mdns: Allow TXT entries larger than 63 bytes; add configurable maximum TXT RDATA size

This can be used to fix device adoption in ESPHome, which uses a long
"package_import_url" string to advertise the location of the sourcecode
for the device's configuration.

doc/mdns.txt

@@ -92,9 +92,9 @@ If this call returns successfully, the following queries will be answered:
      LWIP_ERROR("mdns add service txt failed\n", (res == ERR_OK), return);
    }
 
-  Since a hostname struct is used for TXT storage each single item can be max
-  63 bytes long, and  the total max length (including length bytes for each
-  item) is 255 bytes.
+  Each item is encoded as a length byte followed by the data, so each single
+  item can be max 255 bytes long, and the total max length (including length
+  bytes for each item) is defined by MDNS_TXT_RDATA_SIZE (default 255).
 
 If your device runs a webserver on port 80, an example call might be:
 

src/apps/mdns/mdns.c

@@ -1272,7 +1272,7 @@ mdns_parse_pkt_known_answers(struct netif *netif, struct mdns_packet *pkt,
         } else if (match & REPLY_SERVICE_TXT) {
           mdns_prepare_txtdata(service);
           if (service->txtdata.length == ans.rd_length &&
-              pbuf_memcmp(pkt->pbuf, ans.rd_offset, service->txtdata.name, ans.rd_length) == 0) {
+              pbuf_memcmp(pkt->pbuf, ans.rd_offset, service->txtdata.rdata, ans.rd_length) == 0) {
             LWIP_DEBUGF(MDNS_DEBUG, ("MDNS: Skipping known answer: TXT\n"));
             reply->serv_replies[i] &= ~REPLY_SERVICE_TXT;
           }
@@ -2050,7 +2050,7 @@ mdns_handle_response(struct mdns_packet *pkt, struct netif *netif)
           } else if (ans.info.type == DNS_RRTYPE_TXT) {
             mdns_prepare_txtdata(service);
             if (service->txtdata.length == ans.rd_length &&
-                pbuf_memcmp(pkt->pbuf, ans.rd_offset, service->txtdata.name, ans.rd_length) == 0) {
+                pbuf_memcmp(pkt->pbuf, ans.rd_offset, service->txtdata.rdata, ans.rd_length) == 0) {
               LWIP_DEBUGF(MDNS_DEBUG, ("mDNS: response equals our own TXT record -> no conflict\n"));
               conflict = 0;
             }
@@ -2613,10 +2613,29 @@ mdns_resp_rename_service(struct netif *netif, u8_t slot, const char *name)
   return ERR_OK;
 }
 
+/* Adds an RFC 1035 character-string to TXT RDATA. */
+static err_t
+mdns_txt_add_charstr(struct mdns_txtdata *txtdata, const char *value, u8_t len)
+{
+  if (1 + len + txtdata->length > MDNS_TXT_RDATA_SIZE) {
+    LWIP_DEBUGF(MDNS_DEBUG, ("mdns_txt_add_charstr: adding string would exceed buffer (1+%d+%d > %d). Consider increasing MDNS_TXT_RDATA_SIZE.\n",
+                             len, txtdata->length, MDNS_TXT_RDATA_SIZE));
+    return ERR_MEM;
+  }
+  txtdata->rdata[txtdata->length] = len;
+  txtdata->length++;
+  if (len) {
+    MEMCPY(&txtdata->rdata[txtdata->length], value, len);
+    txtdata->length += len;
+  }
+  return ERR_OK;
+}
+
 /**
  * @ingroup mdns
  * Call this function from inside the service_get_txt_fn_t callback to add text data.
- * Buffer for TXT data is 256 bytes, and each field is prefixed with a length byte.
+ * Buffer for TXT data is MDNS_TXT_RDATA_SIZE (default 256) bytes, and each
+ * field is prefixed with a length byte.
  * @param service The service provided to the get_txt callback
  * @param txt String to add to the TXT field.
  * @param txt_len Length of string
@@ -2629,7 +2648,7 @@ mdns_resp_add_service_txtitem(struct mdns_service *service, const char *txt, u8_
   LWIP_ASSERT("mdns_resp_add_service_txtitem: service != NULL", service);
 
   /* Use a mdns_domain struct to store txt chunks since it is the same encoding */
-  return mdns_domain_add_label(&service->txtdata, txt, txt_len);
+  return mdns_txt_add_charstr(&service->txtdata, txt, txt_len);
 }
 
 #if LWIP_MDNS_SEARCH

src/apps/mdns/mdns_out.c

@@ -62,7 +62,7 @@ static void mdns_clear_outmsg(struct mdns_outmsg *outmsg);
 void
 mdns_prepare_txtdata(struct mdns_service *service)
 {
-  memset(&service->txtdata, 0, sizeof(struct mdns_domain));
+  memset(&service->txtdata, 0, sizeof(struct mdns_txtdata));
   if (service->txt_fn) {
     service->txt_fn(service, service->txt_userdata);
   }
@@ -508,7 +508,7 @@ mdns_add_txt_answer(struct mdns_outpacket *reply, struct mdns_outmsg *msg,
   }
   LWIP_DEBUGF(MDNS_DEBUG, ("MDNS: Responding with TXT record\n"));
   return mdns_add_answer(reply, &service_instance, DNS_RRTYPE_TXT, DNS_RRCLASS_IN,
-                         msg->cache_flush, ttl, (u8_t *) &service->txtdata.name,
+                         msg->cache_flush, ttl, service->txtdata.rdata,
                          service->txtdata.length, NULL);
 }
 

src/include/lwip/apps/mdns_opts.h

@@ -88,6 +88,12 @@
 #define MDNS_OUTPUT_PACKET_SIZE      ((MDNS_MAX_SERVICES == 1) ? 512 : 1450)
 #endif
 
+/** The maximum size of TXT RDATA allocated for each service.
+ */
+#ifndef MDNS_TXT_RDATA_SIZE
+# define MDNS_TXT_RDATA_SIZE 256
+#endif
+
 /** MDNS_RESP_USENETIF_EXTCALLBACK==1: register an ext_callback on the netif
  * to automatically restart probing/announcing on status or address change.
  */

src/include/lwip/apps/mdns_priv.h

@@ -90,10 +90,16 @@ struct mdns_request {
 };
 #endif
 
+/** TXT record data */
+struct mdns_txtdata {
+  u8_t rdata[MDNS_TXT_RDATA_SIZE];
+  u16_t length;
+};
+
 /** Description of a service */
 struct mdns_service {
   /** TXT record to answer with */
-  struct mdns_domain txtdata;
+  struct mdns_txtdata txtdata;
   /** Name of service, like 'myweb' */
   char name[MDNS_LABEL_MAXLEN + 1];
   /** Type of service, like '_http' */