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#dnssd_erlang

dnssd_erlang is an interface to Apple's Bonjour DNS Service Discovery implementation. Bonjour allows applications to browse, resolve and register network services via link-local multicast DNS on the local network and via unicast DNS over the internet. In the later case if the service is running behind a NAT gateway Bonjour will only advertise it if a port forward can be negotiated via NAT-PMP or uPNP (which is attempted automatically).

Prerequisites

Erlang R16A or later. For compatibility with older Erlang releases use dnssd_erlang v0.8.

Apple Bonjour or a compatible API such as Avahi with it's compatibility layer along with the appropriate development files:

OS X - bundled
Windows - Bonjour SDK
BSD/Linux - search for Avahi in your operating systems software manager
Ubuntu - apt-get install avahi-daemon libavahi-compat-libdnssd-dev

Build Process

Build with rebar:

compile: rebar compile
test: rebar eunit
edoc: rebar doc

If you are running Linux with Avahi you will need Avahi's Bonjour compatibility layer installed. If {error,-65537} is returned when starting an operation it may be that avahi-daemon is not running.

If you are running Windows you will need Visual Studio and the Bonjour SDK installed. The project can then be built from a Visual Studio command prompt.

Example use

Eshell V5.8.2  (abort with ^G)
1> dnssd:start().
ok

First start the application via dnssd:start/1 or application:start(dnssd).

Browsing for Services

2> dnssd:browse("_http._tcp").
{ok,#Ref<0.0.0.197>}

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.

3> flush().
Shell got {dnssd,#Ref<0.0.0.197>,
                 {browse,add,
                         {<<"dnsndnsweb">>,<<"_http._tcp.">>,
                          <<"bonjour.tj.id.au.">>}}}
Shell got {dnssd,#Ref<0.0.0.197>,
                 {browse,add,{<<"TIVO">>,<<"_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 {ServiceName, ServiceType, Domain}.

4> dnssd:browse(<<"_http._tcp">>, <<"dns-sd.org">>).
{ok,#Ref<0.0.0.488>}
5> flush().
Shell got {dnssd,#Ref<0.0.0.488>,
                 {browse,add,
                         {<<" * Apple, makers of the iPod">>,
                          <<"_http._tcp.">>,<<"dns-sd.org.">>}}}
Shell got {dnssd,#Ref<0.0.0.488>,
                 {browse,add,
                         {<<" * Google, searching the Web">>,
                          <<"_http._tcp.">>,<<"dns-sd.org.">>}}}
%% snipped %%
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

6> dnssd:resolve(<<" * DNS Service Discovery">>, <<"_http._tcp.">>, <<"dns-sd.org.">>). 
{ok,#Ref<0.0.0.20357>}

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

7> flush().
Shell got {dnssd,#Ref<0.0.0.20357>,
                 {resolve,{<<"dns-sd.org.">>,80,
                           [{<<"txtvers">>,<<"1">>},{<<"path">>,<<"/">>}]}}}
ok

Unlike the other operations results won't be tagged add or remove as the underlying DNSSD 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, TxtStrings} where Hostname is a binary, Port is an integer and TxtStrings 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.

8> 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

9> dnssd:register("_answer._udp",42).
{ok,#Ref<0.0.0.10006>}
10> flush().
Shell got {dnssd,#Ref<0.0.0.10006>,
                 {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 {ServiceName, ServiceType, 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

11> dnssd:enumerate(browse).
{ok,#Ref<0.0.0.15448>}
12> flush().
Shell got {dnssd,#Ref<0.0.0.15448>,{enumerate,add,<<"local.">>}}
Shell got {dnssd,#Ref<0.0.0.15448>,{enumerate,add,<<"bonjour.tj.id.au.">>}}
ok
13> dnssd:enumerate(reg).
{ok,#Ref<0.0.0.15529>}
14> flush().
Shell got {dnssd,#Ref<0.0.0.15529>,{enumerate,add,<<"local.">>}}
Shell got {dnssd,#Ref<0.0.0.15529>,{enumerate,add,<<"bonjour.tj.id.au.">>}}
ok

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

Stopping Operations

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.

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.