MdnsLite

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. The current version of MdnsLite supports two default monitors:

InetMonitor which periodically checks via inet:getifaddrs() for changes in the network. For example, a change could be the re-assignment of IP addresses.
VintageNetMonitor which subscribes to VintageNet address events for all interfaces as they happen.

For configuration values related to the interface monitor, please see the Configuration section below.

MdnsLite recognizes the following query types:

A - Find the IPv4 address of a hostname.
PTR - Given an IPv4 address, find its hostname - reverse lookup. If, however, it receives a request domain of "_services._dns-sd._udp.local", MdnsLite will respond with a list of every service available (and is 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:

Rosetta Home mdns (Elixir)
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.
  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,
    }
  ]

(Note that the configuration changed from v0.2 to v0.3, eliminating a superfluous map.)

The values of host and ttl will be used in the construction of mDNS (DNS) responses.

host can have the value of :hostname in which case the value will be replaced with the value of :inet.gethostname(), otherwise you can provide a string value. You can specify an alias hostname in which case host will be ["hostname", "alias-example"]. The second value must be a string. When you use an alias, an "A" query can be made to alias-example.local as well as to hostname.local. This can also be configured at runtime via MdnsLite.set_host/1:

iex> MdnsLite.set_host([:hostname, "nerves"])
:ok

ttl refers to a Time To Live value in seconds. RFC 6762 - Multicast DNS - recommends a default value of 120 seconds.

As mentioned above, MdnsLite uses an interface monitor. The configuration value ip_address_monitor (not shown) defaults to VintageNetMonitor if VintageNet is added as a dependency to your project using MdnsLite. Otherwise, InetMonitor will be the default. The configuration value excluded_ifnames represents those interfaces that the monitor will not watch. Its default value is ["lo0", "lo"].

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. You may also specify weight and/or host. They each default to a zero value. Please consult the RFC for an explanation of these values. Services can be configured in config.exs as shown above, or at runtime:

iex> 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,
  }
]

iex> MdnsLite.add_mds_services(services)
:ok

Services can also be removed at runtime via remove_mdns_services/1 with the service name to remove:

iex> service_names = ["Web Server", "Secure Socket"]
iex> MdnsLite.remove_mdns_services(services)
:ok

# Remove just a single service
iex> MdnsLite.remove_mdns_services("Secure Socket")
:ok

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, "~> 0.4"}
  ]
end

Usage

MdnsLite is an Elixir/Erlang application; 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 nerves-7fcb.local
...
nerves-7fcb.local. 120  IN  A 192.168.0.106
...
$ dig @224.0.0.251 -p 5353 -x 192.168.0.106
...
106.0.168.192.in-addr.arpa. 120 IN  PTR nerves-7fcb.local.
...
$ dig @nerves-7fcb.local -p 5353 -t PTR _ssh._tcp.local
...
_ssh._tcp.local.  120 IN  PTR nerves-7fcb._ssh._tcp.local.
nerves-7fcb._ssh._tcp.local. 120 IN TXT ""
nerves-7fcb._ssh._tcp.local. 120 IN SRV 0 0 22 nerves-7fcb.local.
nerves-7fcb.local.  120 IN  A 192.168.0.106
...
$ dig @224.0.0.251 -p 5353 -t SRV nerves-7fcb._ssh._tcp.local
...
nerves-7fcb._ssh._tcp.local. 120 IN SRV 0 0 22 nerves-7fcb.local.
nerves-7fcb.local.  120 IN  A 192.168.0.106
...

Although dig is a lookup utility for DNS, it can be used to query MdnsLite. You can use the reserved ip address (224.0.0.251) and port(5353) and query the local domain. Or you can use the local hostname, e.g., nerves-7fcb.local of the host that is providing the mDNS responses along with port 5353.

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

jfcloutier/mdns_lite

MdnsLite

Configuration

Installation

Usage