Fix IPv4 address in replies to A or NS queries (github issue #38)

The destination field in struct query was changed from in_addr_t to
struct sockaddr_storage, wihtout updating the functions sending it
in src/dns.c.

Only add extra A answer for NS queries if destination refers to an
IPv4 address, and fail if trying to encode a reply to an A query
if destination is not IPv4.

This means NS requests received over IPv6 will not contain an address
and A requests will be ignored, unless the -n option is used, or the
www subdomain is requested which sets a fixed address (127.0.0.1).

src/dns.c

@@ -299,21 +299,25 @@ int dns_encode_ns_response(char *buf, size_t buflen, struct query *q,
 	putbyte(&p, 's');
 	putshort(&p, topname);			/* Name Server */
 
-	/* Additional data (A-record of NS server) */
+	/* Do we have an IPv4 address to send? */
-	CHECKLEN(12);
+	if (q->destination.ss_family == AF_INET) {
-	putshort(&p, nsname);			/* Name Server */
+		struct sockaddr_in *dest = (struct sockaddr_in *) &q->destination;
-	putshort(&p, T_A);			/* Type */
+		/* Additional data (A-record of NS server) */
-	putshort(&p, C_IN);			/* Class */
+		CHECKLEN(12);
-	putlong(&p, 3600);			/* TTL */
+		putshort(&p, nsname);		/* Name Server */
-	putshort(&p, 4);			/* Data length */
+		putshort(&p, T_A);		/* Type */
+		putshort(&p, C_IN);		/* Class */
+		putlong(&p, 3600);		/* TTL */
+		putshort(&p, 4);		/* Data length */
 
-	/* ugly hack to output IP address */
+		/* ugly hack to output IP address */
-	ipp = (char *) &q->destination;
+		ipp = (char *) &dest->sin_addr.s_addr;
-	CHECKLEN(4);
+		CHECKLEN(4);
-	putbyte(&p, *(ipp++));
+		putbyte(&p, *(ipp++));
-	putbyte(&p, *(ipp++));
+		putbyte(&p, *(ipp++));
-	putbyte(&p, *(ipp++));
+		putbyte(&p, *(ipp++));
-	putbyte(&p, *ipp);
+		putbyte(&p, *ipp);
+	}
 
 	len = p - buf;
 	return len;
@@ -323,12 +327,17 @@ int dns_encode_ns_response(char *buf, size_t buflen, struct query *q,
  * www.topdomain . Mostly same as dns_encode_ns_response() above */
 int dns_encode_a_response(char *buf, size_t buflen, struct query *q)
 {
+	struct sockaddr_in *dest = (struct sockaddr_in *) &q->destination;
 	HEADER *header;
 	int len;
 	short name;
 	char *ipp;
 	char *p;
 
+	/* Check if we have an IPv4 address to send */
+	if (q->destination.ss_family != AF_INET)
+		return -1;
+
 	if (buflen < sizeof(HEADER))
 		return 0;
 
@@ -355,19 +364,19 @@ int dns_encode_a_response(char *buf, size_t buflen, struct query *q)
 	/* Query section */
 	putname(&p, buflen - (p - buf), q->name); /* Name */
 	CHECKLEN(4);
-	putshort(&p, q->type); /* Type */
+	putshort(&p, q->type);	/* Type */
-	putshort(&p, C_IN); /* Class */
+	putshort(&p, C_IN);	/* Class */
 
 	/* Answer section */
 	CHECKLEN(12);
-	putshort(&p, name); /* Name */
+	putshort(&p, name);	/* Name */
-	putshort(&p, q->type); /* Type */
+	putshort(&p, q->type);	/* Type */
-	putshort(&p, C_IN); /* Class */
+	putshort(&p, C_IN);	/* Class */
-	putlong(&p, 3600); /* TTL */
+	putlong(&p, 3600);	/* TTL */
-	putshort(&p, 4); /* Data length */
+	putshort(&p, 4);	/* Data length */
 
 	/* ugly hack to output IP address */
-	ipp = (char *) &q->destination;
+	ipp = (char *) &dest->sin_addr.s_addr;
 	CHECKLEN(4);
 	putbyte(&p, *(ipp++));
 	putbyte(&p, *(ipp++));

src/iodined.c

@@ -1549,7 +1549,9 @@ handle_ns_request(int dns_fd, struct query *q)
 		/* If ns_ip set, overwrite destination addr with it.
 		 * Destination addr will be sent as additional record (A, IN) */
 		struct sockaddr_in *addr = (struct sockaddr_in *) &q->destination;
+		addr->sin_family = AF_INET;
 		memcpy(&addr->sin_addr, &ns_ip, sizeof(ns_ip));
+		q->dest_len = sizeof(*addr);
 	}
 
 	len = dns_encode_ns_response(buf, sizeof(buf), q, topdomain);
@@ -1577,15 +1579,23 @@ handle_a_request(int dns_fd, struct query *q, int fakeip)
 	if (fakeip) {
 		in_addr_t ip = inet_addr("127.0.0.1");
 		struct sockaddr_in *addr = (struct sockaddr_in *) &q->destination;
+		addr->sin_family = AF_INET;
 		memcpy(&addr->sin_addr, &ip, sizeof(ip));
-
+		q->dest_len = sizeof(*addr);
 	} else if (ns_ip != INADDR_ANY) {
 		/* If ns_ip set, overwrite destination addr with it.
 		 * Destination addr will be sent as additional record (A, IN) */
 		struct sockaddr_in *addr = (struct sockaddr_in *) &q->destination;
+		addr->sin_family = AF_INET;
 		memcpy(&addr->sin_addr, &ns_ip, sizeof(ns_ip));
+		q->dest_len = sizeof(*addr);
 	}
 
+	/* Give up if no IPv4 address known (when A request received over IPv6
+	 * and destination was not overwritten above) */
+	if (q->destination.ss_family != AF_INET)
+		return;
+
 	len = dns_encode_a_response(buf, sizeof(buf), q);
 	if (len < 1) {
 		warnx("dns_encode_a_response doesn't fit");