dns: Support for fallback DNS server

This adds support for list of DNS servers, that could contain manually
configured server that won't be rewritten.
Also if some of the servers in the list is NULL, the algorithm continues
searching with other configured servers up to DNS_MAX_SERVERS.

Ref IDF-4839

.github/workflows/ci-linux.yml

@@ -48,7 +48,7 @@ jobs:
       run: |
         cp ${CONTRIB}/examples/example_app/lwipcfg.h.example ${CONTRIB}/examples/example_app/lwipcfg.h
         cd ${CONTRIB}/ports/unix/example_app
-        export CFLAGS="-DESP_LWIP=LWIP_NETCONN_FULLDUPLEX -DESP_LWIP_IGMP_TIMERS_ONDEMAND=ESP_LWIP -DESP_LWIP_MLD6_TIMERS_ONDEMAND=ESP_LWIP"
+        export CFLAGS="-DESP_LWIP=LWIP_NETCONN_FULLDUPLEX -DESP_LWIP_IGMP_TIMERS_ONDEMAND=ESP_LWIP -DESP_LWIP_MLD6_TIMERS_ONDEMAND=ESP_LWIP -DESP_DNS=ESP_LWIP"
         export LWIPDIR=../../../../src/
         chmod +x iteropts.sh && ./iteropts.sh
 

src/core/dns.c

@@ -314,12 +314,25 @@ const ip_addr_t dns_mquery_v6group = DNS_MQUERY_IPV6_GROUP_INIT;
 void
 dns_init(void)
 {
+#if ESP_DNS
+#ifdef FALLBACK_DNS_SERVER_ADDRESS
+  /* initialize fallback dns DNS server address */
+  ip_addr_t dnsserver;
+  FALLBACK_DNS_SERVER_ADDRESS(&dnsserver);
+  dnsserver.type = IPADDR_TYPE_V4;
+  dns_setserver(DNS_FALLBACK_SERVER_INDEX, &dnsserver);
+#endif /* FALLBACK_DNS_SERVER_ADDRESS */
+
+#else
+
+
 #ifdef DNS_SERVER_ADDRESS
   /* initialize default DNS server address */
   ip_addr_t dnsserver;
   DNS_SERVER_ADDRESS(&dnsserver);
   dns_setserver(0, &dnsserver);
 #endif /* DNS_SERVER_ADDRESS */
+#endif
 
   LWIP_ASSERT("sanity check SIZEOF_DNS_QUERY",
               sizeof(struct dns_query) == SIZEOF_DNS_QUERY);
@@ -1059,7 +1072,11 @@ dns_check_entry(u8_t i)
       entry->server_idx = 0;
       entry->tmr = 1;
       entry->retries = 0;
-
+#if ESP_DNS
+      while((entry->server_idx < DNS_MAX_SERVERS) && ip_addr_isany_val(dns_servers[entry->server_idx])) {
+        entry->server_idx++;
+      }
+#endif
       /* send DNS packet for this entry */
       err = dns_send(i);
       if (err != ERR_OK) {
@@ -1070,6 +1087,12 @@ dns_check_entry(u8_t i)
     case DNS_STATE_ASKING:
       if (--entry->tmr == 0) {
         if (++entry->retries == DNS_MAX_RETRIES) {
+#if ESP_DNS
+          /* skip DNS servers with zero address */
+        while ((entry->server_idx + 1 < DNS_MAX_SERVERS) && ip_addr_isany_val(dns_servers[entry->server_idx + 1])) {
+            entry->server_idx++;
+          }
+#endif
           if (dns_backupserver_available(entry)
 #if LWIP_DNS_SUPPORT_MDNS_QUERIES
               && !entry->is_mdns
@@ -1529,6 +1552,19 @@ dns_gethostbyname(const char *hostname, ip_addr_t *addr, dns_found_callback foun
   return dns_gethostbyname_addrtype(hostname, addr, found, callback_arg, LWIP_DNS_ADDRTYPE_DEFAULT);
 }
 
+#if ESP_DNS
+static bool dns_server_is_set (void)
+{
+  int i = 0;
+  for (i = 0;i < DNS_MAX_SERVERS; i++) {
+    if (!ip_addr_isany_val(dns_servers[i])) {
+      return true;
+    }
+  }
+  return false;
+}
+#endif
+
 /**
  * @ingroup dns
  * Like dns_gethostbyname, but returned address type can be controlled:
@@ -1618,7 +1654,11 @@ dns_gethostbyname_addrtype(const char *hostname, ip_addr_t *addr, dns_found_call
 #endif /* LWIP_DNS_SUPPORT_MDNS_QUERIES */
   {
     /* prevent calling found callback if no server is set, return error instead */
+#if ESP_DNS
+    if (dns_server_is_set() == false) {
+#else
     if (ip_addr_isany_val(dns_servers[0])) {
+#endif
       return ERR_VAL;
     }
   }

src/core/ipv4/dhcp.c

@@ -712,6 +712,11 @@ dhcp_handle_ack(struct netif *netif, struct dhcp_msg *msg_in)
   /* DNS servers */
   for (n = 0; (n < LWIP_DHCP_PROVIDE_DNS_SERVERS) && dhcp_option_given(dhcp, DHCP_OPTION_IDX_DNS_SERVER + n); n++) {
     ip_addr_t dns_addr;
+#if ESP_DNS
+    if (n == DNS_FALLBACK_SERVER_INDEX) {
+        continue;
+    }
+#endif
     ip_addr_set_ip4_u32_val(dns_addr, lwip_htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_DNS_SERVER + n)));
     dns_setserver(n, &dns_addr);
   }

src/include/lwip/dns.h

@@ -111,7 +111,9 @@ err_t            dns_gethostbyname(const char *hostname, ip_addr_t *addr,
 err_t            dns_gethostbyname_addrtype(const char *hostname, ip_addr_t *addr,
                                    dns_found_callback found, void *callback_arg,
                                    u8_t dns_addrtype);
-
+#if ESP_DNS
+void             dns_clear_servers(bool keep_fallback);
+#endif
 
 #if DNS_LOCAL_HOSTLIST
 size_t         dns_local_iterate(dns_found_callback iterator_fn, void *iterator_arg);

test/unit/lwipopts.h

@@ -84,6 +84,7 @@
 #define LWIP_MEM_ILLEGAL_FREE(msg)      /* to nothing */
 
 #ifdef ESP_LWIP
+#define ESP_DNS                          1
 #define LWIP_DHCP_ENABLE_VENDOR_SPEC_IDS 1
 #define LWIP_DHCP_ENABLE_CLIENT_ID 1
 #define LWIP_DHCP_ENABLE_MTU_UPDATE 1
@@ -156,6 +157,7 @@ u32_t esp_random(void);
 
 #else
 #define ESP_LWIP                                0
+#define ESP_DNS                                 0
 #define ESP_LWIP_IGMP_TIMERS_ONDEMAND           0
 #define ESP_LWIP_MLD6_TIMERS_ONDEMAND           0