The dnssd interface

Here you can read about the :dnssd interface. This interface gives you access to all of the DNS-SD interface. It will allow you to register, browse and resolve services on the network.

The examples have been translated to Elixir from the original (Erlang) examples.

In the success case, all functions return a tuple of the form {:ok, reference}. Reference should be retained to pass to :dnssd.stop/1 when no further results are required. Please note that proper handing of references is omitted in the examples below for simplicity and readability.

It’s important to stop operations when no more results are needed to avoid generating needless network traffic. To stop an operation pass the reference returned when you started the operation to :dnssd.stop/1. Operations will also be stopped if your process exits.

Browsing for Services

iex> :dnssd.browse("_http._tcp")
{:ok, #Reference<0.0.2.763>}
iex> flush()
{:dnssd, #Reference<0.0.2.763>,
 {:browse, :add, {"Foo", "_http._tcp.", "local."}}}
:ok

Results will be sent in tuples of the form {:dnssd, reference, {operation, change, result}}. Reference will be the same reference which was used to start the operation. Operation will be one of the atoms :browse, :resolve, :register or :enumerate. Change will be the atom :add or :remove and the result will be an operation specific term. For the browse operation, it will be a tuple containing binaries of the form {service_name, service_type, domain}.

iex> :dnssd.browse("_http._tcp", "dns-sd.org")
{:ok, #Reference<0.0.4.1850>}
iex> flush()
{:dnssd, #Reference<0.0.4.1850>,
 {:browse, :add,
  {" * Apple, makers of the iPod", "_http._tcp.", "dns-sd.org."}}}
{:dnssd, #Reference<0.0.4.1850>,
 {:browse, :add,
  {" * Google, searching the Web", "_http._tcp.", "dns-sd.org."}}}
# …
:ok

Browsing can be limited to a specific domain by specifying the domain as argument two. Both domains and service types may be specified as lists or binaries.

Resolving a Service Instance

To resolve a service, supply it’s name, registration type and domain to the resolve function.

iex> :dnssd.resolve(" * DNS Service Discovery", "_http._tcp.", "dns-sd.org.")
{:ok, #Reference<0.0.4.1888>}
iex> flush()
{:dnssd, #Reference<0.0.4.1888>,
 {:resolve, {"dns-sd.org.", 80, [{"txtvers", "1"}, {"path", "/"}]}}}
:ok

Unlike the other operations results won’t be tagged add or remove as the underlying DNS-SD API does not provide this information. As resolve is generally called just prior to connecting to a service this shouldn’t pose a problem. The Result term for this operation is a tuple of the form {hostname, port, txt_strings} where hostname is a binary, port is an integer and txt_strings is a list containing either binaries or should a given string contain an equals sign, a {key, value} tuple wherein key is everything up to the first equals sign and the remainder of the string is the value.

iex> :dnssd.resolve_sync(" * DNS Service Discovery", "_http._tcp.", "dns-sd.org.")
{:ok, {"dns-sd.org.", 80, [{"txtvers", "1"}, {"path", "/"}]}}

A synchronous wrapper to resolve is also provided. A timeout in milliseconds can also be specified by adding a fourth argument. The default timeout is 5 seconds. {:error, :timeout} will be returned should the operation timeout.

Registering Services

iex> :dnssd.register("_answer._udp", 42)
{:ok, #Reference<0.0.4.1929>}
iex> flush()
{:dnssd, #Reference<0.0.4.1929>,
 {:register, :add, {"atj-mbp", "_answer._udp.", "local."}}}
:ok

The minimum arguments needed to register a service are the service type and port. If no service name is supplied, the machines name is used (in the example above, that’s "atj-mbp"). The result term for this operation is a tuple containing binaries of the form {service_name, service_type, domain}.

For brevity, the alternative invocations of register are:

:dnssd.register(name, type, port)
:dnssd.register(type, port, txt)
:dnssd.register(name, type, port, txt)
:dnssd.register(name, type, port, txt, host, domain)

Wherein:

txt is a TXT record data in either binary form (a sequence of <<size, string:size/binary>>), a list of atoms, strings or binaries or tuples of the form {key, value} where key and value are atoms, strings or binaries.
host is the hostname of the machine running the service. Pass an empty string or binary for the local machine.
domain is the domain to register the service within. Pass an empty string or binary for all domains.

Note: A service may be renamed if it conflicts with another service. Check the Results tuple to determine what name a service has been assigned.

Local Registrations

If localhost is passed as host to :dnssd.register/6 the service will be registered only in the local domain (regardless of the domain argument) and only on the local machine.

Enumerating Domains

iex> :dnssd.enumerate(:browse)
{:ok, #Reference<0.0.4.1947>}
iex> flush()
{:dnssd, #Reference<0.0.4.1947>, {:enumerate, :add, "local."}}
{:dnssd, #Reference<0.0.0.1947>, {:enumerate, :add, "bonjour.tj.id.au."}}
:ok

iex> :dnssd.enumerate(:reg)
{:ok, #Reference<0.0.4.1962>}
iex> flush()
{:dnssd, #Reference<0.0.4.1962>, {:enumerate, :add, "local."}}
{:dnssd, #Reference<0.0.4.1962>, {:enumerate, :add, "bonjour.tj.id.au."}}
:ok

The result term for this operation is a binary containing the browse or registration domain.

Retrieving Results

Results from a running operation can be retrieved by calling :dnssd.results(ref). For resolve operations this will only return the last result. For all other operations it will return all current results.

nerves_dnssd