mDNS Service

Overview

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.

mDNS Properties

  • hostname: the hostname that the device will respond to. If not set, the hostname will be read from the interface. Example: my-esp32 will resolve to my-esp32.local
  • default_instance: friendly name for your device, like Jhon'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:

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 Services

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 on
  • instance: friendly name for your service, like Jhon'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 Query

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");
}

Application Example

mDNS server/scanner example: protocols/mdns.

API Reference

Header Files

Macros

Type Definitions

Warning

doxygentypedef: Cannot find typedef “mdns_server_t” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygentypedef: Cannot find typedef “mdns_result_t” in doxygen xml output for project “esp32-idf” from directory: xml/

Enumerations

Structures

Warning

doxygenstruct: Cannot find class “mdns_result_s” in doxygen xml output for project “esp32-idf” from directory: xml/

Functions

Warning

doxygenfunction: Cannot find function “mdns_init” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_free” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_set_hostname” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_set_instance” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_service_add” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_service_remove” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_service_instance_set” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_service_txt_set” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_service_port_set” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_service_remove_all” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_query” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_query_end” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_result_get_count” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_result_get” in doxygen xml output for project “esp32-idf” from directory: xml/

Warning

doxygenfunction: Cannot find function “mdns_result_free” in doxygen xml output for project “esp32-idf” from directory: xml/