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 File

Functions

esp_err_t mdns_init(tcpip_adapter_if_t tcpip_if, mdns_server_t **server)

Initialize mDNS on given interface.

Return
  • ESP_OK on success
  • ESP_ERR_INVALID_ARG when bad tcpip_if is given
  • ESP_ERR_INVALID_STATE when the network returned error
  • ESP_ERR_NO_MEM on memory error
  • ESP_ERR_WIFI_NOT_INIT when WiFi is not initialized by eps_wifi_init
Parameters
  • tcpip_if: Interface that the server will listen on
  • server: Server pointer to populate on success

void mdns_free(mdns_server_t *server)

Stop and free mDNS server.

Parameters
  • server: mDNS Server to free

esp_err_t mdns_set_hostname(mdns_server_t *server, const char *hostname)

Set the hostname for mDNS server.

Return
  • ESP_OK success
  • ESP_ERR_INVALID_ARG Parameter error
  • ESP_ERR_NO_MEM memory error
Parameters
  • server: mDNS Server
  • hostname: Hostname to set

esp_err_t mdns_set_instance(mdns_server_t *server, const char *instance)

Set the default instance name for mDNS server.

Return
  • ESP_OK success
  • ESP_ERR_INVALID_ARG Parameter error
  • ESP_ERR_NO_MEM memory error
Parameters
  • server: mDNS Server
  • instance: Instance name to set

esp_err_t mdns_service_add(mdns_server_t *server, const char *service, const char *proto, uint16_t port)

Add service to mDNS server.

Return
  • ESP_OK success
  • ESP_ERR_INVALID_ARG Parameter error
  • ESP_ERR_NO_MEM memory error
Parameters
  • server: mDNS Server
  • service: service type (_http, _ftp, etc)
  • proto: service protocol (_tcp, _udp)
  • port: service port

esp_err_t mdns_service_remove(mdns_server_t *server, const char *service, const char *proto)

Remove service from mDNS server.

Return
  • ESP_OK success
  • ESP_ERR_INVALID_ARG Parameter error
  • ESP_ERR_NOT_FOUND Service not found
  • ESP_FAIL unknown error
Parameters
  • server: mDNS Server
  • service: service type (_http, _ftp, etc)
  • proto: service protocol (_tcp, _udp)

esp_err_t mdns_service_instance_set(mdns_server_t *server, const char *service, const char *proto, const char *instance)

Set instance name for service.

Return
  • ESP_OK success
  • ESP_ERR_INVALID_ARG Parameter error
  • ESP_ERR_NOT_FOUND Service not found
  • ESP_ERR_NO_MEM memory error
Parameters
  • server: mDNS Server
  • service: service type (_http, _ftp, etc)
  • proto: service protocol (_tcp, _udp)
  • instance: instance name to set

esp_err_t mdns_service_txt_set(mdns_server_t *server, const char *service, const char *proto, uint8_t num_items, const char **txt)

Set TXT data for service.

Return
  • ESP_OK success
  • ESP_ERR_INVALID_ARG Parameter error
  • ESP_ERR_NOT_FOUND Service not found
  • ESP_ERR_NO_MEM memory error
Parameters
  • server: mDNS Server
  • service: service type (_http, _ftp, etc)
  • proto: service protocol (_tcp, _udp)
  • num_items: number of items in TXT data
  • txt: string array of TXT data (eg. {“var=val”,”other=2”})

esp_err_t mdns_service_port_set(mdns_server_t *server, const char *service, const char *proto, uint16_t port)

Set service port.

Return
  • ESP_OK success
  • ESP_ERR_INVALID_ARG Parameter error
  • ESP_ERR_NOT_FOUND Service not found
Parameters
  • server: mDNS Server
  • service: service type (_http, _ftp, etc)
  • proto: service protocol (_tcp, _udp)
  • port: service port

esp_err_t mdns_service_remove_all(mdns_server_t *server)

Remove and free all services from mDNS server.

Return
  • ESP_OK success
  • ESP_ERR_INVALID_ARG Parameter error
Parameters
  • server: mDNS Server

size_t mdns_query(mdns_server_t *server, const char *service, const char *proto, uint32_t timeout)

Query mDNS for host or service.

Return
the number of results found
Parameters
  • server: mDNS Server
  • service: service type or host name
  • proto: service protocol or NULL if searching for host
  • timeout: time to wait for answers. If 0, mdns_query_end MUST be called to end the search

size_t mdns_query_end(mdns_server_t *server)

Stop mDNS Query started with timeout = 0.

Return
the number of results found
Parameters
  • server: mDNS Server

size_t mdns_result_get_count(mdns_server_t *server)

get the number of results currently in memoty

Return
the number of results
Parameters
  • server: mDNS Server

const mdns_result_t *mdns_result_get(mdns_server_t *server, size_t num)

Get mDNS Search result with given index.

Return
the result or NULL if error
Parameters
  • server: mDNS Server
  • num: the index of the result

esp_err_t mdns_result_free(mdns_server_t *server)

Remove and free all search results from mDNS server.

Return
  • ESP_OK success
  • ESP_ERR_INVALID_ARG Parameter error
Parameters
  • server: mDNS Server

Structures

struct mdns_result_s

mDNS query result structure

Public Members

const char *host

hostname

const char *instance

instance

const char *txt

txt data

uint16_t priority

service priority

uint16_t weight

service weight

uint16_t port

service port

struct ip4_addr addr

ip4 address

struct ip6_addr addrv6

ip6 address

const struct mdns_result_s *next

next result, or NULL for the last result in the list

Type Definitions

typedef struct mdns_server_s mdns_server_t
typedef struct mdns_result_s mdns_result_t

mDNS query result structure