Add DNS server example

PUBLISHED_FROM=0d71533fd475c152c29fe304ca95559512a6059d

docs/dns/server_example.md

@@ -2,4 +2,72 @@
 title: DNS server example
 ---
 
-TBD
+To create a DNS server, follow this pattern:
+
+1. Create a listening UDP connection by calling `mg_bind()` or `mg_bind_opt()`
+2. Call `mg_set_protocol_dns()` for that listening connection.
+  That attaches a built-in DNS event handler which parses incoming
+  data and triggers DNS-specific events.
+3. Create an event handler function.
+
+Here is an example of a simpe DNS server. It is a captive DNS server, meaning
+that it replies with the same IP address on all queries, regardless of what
+exactly host name is being resolved. Error checking is omitted for
+the sake of clarity:
+
+```c
+#include "mongoose.h"
+#include <stdio.h>
+
+static const char *s_listening_addr = "udp://:5353";
+
+static void ev_handler(struct mg_connection *nc, int ev, void *ev_data) {
+  struct mg_dns_message *msg;
+  struct mg_dns_resource_record *rr;
+  struct mg_dns_reply reply;
+  int i;
+
+  switch (ev) {
+    case MG_DNS_MESSAGE: {
+      struct mbuf reply_buf;
+      mbuf_init(&reply_buf, 512);
+      msg = (struct mg_dns_message *) ev_data;
+      reply = mg_dns_create_reply(&reply_buf, msg);
+
+      for (i = 0; i < msg->num_questions; i++) {
+        char rname[256];
+        rr = &msg->questions[i];
+        mg_dns_uncompress_name(msg, &rr->name, rname, sizeof(rname) - 1);
+        fprintf(stdout, "Q type %d name %s\n", rr->rtype, rname);
+        if (rr->rtype == MG_DNS_A_RECORD) {
+          in_addr_t addr = inet_addr("127.0.0.1");
+          mg_dns_reply_record(&reply, rr, NULL, rr->rtype, 10, &addr, 4);
+        }
+      }
+
+      mg_dns_send_reply(nc, &reply);
+      nc->flags |= MG_F_SEND_AND_CLOSE;
+      mbuf_free(&reply_buf);
+      break;
+    }
+  }
+}
+
+int main(int argc, char *argv[]) {
+  struct mg_mgr mgr;
+  struct mg_connection *c;
+
+  mg_mgr_init(&mgr, NULL);
+  c = mg_bind(&mgr, s_listening_addr, ev_handler);
+  mg_set_protocol_dns(c);
+
+  for (;;) {
+    mg_mgr_poll(&mgr, 1000);
+  }
+  mg_mgr_free(&mgr);
+
+  return 0;
+}
+```
+
+See full [Captive DNS server example](https://github.com/cesanta/mongoose/tree/master/examples/captive_dns_server).

docs/http/server_example.md

@@ -51,3 +51,5 @@ int main(void) {
   return 0;
 }
 ```
+
+See full [HTTP server example](https://github.com/cesanta/mongoose/tree/master/examples/simplest_web_server).