114 lines
4.3 KiB
Text
114 lines
4.3 KiB
Text
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Multicast DNS for lwIP
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Author: Erik Ekman
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Note! The MDNS responder does not have all features required by the standards.
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See notes in src/apps/mdns/mdns.c for what is left. It is however usable in normal
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cases - but watch out if many devices on the same network try to use the same
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host/service instance names.
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How to enable:
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==============
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MDNS support does not depend on DNS.
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MDNS supports using IPv4 only, v6 only, or v4+v6.
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To enable MDNS responder, set
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LWIP_MDNS_RESPONDER = 1
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in lwipopts.h and add src/apps/mdns/mdns.c to your list of files to build.
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The max number of services supported per netif is defined by MDNS_MAX_SERVICES,
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default is 1.
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Increase MEMP_NUM_UDP_PCB by 1. MDNS needs one PCB.
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Increase LWIP_NUM_NETIF_CLIENT_DATA by 1 (MDNS needs one entry on netif).
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MDNS with IPv4 requires LWIP_IGMP = 1, and preferably LWIP_AUTOIP = 1.
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MDNS with IPv6 requires LWIP_IPV6_MLD = 1, and that a link-local address is
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generated.
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The MDNS code puts its structs on the stack where suitable to reduce dynamic
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memory allocation. It may use up to 1kB of stack.
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MDNS needs a strncasecmp() implementation. If you have one, define
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LWIP_MDNS_STRNCASECMP to it. Otherwise the code will provide an implementation
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for you.
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How to use:
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===========
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Call mdns_resp_init() during system initialization.
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This opens UDP sockets on port 5353 for IPv4 and IPv6.
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To start responding on a netif, run
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mdns_resp_add_netif(struct netif *netif, char *hostname, u32_t dns_ttl)
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The hostname will be copied. If this returns successfully, the netif will join
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the multicast groups and any MDNS/legacy DNS requests sent unicast or multicast
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to port 5353 will be handled:
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- <hostname>.local type A, AAAA or ANY returns relevant IP addresses
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- Reverse lookups (PTR in-addr.arpa, ip6.arpa) of netif addresses
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returns <hostname>.local
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Answers will use the supplied TTL (in seconds)
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MDNS allows UTF-8 names, but it is recommended to stay within ASCII,
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since the default case-insensitive comparison assumes this.
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It is recommended to call this function after an IPv4 address has been set,
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since there is currently no check if the v4 address is valid.
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Call mdns_resp_netif_settings_changed() every time the IP address
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on the netif has changed.
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To stop responding on a netif, run
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mdns_resp_remove_netif(struct netif *netif)
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Adding services:
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================
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The netif first needs to be registered. Then run
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mdns_resp_add_service(struct netif *netif, char *name, char *service,
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u16_t proto, u16_t port, u32_t dns_ttl,
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service_get_txt_fn_t txt_fn, void *txt_userdata);
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The name and service pointers will be copied. Name refers to the name of the
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service instance, and service is the type of service, like _http
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proto can be DNSSD_PROTO_UDP or DNSSD_PROTO_TCP which represent _udp and _tcp.
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If this call returns successfully, the following queries will be answered:
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- _services._dns-sd._udp.local type PTR returns <service>.<proto>.local
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- <service>.<proto>.local type PTR returns <name>.<service>.<proto>.local
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- <name>.<service>.<proto>.local type SRV returns hostname and port of service
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- <name>.<service>.<proto>.local type TXT builds text strings by calling txt_fn
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with the supplied userdata. The callback adds strings to the reply by calling
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mdns_resp_add_service_txtitem(struct mdns_service *service, char *txt,
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int txt_len). Example callback method:
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static void srv_txt(struct mdns_service *service, void *txt_userdata)
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{
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res = mdns_resp_add_service_txtitem(service, "path=/", 6);
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LWIP_ERROR("mdns add service txt failed\n", (res == ERR_OK), return);
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}
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Since a hostname struct is used for TXT storage each single item can be max
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63 bytes long, and the total max length (including length bytes for each
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item) is 255 bytes.
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If your device runs a webserver on port 80, an example call might be:
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mdns_resp_add_service(netif, "myweb", "_http"
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DNSSD_PROTO_TCP, 80, 3600, srv_txt, NULL);
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which will publish myweb._http._tcp.local for any hosts looking for web servers,
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and point them to <hostname>.local:80
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Relevant information will be sent as additional records to reduce number of
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requests required from a client.
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Removing services is currently not supported. Services are removed when the
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netif is removed.
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