opt.h, init.c, tcpip.c, dhcp.c, dns.h, dns.c: add DNS client for simple name requests with RAW api interface. Initialization is done in lwip_init() with build time options. DNS timer is added in tcpip_thread context. DHCP can set DNS server ip addresses when options are received. You need to set LWIP_DNS=1 in your lwipopts.h file (LWIP_DNS=0 in opt.h). DNS_DEBUG can be set to get some traces with LWIP_DEBUGF. Sanity check have been added. There is a "todo" list with points to improve.

src/api/tcpip.c

@@ -48,6 +48,7 @@
 #include "lwip/autoip.h"
 #include "lwip/dhcp.h"
 #include "lwip/igmp.h"
+#include "lwip/dns.h"
 #include "lwip/tcpip.h"
 #include "lwip/init.h"
 #include "netif/etharp.h"
@@ -202,6 +203,22 @@ igmp_timer(void *arg)
 }
 #endif /* LWIP_IGMP */
 
+#if LWIP_DNS
+/**
+ * Timer callback function that calls dns_tmr() and reschedules itself.
+ *
+ * @param arg unused argument
+ */
+static void
+dns_timer(void *arg)
+{
+  LWIP_UNUSED_ARG(arg);
+  LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip: dns_tmr()\n"));
+  dns_tmr();
+  sys_timeout(DNS_TMR_INTERVAL, dns_timer, NULL);
+}
+#endif /* LWIP_DNS */
+
 /**
  * The main lwIP thread. This thread has exclusive access to lwIP core functions
  * (unless access to them is not locked). Other threads communicate with this
@@ -234,6 +251,9 @@ tcpip_thread(void *arg)
 #if LWIP_IGMP
   sys_timeout(IGMP_TMR_INTERVAL, igmp_timer, NULL);
 #endif /* LWIP_IGMP */
+#if LWIP_DNS
+  sys_timeout(DNS_TMR_INTERVAL, dns_timer, NULL);
+#endif /* LWIP_DNS */
 
   if (tcpip_init_done != NULL) {
     tcpip_init_done(tcpip_init_done_arg);