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/include/lwip/dns.h

@@ -0,0 +1,90 @@
+/**
+ * lwip DNS resolver header file.
+
+ * Author: Jim Pettinato 
+ *   April 2007
+
+ * ported from uIP resolv.c Copyright (c) 2002-2003, Adam Dunkels.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote
+ *    products derived from this software without specific prior
+ *    written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS
+ * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
+ * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef __LWIP_DNS_H__
+#define __LWIP_DNS_H__
+
+#include "lwip/opt.h"
+
+#if LWIP_DNS /* don't build if not configured for use in lwipopts.h */
+
+/** DNS timer period */
+#ifndef DNS_TMR_INTERVAL
+#define DNS_TMR_INTERVAL 1000
+#endif
+
+/* enumerated list of possible result values returned by dns_gethostname() */
+typedef enum dns_result {
+  DNS_QUERY_INVALID,
+  DNS_QUERY_QUEUED,
+  DNS_COMPLETE
+}DNS_RESULT;
+
+/* initializes the resolver */
+err_t dns_init(void);
+
+/* handles requests, retries and timeouts - call every DNS_TMR_INTERVAL tick */
+void  dns_tmr(void);
+
+/* initializes DNS server IP address */
+void dns_setserver(u8_t numdns, struct ip_addr *dnsserver);
+
+/* returns configured DNS server IP address */
+u32_t dns_getserver(u8_t numdns);
+
+/* returns IP for host 'name' only if already in table */
+u32_t dns_lookup(char *name);
+
+/* resolove a host 'name' in ip address */
+DNS_RESULT dns_gethostbyname(char *hostName, struct ip_addr *addr, 
+                             void (*found)(char *name, struct ip_addr *ipaddr, void *arg),
+                             void *arg);
+
+/* dns_gethostbyname() - Returns immediately with one of DNS_RESULT return codes
+ *                       Return value will be DNS_COMPLETE if hostName is a valid
+ *                       IP address string or the host name is already in the local
+ *                       names table. Returns DNS_REQUEST_QUEUED and queues a
+ *                       request to be sent to the DNS server for resolution if no
+ *                       errors are present.
+ */
+
+/* dns_found_func() - Callback which is invoked when a hostname is found.
+ * This function should be implemented by the application using the DNS resolver.
+ *  param 'name'   - pointer to the name that was looked up.
+ *  param 'ipaddr' - pointer to a struct ip_addr containing the IP address of the
+ *                   hostname, or NULL if the name could not be found.
+*/
+
+#endif /* LWIP_DNS */
+
+#endif /* __LWIP_DNS_H__ */

src/include/lwip/opt.h

@@ -546,6 +546,19 @@
 #define LWIP_IGMP                       0
 #endif
 
+/*
+   ----------------------------------
+   ---------- DNS options -----------
+   ----------------------------------
+*/
+/**
+ * LWIP_DNS==1: Turn on DNS module. UDP must be available for DNS
+ * transport.
+ */
+#ifndef LWIP_DNS
+#define LWIP_DNS                        0
+#endif
+
 /*
    ---------------------------------
    ---------- UDP options ----------
@@ -1515,4 +1528,11 @@
 #define SNMP_MIB_DEBUG                  LWIP_DBG_OFF
 #endif
 
+/**
+ * DNS_DEBUG: Enable debugging for DNS.
+ */
+#ifndef DNS_DEBUG
+#define DNS_DEBUG                       LWIP_DBG_OFF
+#endif
+
 #endif /* __LWIP_OPT_H__ */