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+Multicast DNS for lwIP
+
+Author: Erik Ekman
+
+
+Note! The MDNS responder does not have all features required by the standards.
+See notes in src/apps/mdns/mdns.c for what is left. It is however usable in normal
+cases - but watch out if many devices on the same network try to use the same
+host/service instance names.
+
+
+How to enable:
+==============
+
+MDNS support does not depend on DNS.
+MDNS supports using IPv4 only, v6 only, or v4+v6.
+
+To enable MDNS responder, set
+  LWIP_MDNS_RESPONDER = 1
+in lwipopts.h and add src/apps/mdns/mdns.c to your list of files to build.
+
+The max number of services supported per netif is defined by MDNS_MAX_SERVICES,
+default is 1.
+
+Increase MEMP_NUM_UDP_PCB by 1. MDNS needs one PCB.
+Increase LWIP_NUM_NETIF_CLIENT_DATA by 1 (MDNS needs one entry on netif).
+
+MDNS with IPv4 requires LWIP_IGMP = 1, and preferably LWIP_AUTOIP = 1.
+MDNS with IPv6 requires LWIP_IPV6_MLD = 1, and that a link-local address is
+generated.
+
+The MDNS code puts its structs on the stack where suitable to reduce dynamic
+memory allocation. It may use up to 1kB of stack.
+
+MDNS needs a strncasecmp() implementation. If you have one, define
+LWIP_MDNS_STRNCASECMP to it. Otherwise the code will provide an implementation
+for you.
+
+
+How to use:
+===========
+
+Call mdns_resp_init() during system initialization.
+This opens UDP sockets on port 5353 for IPv4 and IPv6.
+
+
+To start responding on a netif, run
+  mdns_resp_add_netif(struct netif *netif, char *hostname, u32_t dns_ttl)
+
+The hostname will be copied. If this returns successfully, the netif will join
+the multicast groups and any MDNS/legacy DNS requests sent unicast or multicast
+to port 5353 will be handled:
+- <hostname>.local type A, AAAA or ANY returns relevant IP addresses
+- Reverse lookups (PTR in-addr.arpa, ip6.arpa) of netif addresses
+  returns <hostname>.local
+Answers will use the supplied TTL (in seconds)
+MDNS allows UTF-8 names, but it is recommended to stay within ASCII,
+since the default case-insensitive comparison assumes this.
+
+It is recommended to call this function after an IPv4 address has been set,
+since there is currently no check if the v4 address is valid.
+
+Call mdns_resp_netif_settings_changed() every time the IP address
+on the netif has changed.
+
+To stop responding on a netif, run
+  mdns_resp_remove_netif(struct netif *netif)
+
+
+Adding services:
+================
+
+The netif first needs to be registered. Then run
+  mdns_resp_add_service(struct netif *netif, char *name, char *service,
+      u16_t proto, u16_t port, u32_t dns_ttl,
+      service_get_txt_fn_t txt_fn, void *txt_userdata);
+
+The name and service pointers will be copied. Name refers to the name of the
+service instance, and service is the type of service, like _http
+proto can be DNSSD_PROTO_UDP or DNSSD_PROTO_TCP which represent _udp and _tcp.
+If this call returns successfully, the following queries will be answered:
+- _services._dns-sd._udp.local type PTR returns <service>.<proto>.local
+- <service>.<proto>.local type PTR returns <name>.<service>.<proto>.local
+- <name>.<service>.<proto>.local type SRV returns hostname and port of service
+- <name>.<service>.<proto>.local type TXT builds text strings by calling txt_fn
+  with the supplied userdata. The callback adds strings to the reply by calling
+  mdns_resp_add_service_txtitem(struct mdns_service *service, char *txt,
+   int txt_len). Example callback method:
+
+   static void srv_txt(struct mdns_service *service, void *txt_userdata)
+   {
+     res = mdns_resp_add_service_txtitem(service, "path=/", 6);
+     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.
+
+If your device runs a webserver on port 80, an example call might be:
+
+  mdns_resp_add_service(netif, "myweb", "_http"
+      DNSSD_PROTO_TCP, 80, 3600, srv_txt, NULL);
+
+which will publish myweb._http._tcp.local for any hosts looking for web servers,
+and point them to <hostname>.local:80
+
+Relevant information will be sent as additional records to reduce number of
+requests required from a client.
+
+Removing services is currently not supported. Services are removed when the
+netif is removed.
+