Public domain mDNS/DNS-SD library in C
This library provides a header only cross-platform mDNS and DNS-DS library in C. The latest source code is always available at
https://github.com/mjansson/mdns
This library is put in the public domain; you can redistribute it and/or modify it without any restrictions.
Created by Mattias Jansson (@maniccoder)
Discord server for discussions https://discord.gg/M8BwTQrt6c
Features
The library does DNS-SD discovery and service as well as one-shot single record mDNS query and response. There are no memory allocations done by the library, all buffers used must be passed in by the caller. Custom data for use in processing can be passed along using a user data opaque pointer.
Usage
The mdns.c test executable file demonstrates the use of all features, including discovery, query and service response. The documentation here is intentionally sparse, the example code is well documented and should provide all the details.
Sockets
Socket for mDNS communication can either be opened by the library by using mdns_socket_open_ipv4 or mdns_socket_open_ipv6, or by initializing an existing socket with mdns_socket_setup_ipv4 or mdns_socket_setup_ipv6. The socket open/setup functions will initialize the socket with multicast membership (including loopback) and set to non-blocking mode.
Call mdns_socket_close to close a socket opened with mdns_socket_open_ipv4 or mdns_socket_open_ipv6.
Port
To open/setup the socket for one-shot queries you can pass a null pointer socket address, or set the port in the passed socket address to 0. This will bind the socket to a random ephemeral local UDP port as required by the RFCs for one-shot queries. You should NOT bind to port 5353 when doing one-shot queries (see the RFC for details).,
To open/setup the socket for service, responding to incoming queries, you need pass in a socket address structure with the port set to 5353 (defined by MDNS_PORT in the header). You cannot pick any other port or you will not recieve any incoming queries.
Network interface
To do discovery or queries on the default network interface, you can pass a null pointer as socket address in the socket create/setup functions. This will bind the socket to the default network interface. Otherwise you should enumerate the available interfaces and pass the appropriate socket address to the create/setup function. See the example program in mdns.c for an example implementation of doing this for both IPv4 and IPv6.
If you want to do mDNS service response to incoming queries, you do not need to enumerate interfaces to do service response on all interfaces as sockets receive data from all interfaces. See the example program in mdns.c for an example of setting up a service socket for both IPv4 and IPv6.
Discovery
To send a DNS-SD service discovery request use mdns_discovery_send. This will send a single multicast packet (single PTR question record for _services._dns-sd._udp.local.) requesting a unicast response.
To read discovery responses use mdns_discovery_recv. All records received since last call will be piped to the callback supplied in the function call. The entry type will be one of MDNS_ENTRYTYPE_ANSWER, MDNS_ENTRYTYPE_AUTHORITY and MDNS_ENTRYTYPE_ADDITIONAL.
Query
To send a one-shot mDNS query for a single record use mdns_query_send. This will send a single multicast packet for the given record and name (for example PTR record for _http._tcp.local.). You can optionally pass in a query ID for the query for later filtering of responses (even though this is discouraged by the RFC), or pass 0 to be fully compliant. The function returns the query ID associated with this query, which if non-zero can be used to filter responses in mdns_query_recv. If the socket is bound to port 5353 a multicast response is requested, otherwise a unicast response.
To read query responses use mdns_query_recv. All records received since last call will be piped to the callback supplied in the function call. If query_id parameter is non-zero the function will filter out any response with a query ID that does not match the given query ID. The entry type will be one of MDNS_ENTRYTYPE_ANSWER, MDNS_ENTRYTYPE_AUTHORITY and MDNS_ENTRYTYPE_ADDITIONAL.
Note that a socket opened for one-shot queries from an emphemeral port will not recieve any unsolicited answers (announces) as these are sent as a multicast on port 5353.
To send multiple queries in the same packet use mdns_multiquery_send which takes an array and count of service names and record types to query for.
Service
To listen for incoming DNS-SD requests and mDNS queries the socket can be opened/setup on the default interface by passing 0 as socket address in the call to the socket open/setup functions (the socket will receive data from all network interfaces). 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 indicates which record to respond with. The example program responds to SRV, PTR, A and AAAA records. Use the mdns_string_extract 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_unicast or mdns_query_answer_multicast depending on the response type flag in the question 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.
Announce
If you provide a mDNS service listening and answering queries on port 5353 it is encouraged to send announcement on startup of your service (as an unsolicited answer). Use the mdns_announce_multicast to announce the records for your service at startup, and mdns_goodbye_multicast to announce the end of service on termination.
Test executable
The mdns.c file contains a test executable implementation using the library to do DNS-SD and mDNS queries. Compile into an executable and run to see command line options for discovery, query and service modes.
Windows
Microsoft compiler
cl mdns.c /Zi /Fdmdns.pdb /link /out:mdns.exe ws2_32.lib iphlpapi.lib
Linux
GCC
gcc -o mdns mdns.c
clang
clang -o mdns mdns.c
Using with cmake, conan or vcpkg
- use cmake with
FetchContentor install andfind_package - use conan with dependency name
mdns/20200130, andfind_package-> https://conan.io/center/mdns/20200130 - use with vcpkg and cmake:
vcpkg install mdnsandfind_package