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

2 files changed, 70 insertions, 0 deletions

diff --git a/docs/libcurl/opts/CURLOPT_DNS_SHUFFLE_ADDRESSES.3 b/docs/libcurl/opts/CURLOPT_DNS_SHUFFLE_ADDRESSES.3
new file mode 100644
index 000000000..8af2d0aa2
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_DNS_SHUFFLE_ADDRESSES.3
@@ -0,0 +1,69 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * 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
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.\"
+.TH CURLOPT_DNS_SHUFFLE_ADDRESSES 3 "3 March 2018" "libcurl 7.60.0" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_DNS_SHUFFLE_ADDRESSES \- Shuffle addresses when a hostname returns more than one
+.SH SYNOPSIS
+.nf
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_DNS_SHUFFLE_ADDRESSES, long onoff);
+.fi
+.SH DESCRIPTION
+When a name is resolved and more than one IP address is returned, shuffle the
+order of all returned addresses so that they will be used in a random order.
+This is similar to the ordering behavior of gethostbyname which is no longer
+used on most platforms.
+
+Addresses will not be reshuffled if a name resolution is completed using the
+DNS cache. \fICURLOPT_DNS_CACHE_TIMEOUT(3)\fP can be used together with this
+option to reduce DNS cache timeout or disable caching entirely if frequent
+reshuffling is needed.
+
+Since the addresses returned will be reordered randomly, their order will not
+be in accordance with RFC 3484 or any other deterministic order that may be
+generated by the system's name resolution implementation. This may have
+performance impacts and may cause IPv4 to be used before IPv6 or vice versa.
+.SH DEFAULT
+0 (disabled)
+.SH PROTOCOLS
+All
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
+  curl_easy_setopt(curl, CURLOPT_DNS_SHUFFLE_ADDRESSES, 1L);
+
+  curl_easy_perform(curl);
+
+  /* always cleanup */
+  curl_easy_cleanup(curl);
+}
+.fi
+.SH AVAILABILITY
+Added in 7.60.0
+.SH RETURN VALUE
+CURLE_OK or an error such as CURLE_UNKNOWN_OPTION.
+.SH "SEE ALSO"
+.BR CURLOPT_DNS_CACHE_TIMEOUT "(3), " CURLOPT_IPRESOLVE "(3), "
diff --git a/docs/libcurl/opts/Makefile.inc b/docs/libcurl/opts/Makefile.inc
index b370082d6..80488bfa3 100644
--- a/docs/libcurl/opts/Makefile.inc
+++ b/docs/libcurl/opts/Makefile.inc
@@ -112,6 +112,7 @@ man_MANS =                                      \
   CURLOPT_DNS_LOCAL_IP4.3                       \
   CURLOPT_DNS_LOCAL_IP6.3                       \
   CURLOPT_DNS_SERVERS.3                         \
+  CURLOPT_DNS_SHUFFLE_ADDRESSES.3               \
   CURLOPT_DNS_USE_GLOBAL_CACHE.3                \
   CURLOPT_EGDSOCKET.3                           \
   CURLOPT_ERRORBUFFER.3                         \