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A simple implementation of an mDNS (multicast Domain Name System) server.
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README.md

MdnsLite

CircleCI

MdnsLite is a simple, limited, no frills implementation of an mDNS (multicast Domain Name System) server. It operates like a DNS server, the difference being that it uses multicast instead of unicast and is meant to be the DNS server for the .local domain. MdnsLite also provides for the advertising (discovery) of services offered by the host system. Examples of services are an HTTP or an SSH server. Read about configuring services in the Configuration section below.

MdnsLite employs a network interface monitor that can dynamically adjust to network changes, e.g., assignment of a new IP address to a host. You can add your own Network Monitor; a future release will allow the use of another implementation.

It recognizes the following query types:

  • A - Find the IPv4 address of a hostname.
  • PTR - Given an IPv4 address, find its hostname. If, however, it receives a request domain of "_services._dns-sd._udp.local", MdnsLite will respond with a list of every service available (and specified in the configuration) on the host.
  • SRV - Service Locator

If you want to know the details of the various DNS/mDNS record types and their fields, a good source is zytrax.com/books/dns.

There are at least a couple of other Elixir/Erlang implementations of mDNS servers:

  1. Rosetta Home mdns (Elixir)
  2. Shortishly mdns (Erlang)

These implementations provided valuable guidance in the building of MdnsLite.

Configuration

A typical configuration in the config.exs file looks like:

config :mdns_lite,
  # Use these values to construct the DNS resource record responses
  # to a DNS query.
  mdns_config: %{
    host: :hostname,
    ttl: 120
  },
  services: [
    # service type: _http._tcp.local - used in match
    %{
      name: "Web Server",
      protocol: "http",
      transport: "tcp",
      port: 80,
    },
    # service_type: _ssh._tcp.local - used in match
    %{
      name: "Secure Socket",
      protocol: "ssh",
      transport: "tcp",
      port: 22,
    }
  ]

The mdns_config section is where you specify values that will be used in the construction of mDNS (DNS) responses. host can be :hostname in which case the value will be replaced with the value of :inet.gethostname(), otherwise you can provide a string value. ttl refers to a Time To Live value in seconds. RFC 6762 - Multicast DNS - recommends a default value of 120 seconds.

The services section lists the services that the host offers, such as providing an HTTP server. You must supply the protocol, transport and port values for each service.

MdnsLite uses a "network monitor", a module that listens for changes in a network. Its purpose is to ensure that the network interfaces are up to date. The current version of MdnsLite has an InetMonitor which periodically checks via inet:getifaddrs() for changes in the network. A change could be the re-assignment of IP addresses.

Installation

If available in Hex, the package can be installed by adding mdns_lite to your list of dependencies in mix.exs:

def deps do
  [
    {:mdns_lite, "~> 0.1.0"}
  ]
end

Usage

MdnsLite is an Elixir/Erlang application, hence it will start up automatically when its enclosing application starts.

When MdnsLite is running, it can be tested using the linux dig utility:

dig @224.0.0.251 -p 5353 -t A petes-pt.local => 192.168.0.102

dig @224.0.0.251 -p 5353 -x 192.168.0.102 => petes-pt.local (reverse lookup)

dig @224.0.0.251 -p 5353 -t SRV _http._tcp.local => Depends on the service(s) available

Although dig is a lookup utility for DNS, it will send requests that MdnsLite can receive. Note, however, that you need to specify the ip address (224.0.0.251) and port(5353) when using dig to get mDNS responses.

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/mdns_lite.

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