fix(dns): Fix IPv6-only network, by IPv6 preference if you have public address

Work around because AF_UNSPEC does not check available addresses when
determining result.

If you have a global scope IPv6 address, then first check for IPv6 DNS result;
if you don't have an IPv6, or there is no IPv6 result, then check IPv4.

This allows IPv6-only networks to connect to dual-stack destinations, as they
will get the IPv6 address (rather than the unusable IPv4).

It also means a dual-stack host to a dual-stack destination will preference
IPv6.

There is no effect if you are on an IPv4-only network, or it is an IPv4-only
destination.

Author: sgryphon

libraries/Network/src/NetworkManager.cpp

@@ -15,6 +15,8 @@ NetworkManager::NetworkManager(){
 
 }
 
+NetworkInterface * getNetifByID(Network_Interface_ID id);
+
 bool NetworkManager::begin(){
     static bool initialized = false;
     if(!initialized){
@@ -45,28 +47,62 @@ bool NetworkManager::begin(){
  */
 int NetworkManager::hostByName(const char* aHostname, IPAddress& aResult)
 {
+    static bool hasGlobalV6 = false;
     err_t err = ERR_OK;
+    const char *servname = "0";
+    struct addrinfo *res;
+    aResult = static_cast<uint32_t>(0);
 
-    // This should generally check if we have a global address assigned to one of the interfaces.
-    // If such address is not assigned, there is no point in trying to get V6 from DNS as we will not be able to reach it.
-    // That is of course, if 'preferV6' is not set to true
-    static bool hasGlobalV6 = false;
+    // First check if the host parses as a literal address
+    if (aResult.fromString(aHostname)) {
+        return 1;
+    }
+
+    // **Workaround**
+    // LWIP AF_UNSPEC always prefers IPv4 and doesn't check what network is
+    // available. See https://github.com/espressif/esp-idf/issues/13255
+    // Until that is fixed, as a work around if we have a global scope IPv6,
+    // then we check IPv6 only first.
+
+    // This checks if we have a global address assigned to one of the interfaces.
+    // If such address is assigned, then we trying to get V6 from DNS first.
     bool hasGlobalV6Now = false;//ToDo: implement this!
+    for (int i = 0; i < ESP_NETIF_ID_MAX; ++i){
+        NetworkInterface * iface = getNetifByID((Network_Interface_ID)i);
+        if(iface != NULL && iface->hasGlobalIPv6()){
+            hasGlobalV6Now = true;
+        }
+        if (hasGlobalV6Now){
+            break;
+        }
+    }
+
+    // Clear DNS cache if the flag has changed
     if(hasGlobalV6 != hasGlobalV6Now){
         hasGlobalV6 = hasGlobalV6Now;
         dns_clear_cache();
         log_d("Clearing DNS cache");
     }
 
-    aResult = static_cast<uint32_t>(0);
+    if (hasGlobalV6) {
+        const struct addrinfo hints6 = {
+            .ai_family = AF_INET6,
+            .ai_socktype = SOCK_STREAM,
+        };
+        err = lwip_getaddrinfo(aHostname, servname, &hints6, &res);
 
-    // First check if the host parses as a literal address
-    if (aResult.fromString(aHostname)) {
-        return 1;
+        if (err == ERR_OK)
+        {
+            struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)res->ai_addr;
+            // As an array of u8_t
+            aResult = IPAddress(IPv6, ipv6->sin6_addr.s6_addr);
+            log_d("DNS found first IPv6 %s", aResult.toString().c_str());
+            lwip_freeaddrinfo(res);
+            return 1;
+        }
     }
+    // **End Workaround**
 
-    const char *servname = "0";
-    struct addrinfo *res;
     const struct addrinfo hints = {
         .ai_family = AF_UNSPEC,
         .ai_socktype = SOCK_STREAM,
@@ -130,8 +166,6 @@ bool NetworkManager::setHostname(const char * name)
     return true;
 }
 
-NetworkInterface * getNetifByID(Network_Interface_ID id);
-
 size_t NetworkManager::printTo(Print & out) const {
     size_t bytes = 0;
 
@@ -144,5 +178,4 @@ size_t NetworkManager::printTo(Print & out) const {
     return bytes;
 }
 
-
 NetworkManager Network;