lua-mdns

Multicast DNS (mDNS) service browser implemented in pure Lua. mDNS provides the ability to provide and find DNS names for local devices. For more information on mDNS and DNS Service discovery, see http://www.dns-sd.org.

Installing

Prerequisites

• Lua 5.2 or later
• LuaSocket 2.1 or later (currently tested on 3.0rc1)
• LDoc 1.5 or later - for source code documentation

Installation using LuaRocks package manager

$ luarocks make mdns-2.0-1.rockspec

Generate source code documentation

$ luarocks install ldoc
$ ldoc .

Example Usage

The code below queries all mDNS services available on the local network.

local mdns = require('mdns')

-- DNS service discovery (defaults: all services, 2 seconds timeout)
local res = mdns.query()
if res then
    for k, v in pairs(res) do
        -- output key name
        print(k)
        local function print_table(t, indent)
            for k, v in pairs(t) do
                -- output service descriptor fields
                if type(v) == 'table' then
                    print(string.rep('  ', indent)..k..':')
                    print_table(v, indent + 1)
                else
                    print(string.rep('  ', indent)..k..': '..v)
                end
            end
        end
        print_table(v, 1)
    end
else
    print('no result')
end

If called without parameters, mdns.query returns all available services after the default timeout of 2 seconds. Additional examples can be found in the examples subdirectory.

Reference

The module exports mdns.query, mdns.query_async, and mdns.socket

mdns.query

Usage

result = mdns.query([<service>, [<timeout>]])

Parameters

mdns.query takes up to two parameters:

• service: mDNS service name (e.g. _printers._tcp.local). The .local suffix may be omitted. If this parameter is missing or if it evaluates to nil, mdns.query queries all available mDNS services by using enumerating the _services._dns-sd._udp.local service.

• timeout: Timeout in seconds waiting for mDNS responses. If this parameter is missing or if it evaluates to nil, mdns.query uses the dafault timeout of 2 seconds.

Return value

If mdns.resolve succeeds, an associateve array of service descriptors is returned as a Lua table. Please note that the array may be empty if there is no mDNS service available on the local network. In case of error, the function either asserts, or it returns nil.

Service descriptors returned by mdns.query may contain a combination of the following fields:

• name: mDNS service name (e.g. HP Laserjet 4L @ server.example.com)
• service: mDNS service type (e.g. _ipps._tcp.local)
• hostname: hostname
• port: port number
• ipv4: IPv4 address
• ipv6: IPv6 address
• text: Table of text record(s)

mdns.resolve returns whatever information the mDNS daemons provide. The presence of certain fields doesn't imply that the system running lua-mdns supports all features. For example, an IPv6 address may be returned even though the LuaSocket library installed on the system may not support IPv6. Resolving such potetial mismatches is beyond the scope of lua-mdns.

mdns.query_async

Allows the use of platform appropriate timer options. Specifically, if available, an asynchronous timer.

Usage

tick, finalise = mdns.query_async([<service>])

Parameters

mdns.query_async takes up to one parameter:

• service: see mdns.query

Return value

mdns.query_async returns two functions. These are indented to be called by a platform appropriate timer.

• tick(): This should be called frequently as possible until the user operation timeout.

• finalise(): Once the user operation has timed out, this function should be called.
  It will clean up and return a table of services. Return value

  • services: Table of service descriptors or nil, see mdns.query

Example

The timer library here is given as an example, please use a platform appropriate library:

local mdns = require('mdns')
local ticker = require('timer')
local timeout = require('timer')

-- Query all MDNS services
local mdns_tick, mdns_finalise = mdns.query_async()

-- Setup ticker
ticker.single_shot = false
ticker.interval = 0.01
ticker.handler = function()
    mdns_tick()
end
ticker:start()

-- Setup timeout
timeout.single_shot = true
timeout.interval = 2.0
timeout.handler = function()
    ticker:stop()

    local res = mdns_finalise()
    if (res) then
        for k,v in pairs(res) do
            <... use results>
        end
    end
end
timeout:start()

mdns.socket

Allows the user to set custom socket operations. e.g. Use IPv6, unicast IPv4, some other transport, or even a socket library other than LuaSocket.

Members

• mdns.socket.PEER.IP MDNS Query destination IP. Default: IPv4 multicast address '224.0.0.251'

• mdns.socket.PEER.PORT MDNS Query destination port. Default: 5353

• mdns.socket:setup() Setup the socket

• mdns.socket:send() Send datagram
  Parameters

  • datagram: Datagram string to send to the peer

• mdns.socket:recv() Receive response datagram
  Return value

  • datagram: Response datagram, or nil

• mdns.socket:teardown() Tear down the socket

Example (IPv6)

local mdns = require('mdns')

mdns.socket.PEER.IP = 'ff02::fb'
mdns.socket.setup = function(self, timeout)
    local socket = require('socket')
    self.udp = socket.udp6()
    assert(self.udp:setoption('ipv6-add-membership', { multiaddr = self.PEER.IP }))
    assert(self.udp:settimeout(0.1))
end
mdns.socket.teardown = function(self)
    assert(self.udp:setoption("ipv6-drop-membership", { multiaddr = self.PEER.IP }))
    assert(self.udp:close())
    self.udp = nil
end

local res = mdns.resolve()

License

lua-mdns is released under the MIT license.

Copyright (c) 2015 Frank Edelhaeuser

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.