resolve: add CURLOPT_DNS_SHUFFLE_ADDRESSES

This patch adds CURLOPT_DNS_SHUFFLE_ADDRESSES to explicitly request
shuffling of IP addresses returned for a hostname when there is more
than one. This is useful when the application knows that a round robin
approach is appropriate and is willing to accept the consequences of
potentially discarding some preference order returned by the system's
implementation.

Closes #1694

1 files changed, 12 insertions, 1 deletions

diff --git a/lib/hostip.h b/lib/hostip.h
index 298eeeee3..1de4bee8d 100644
--- a/lib/hostip.h
+++ b/lib/hostip.h
@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -182,6 +182,17 @@ struct Curl_dns_entry *
 Curl_fetch_addr(struct connectdata *conn,
                 const char *hostname,
                 int port);
+
+/*
+ * Curl_shuffle_addr() shuffles the order of addresses in a 'Curl_addrinfo'
+ * struct by re-linking its linked list.
+ *
+ * The addr argument should be the address of a pointer to the head node of a
+ * `Curl_addrinfo` list and it will be modified to point to the new head after
+ * shuffling.
+ */
+CURLcode Curl_shuffle_addr(struct Curl_easy *data, Curl_addrinfo **addr);
+
 /*
  * Curl_cache_addr() stores a 'Curl_addrinfo' struct in the DNS cache.
  *