davidgraeff / mdnscpp

MDNS Service discovery and publish library for C++17

Public domain mDNS/DNS-SD library for C++

This is a mDNS / service discovery and publish header-only C++17 library. Optional parts of C++20 are used (concepts).

This library is put in the public domain; you can redistribute it and/or modify it without any restrictions.

Features

The library does DNS-SD discovery and service as well as single record mDNS query and response.

• Memory management is customizable via templates, with a pre-allocated buffer default implementation.
• Socket communication is customizable via templates. Either existing BSD Socket API handlers can be used or the library manages sockets for you with the default implementation. The socket is initialized with multicast membership (including loopback) and set to non-blocking mode.

Usage

The examples/discovery.cpp and examples/publish.cpp demonstrates the use of all features, including discovery, query and service response. Build this repo with CMake and find the example executables in your build directory.

Initialize the library

First create a custom type with the features that you require:

using Mdns = MdnsCpp<MdnsFixedSizeBuffer<3>, MdnsManagedSocket, MdnsThreadSafety>

The first template argument defines the memory management strategy. You can chose between MdnsFixedSizeBuffer and MdnsDynamicMemory.

The second argument is either MdnsManagedSocket or a custom type that implements the same methods and sub-types like MdnsManagedSocket.

Discovery

To send a DNS-SD service discovery request use mdns.discover(). This will send a single multicast packet (single question record for _services._dns-sd._udp.local.).

Use the returned Mdns::QueryProcess type and the method receive() -> Optional<Mdns::QueryProcess> to query for responses. If there is another record available since the last call, it will be returned.

The second entry type will be one of MDNS_ENTRYTYPE_ANSWER, MDNS_ENTRYTYPE_AUTHORITY and MDNS_ENTRYTYPE_ADDITIONAL.

Query

To send a mDNS query for a single record use mdns.query(record : string_view) with record = _http._tcp.local. for example. This will send a single multicast query packet for the given record with a unique transaction id.

Use the returned Mdns::QueryProcess type and the method receive() -> Optional<Mdns::QueryResult> to query for responses (for the unique transaction id determined earlier). If there is another record available since the last call, it will be returned.

The second entry type will be one of MDNS_ENTRYTYPE_ANSWER, MDNS_ENTRYTYPE_AUTHORITY and MDNS_ENTRYTYPE_ADDITIONAL.

Service

If you use the default socket implementation, using this library in service / publish mode is straight-forward.

Call mdns.publish(service) to

To listen for incoming DNS-SD requests and mDNS queries the socket should be opened on port 5353 (default) in call to the socket open/setup functions. Then call mdns_socket_listen either on notification of incoming data, or by setting blocking mode and calling mdns_socket_listen to block until data is available and parsed.

The entry type passed to the callback will be MDNS_ENTRYTYPE_QUESTION and record type MDNS_RECORDTYPE_PTR. Use the mdns_record_parse_ptr function to get the name string of the service record that was asked for.

If service record name is _services._dns-sd._udp.local. you should use mdns_discovery_answer to send the records of the services you provide (DNS-SD).

If the service record name is a service you provide, use mdns_query_answer to send the service details back in response to the query.

See the test executable implementation for more details on how to handle the parameters to the given functions.