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https://github.com/spider-gazelle/mdns

Crystal Lang mDNS and DNS-SD Support
https://github.com/spider-gazelle/mdns

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Crystal Lang mDNS and DNS-SD Support

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# Crystal Lang mDNS Support



[![CI](https://github.com/spider-gazelle/mdns/actions/workflows/ci.yml/badge.svg)](https://github.com/spider-gazelle/mdns/actions/workflows/ci.yml)



* Discover services using DNS-SD



DNS-SD is used where there might be multiple devices implementing a service.



* Lookup devices using mDNS



mDNS is used to find the IP address of device, like a Raspberry Pi



## Installation



1. Add the dependency to your `shard.yml`:



   ```yaml

   dependencies:

     mDNS:

       github: spider-gazelle/mdns

   ```



2. Run `shards install`



## Single Shot usage



As described in [the rfc](https://tools.ietf.org/html/rfc6762#section-5.1)



### mDNS query



If we are looking for the IP address of a device for instance.



```crystal



# We're requesting the IPv4 address of the phone (A record)

# We could also do a multi-request and get the IPv6 address too (AAAA record)

results = MDNS.one_shot "Steves-iPhone.local", type: MDNS::Type::A



if address_response = results.first?

  # NOTE:: the `_address` here is the device that responded, it might not be an

  # authoritative answer (i.e. something replying on behalf of the device) so

  # extract the IP address from the payload (mDNS only, for DNS-SD use the `_address`)

  _address, response = address_response



  # This is assuming answers[0] is an A or AAAA record

  response.answers[0].payload.as(Socket::IPAddress).address # => "192.168.40.150"

end



```



Another common request, if you want the port of the service, is the SRV query.

Use the SRV query in tandem with the DNS-SD request described below



### Simple DNS-SD query



To perform a simple query for a service on the network



```crystal



require "mdns"



# Look up homekit devices on the network

results = MDNS.one_shot "_hap._tcp.local"



results.each do |(address, response)|

  address # => Socket::IPAddress

  response # => MDNS::Message



  response.is_response # => true

  response.authoritative_answer # => true / false

  response.response_code # => ResponseCode::NoError

  response.answer_count # => 1

  response.answers # => Array(MDNS::Resource)



  # The answers will most likely be PTRs to the devices local domain names

  # Then you can perform a SRV query for the service details of the pointer

end



```



### Multiple service queries



In a single request



```crystal



# Look up homekit devices and hubs on the network

results = MDNS.one_shot do |message|

  message.query "_hap._tcp.local"

  message.query "_homekit._tcp.local"

end



# You will need to inspect the answers to differentiate between devices and hubs

results.each do |(address, response)|

  is_hap = false

  is_kit = false



  response.answers.each do |answer|

    domain_name = answer.domain_name



    if domain_name == "_hap._tcp.local"

      is_hap = true

    elsif domain_name == "_homekit._tcp.local"

      is_kit = true

    end

  end

end



```



## Continuous Multicast DNS Querying



This is only the most basic of servers implementing the transport layer.

It could definitely be used as the basis for a DNS cache as described by the RFC



```crystal



require "mdns"



server = MDNS::Server.new(MDNS::IPv4)

loop do

  break if server.closed?

  address, message = server.receive



  if message.query?

    # process a query here

  else

    # process responses here

  end



  # This code prints the details of the message

  puts Time.utc.to_s("%X")

  puts "QUERY:"

  puts message.queries.map(&.domain_name).join("\n")

  puts "ANSWERS:"

  puts message.answers.map { |answer|

    String.build do |str|

      str << answer.domain_name

    	str << " => (#{answer.type}) "

      str << answer.payload.inspect

    end

  }.join("\n")

  puts "\n\n\n"

end



```



See the [example](https://github.com/spider-gazelle/mdns/tree/master/examples) for how to register a service on your local network.