NOT contain any questions in the
+		 * Question Section. */
+		orig_buffer = NULL;
+		orig_len = 0;
 	}
 
 	DBG(1, "Q -> %s %s\n", dns_type_string(q->type), name);
@@ -465,10 +472,6 @@ dns_handle_packet(struct interface *iface, struct sockaddr *from, uint16_t port,
 		return;
 	}
 
-	if (h->questions && !interface_multicast(iface) && port != MCAST_PORT)
-		/* silently drop unicast questions that dont originate from port 5353 */
-		return;
-
 	while (h->questions-- > 0) {
 		char *name = dns_consume_name(buffer, len, &b, &rlen);
 		struct dns_question *q;