mDNS is a multicast UDP service that is used to provide local network service and host discovery.
mDNS is installed by default on most operating systems or is available as separate package. On Mac OS
it is installed by default and is called Bonjour
. Apple releases an installer for Windows
that can be found on Apple's support page. On Linux
, mDNS is provided by avahi and is usually installed by default.
hostname
: the hostname that the device will respond to. If not set, thehostname
will be read from the interface. Example:my-esp32
will resolve tomy-esp32.local
default_instance
: friendly name for your device, likeJhon's ESP32 Thing
. If not set,hostname
will be used.
Example method to start mDNS for the STA interface and set hostname
and default_instance
:
c
mdns_server_t * mdns = NULL;
void start_mdns_service()
{
//initialize mDNS service on STA interface
esp_err_t err = mdns_init(TCPIP_ADAPTER_IF_STA, &mdns);
if (err) {
printf("MDNS Init failed: %d\n", err);
return;
}
//set hostname
mdns_set_hostname(mdns, "my-esp32");
//set default instance
mdns_set_instance(mdns, "Jhon's ESP32 Thing");
}
mDNS can advertise information about network services that your device offers. Each service is defined by a few properties.
service
: (required) service type, prepended with underscore. Some common types can be found here.proto
: (required) protocol that the service runs on, prepended with underscore. Example:_tcp
or_udp
port
: (required) network port that the service runs oninstance
: friendly name for your service, likeJhon's ESP32 Web Server
. If not defined,default_instance
will be used.txt
:var=val
array of strings, used to define properties for your service
Example method to add a few services and different properties:
void add_mdns_services()
{
//add our services
mdns_service_add(mdns, "_http", "_tcp", 80);
mdns_service_add(mdns, "_arduino", "_tcp", 3232);
mdns_service_add(mdns, "_myservice", "_udp", 1234);
//NOTE: services must be added before their properties can be set
//use custom instance for the web server
mdns_service_instance_set(mdns, "_http", "_tcp", "Jhon's ESP32 Web Server");
const char * arduTxtData[4] = {
"board=esp32",
"tcp_check=no",
"ssh_upload=no",
"auth_upload=no"
};
//set txt data for service (will free and replace current data)
mdns_service_txt_set(mdns, "_arduino", "_tcp", 4, arduTxtData);
//change service port
mdns_service_port_set(mdns, "_myservice", "_udp", 4321);
}
mDNS provides methods for browsing for services and resolving host's IP/IPv6 addresses.
Results are returned as a linked list of mdns_result_t
objects. If the result is from host query, it will contain only addr
and addrv6
if found. Service queries will populate all fields in a result that were found.
Example method to resolve host IPs:
void resolve_mdns_host(const char * hostname)
{
printf("mDNS Host Lookup: %s.local\n", hostname);
//run search for 1000 ms
if (mdns_query(mdns, hostname, NULL, 1000)) {
//results were found
const mdns_result_t * results = mdns_result_get(mdns, 0);
//itterate through all results
size_t i = 1;
while(results) {
//print result information
printf(" %u: IP:" IPSTR ", IPv6:" IPV6STR "\n", i++
IP2STR(&results->addr), IPV62STR(results->addrv6));
//load next result. Will be NULL if this was the last one
results = results->next;
}
//free the results from memory
mdns_result_free(mdns);
} else {
//host was not found
printf(" Host Not Found\n");
}
}
Example method to resolve local services:
void find_mdns_service(const char * service, const char * proto)
{
printf("mDNS Service Lookup: %s.%s\n", service, proto);
//run search for 1000 ms
if (mdns_query(mdns, service, proto, 1000)) {
//results were found
const mdns_result_t * results = mdns_result_get(mdns, 0);
//itterate through all results
size_t i = 1;
while(results) {
//print result information
printf(" %u: hostname:%s, instance:\"%s\", IP:" IPSTR ", IPv6:" IPV6STR ", port:%u, txt:%s\n", i++,
(results->host)?results->host:"NULL", (results->instance)?results->instance:"NULL",
IP2STR(&results->addr), IPV62STR(results->addrv6),
results->port, (results->txt)?results->txt:"\r");
//load next result. Will be NULL if this was the last one
results = results->next;
}
//free the results from memory
mdns_result_free(mdns);
} else {
//service was not found
printf(" Service Not Found\n");
}
}
Example of using the methods above:
void my_app_some_method(){
//search for esp32-mdns.local
resolve_mdns_host("esp32-mdns");
//search for HTTP servers
find_mdns_service("_http", "_tcp");
//or file servers
find_mdns_service("_smb", "_tcp"); //windows sharing
find_mdns_service("_afpovertcp", "_tcp"); //apple sharing
find_mdns_service("_nfs", "_tcp"); //NFS server
find_mdns_service("_ftp", "_tcp"); //FTP server
//or networked printer
find_mdns_service("_printer", "_tcp");
find_mdns_service("_ipp", "_tcp");
}
mDNS server/scanner example: protocols/mdns
.