Add possibility to prefer IPv6, IPv4 or unspecified

hiredis.c

@@ -814,6 +814,12 @@ redisContext *redisConnectWithOptions(const redisOptions *options) {
     if (options->options & REDIS_OPT_NOAUTOFREEREPLIES) {
         c->flags |= REDIS_NO_AUTO_FREE_REPLIES;
     }
+    if (options->options & REDIS_OPT_PREFER_IPV4) {
+        c->flags |= REDIS_PREFER_IPV4;
+    }
+    if (options->options & REDIS_OPT_PREFER_IPV6) {
+        c->flags |= REDIS_PREFER_IPV6;
+    }
 
     /* Set any user supplied RESP3 PUSH handler or use freeReplyObject
      * as a default unless specifically flagged that we don't want one. */

hiredis.h

@@ -92,6 +92,11 @@ typedef long long ssize_t;
 /* Flag that indicates the user does not want replies to be automatically freed */
 #define REDIS_NO_AUTO_FREE_REPLIES 0x400
 
+/* Flags to prefer IPv6 or IPv4 when doing DNS lookup. (If both are set,
+ * AF_UNSPEC is used.) */
+#define REDIS_PREFER_IPV4 0x800
+#define REDIS_PREFER_IPV6 0x1000
+
 #define REDIS_KEEPALIVE_INTERVAL 15 /* seconds */
 
 /* number of times we retry to connect in the case of EADDRNOTAVAIL and
@@ -149,6 +154,8 @@ struct redisSsl;
 
 #define REDIS_OPT_NONBLOCK 0x01
 #define REDIS_OPT_REUSEADDR 0x02
+#define REDIS_OPT_PREFER_IPV4 0x04
+#define REDIS_OPT_PREFER_IPV6 0x08
 
 /**
  * Don't automatically free the async object on a connection failure,

net.c

@@ -439,17 +439,25 @@ static int _redisContextConnectTcp(redisContext *c, const char *addr, int port,
     hints.ai_family = AF_INET;
     hints.ai_socktype = SOCK_STREAM;
 
-    /* Try with IPv6 if no IPv4 address was found. We do it in this order since
-     * in a Redis client you can't afford to test if you have IPv6 connectivity
-     * as this would add latency to every connect. Otherwise a more sensible
-     * route could be: Use IPv6 if both addresses are available and there is IPv6
-     * connectivity. */
-    if ((rv = getaddrinfo(c->tcp.host,_port,&hints,&servinfo)) != 0) {
-         hints.ai_family = AF_INET6;
-         if ((rv = getaddrinfo(addr,_port,&hints,&servinfo)) != 0) {
-            __redisSetError(c,REDIS_ERR_OTHER,gai_strerror(rv));
-            return REDIS_ERR;
-        }
+    /* DNS lookup. To use dual stack, set both flags to prefer both IPv4 and
+     * IPv6. By default, for historical reasons, we try IPv4 first and then we
+     * try IPv6 only if no IPv4 address was found. */
+    if (c->flags & REDIS_PREFER_IPV6 && c->flags & REDIS_PREFER_IPV4)
+        hints.ai_family = AF_UNSPEC;
+    else if (c->flags & REDIS_PREFER_IPV6)
+        hints.ai_family = AF_INET6;
+    else
+        hints.ai_family = AF_INET;
+
+    rv = getaddrinfo(c->tcp.host, _port, &hints, &servinfo);
+    if (rv != 0 && hints.ai_family != AF_UNSPEC) {
+        /* Try again with the other IP version. */
+        hints.ai_family = (hints.ai_family == AF_INET) ? AF_INET6 : AF_INET;
+        rv = getaddrinfo(c->tcp.host, _port, &hints, &servinfo);
+    }
+    if (rv != 0) {
+        __redisSetError(c, REDIS_ERR_OTHER, gai_strerror(rv));
+        return REDIS_ERR;
     }
     for (p = servinfo; p != NULL; p = p->ai_next) {
 addrretry: