fix(dns): Fix IPv6 only networks by preferencing IPv6 if you have 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

@@ -47,55 +47,90 @@ int NetworkManager::hostByName(const char* aHostname, IPAddress& aResult, bool p
 {
     // IDEA: Rename to getAddressInfo() ?
 
-    err_t err = ERR_OK;
-
-    // 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
+    err_t err = -1;
+    const char *servname = "0";
+    struct addrinfo *res;
     static bool hasGlobalV6 = false;
-    bool hasGlobalV6Now = false;//ToDo: implement this!
+
+    aResult = static_cast<uint32_t>(0);
+
+    // 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.
+
+    // Iterate over active interfaces, and find if we have any global scope IPv6
+    bool hasGlobalV6Now = false;
+    esp_netif_t *netif = NULL;
+    while ((netif = esp_netif_next_unsafe(netif)) != NULL) {
+        esp_ip6_addr_t ip6[CONFIG_LWIP_IPV6_NUM_ADDRESSES];
+        int ip6_addrs = esp_netif_get_all_ip6(netif, ip6);
+        for (int j = 0; j < ip6_addrs; ++j) {
+            // Both global and unique local addresses have global scope.
+            // ULA assumes either private DNS or NAT66 (same assumption as IPv4 private address ranges).
+            esp_ip6_addr_type_t ipv6_type = esp_netif_ip6_get_addr_type(&(ip6[j]));
+            if (ipv6_type == ESP_IP6_ADDR_IS_GLOBAL || ipv6_type == ESP_IP6_ADDR_IS_UNIQUE_LOCAL) {
+                hasGlobalV6Now = true;
+                break;
+            }
+        }
+        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);
-
-    // First check if the host parses as a literal address
-    if (!aResult.fromString(aHostname)) {
-        const char *servname = "0";
-        struct addrinfo *res;
-        const struct addrinfo hints = {
-            .ai_family = AF_UNSPEC,
+    if (hasGlobalV6) {
+        const struct addrinfo hints6 = {
+            .ai_family = AF_INET6,
             .ai_socktype = SOCK_STREAM,
         };
-        err = lwip_getaddrinfo(aHostname, servname, &hints, &res);
-        if (err == 0)
-        {
-            if (res->ai_family == AF_INET6)
-            {
-                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 IPv6 %s", aResult.toString().c_str());
-            }
-            else if (res->ai_family == AF_INET)
-            {
-                struct sockaddr_in *ipv4 = (struct sockaddr_in *)res->ai_addr;
-                // As a single u32_t
-                aResult = IPAddress(ipv4->sin_addr.s_addr);
-                log_d("DNS found IPv4 %s", aResult.toString().c_str());
-            }
-            else
-            {
-                err = -1;
-            }
+        err = lwip_getaddrinfo(aHostname, servname, &hints6, &res);
 
+        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;
         }
     }
-    if (err == ERR_OK) {
+    // **End Workaround**
+
+    const struct addrinfo hints = {
+        .ai_family = AF_UNSPEC,
+        .ai_socktype = SOCK_STREAM,
+    };
+    err = lwip_getaddrinfo(aHostname, servname, &hints, &res);
+    if (err == ERR_OK)
+    {
+        if (res->ai_family == AF_INET6)
+        {
+            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 any IPv6 %s", aResult.toString().c_str());
+        }
+        else
+        {
+            struct sockaddr_in *ipv4 = (struct sockaddr_in *)res->ai_addr;
+            // As a single u32_t
+            aResult = IPAddress(ipv4->sin_addr.s_addr);
+            log_d("DNS found any IPv4 %s", aResult.toString().c_str());
+        }
+        lwip_freeaddrinfo(res);
         return 1;
     }
     log_e("DNS Failed for '%s' with error '%d'", aHostname, err);