add dns_message_peekheader()

lib/dns/include/dns/message.h

@@ -627,6 +627,30 @@ dns_message_puttemprdatalist(dns_message_t *msg, dns_rdatalist_t **item);
  *	*item == NULL
  */
 
+dns_result_t
+dns_message_peekheader(isc_buffer_t *source, dns_messageid_t *idp,
+		       unsigned int *flagsp);
+/*
+ * Assume the remaining region of "source" is a DNS message.  Peek into
+ * it and fill in "*idp" with the message id, and "*flagsp" with the flags.
+ *
+ * Requires:
+ *
+ *	source != NULL
+ *
+ * Ensures:
+ *
+ *	if (idp != NULL) *idp == message id.
+ *
+ *	if (flagsp != NULL) *flagsp == message flags.
+ *
+ * Returns:
+ *
+ *	DNS_R_SUCCESS		-- all is well.
+ *
+ *	DNS_R_UNEXPECTEDEND	-- buffer doesn't contain enough for a header.
+ */
+
 ISC_LANG_ENDDECLS
 
 #endif	/* DNS_DNS_H */

lib/dns/message.c

@@ -1539,3 +1539,32 @@ dns_message_puttemprdatalist(dns_message_t *msg, dns_rdatalist_t **item)
 	releaserdatalist(msg, *item);
 	*item = NULL;
 }
+
+dns_result_t
+dns_message_peekheader(isc_buffer_t *source, dns_messageid_t *idp,
+		       unsigned int *flagsp)
+{
+	isc_region_t r;
+	isc_buffer_t buffer;
+	dns_messageid_t id;
+	unsigned int flags;
+
+	REQUIRE(source != NULL);
+
+	buffer = *source;
+
+	isc_buffer_remaining(&buffer, &r);
+	if (r.length < DNS_MESSAGE_HEADERLEN)
+		return (DNS_R_UNEXPECTEDEND);
+
+	id = isc_buffer_getuint16(source);
+	flags = isc_buffer_getuint16(source);
+	flags &= DNS_MESSAGE_FLAG_MASK;
+
+	if (flagsp != NULL)
+		*flagsp = flags;
+	if (idp != NULL)
+		*idp = id;
+
+	return (DNS_R_SUCCESS);
+}