Querying DNS SRV for broker address

[Roger Light]

Hi all,

I've been playing a little with using SRV DNS records for locating
MQTT brokers and thought others might be interested. If you have code
that supports it you can now perform a lookup à la "dig
_mqtt._tcp.mosquitto.org srv" and receive the answer that you should
connect to test.mosquitto.org:1883.

There is early code at
https://bitbucket.org/oojah/mosquitto-experimental that lets you do
this:

mosquitto_sub -h mosquitto.org -t '$SYS/#' -v -S

You can also request _mqtts._tcp.mosquitto.org or
_smqtt._tcp.mosquitto.org and get the answer test.mosquitto.org:8883.

mosquitto_sub -h mosquitto.org -t '$SYS/#' -v -S --cafile mosquitto.org.crt

Like I said, it's very early days and is broken and leaky if the
server referred to doesn't exist for example.

Cheers,

Roger

[Paul Fremantle]

Roger

I think this is fantastic. It is one of the best aspects of XMPP for example. 

I think this really motivates a nicely defined URI scheme for MQTT topics that could incorporate this.

Paul

PS Bizarrely I was just talking to someone about 2 hours ago thinking about this exact idea. Unfortunately I didn't actually do anything about it!

[Nicholas Humfrey]

Very cool! Wish people would make better use of SRV records.

If web browsers supported it, we won't have this problem:
https://devcenter.heroku.com/articles/apex-domains

With an A record on root of a domain, I think it is quite arrogant to
assume HTTP is the default thing we want to do ;-)


nick.

[Andy Piper]

There are now a bunch of techniques which are described for adding this to an MQTT server, and an existing discussion in the mosquitto bug tracker :-)

http://mqtt.org/wiki/doku.php/broker_auto-discovery

http://joergwende.wordpress.com/2013/11/10/using-jmdns-and-mqtt-together/

https://bugs.launchpad.net/mosquitto/+bug/902916

Pleased to see some movement on this from Joerg and Roger independently!

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[Dave C-J]

Just as well I got other Andy to register here a while ago ... http://www.dns-sd.org/ServiceTypes.html

[Jan-Piet Mens]

Roger,

> I've been playing a little with using SRV DNS records for locating
> MQTT brokers and thought others might be interested.

As I mentioned to you yesterday, this is excellent! It's a shame not
more use is made of DNS SRV records.

FWIW, I've hacked up [1] preliminary support for SRV records to
mosquitto.py. Not quite sure of the full implications, but it's a start.

Keep up the great work!

-JP

[1] https://gist.github.com/jpmens/7445275

[Andy Piper]

Can you submit that as a patch to Paho Python's client (which is mosquitto Python) via Eclipse bugzilla? :-)

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[Jan-Piet Mens]

Andy,

> Can you submit that as a patch to Paho Python's client (which is mosquitto
> Python) via Eclipse bugzilla? :-)

Roger showed me the Eclipse Legal Process [1] yesterday, which is why I
put the patch in the Public Domain. ;-)

(My non-legal thinking / hope is that Roger can then just
copy/adapt/improve the code to best suit.)

-JP

[1] http://www.eclipse.org/legal/EclipseLegalProcessPoster.pdf

[Jan-Piet Mens]

Thanks to Roger for twisting my arm really really hard, I've "signed"
all the stuff, and have pushed my meager contribution for DNS SRV
support in client.py to [1].

-JP

[1] https://github.com/jpmens/paho-mqtt-dns-srv/commit/10c2ad7a224ba02f15133854187bd0a789c049bb

[Roger Light]

Thanks Jan-Piet,

As it's the first time I'll be taking a contribution I'll be making
sure I'm covering everything. Rest assured it'll be quicker next time!

I've cross posted to paho-dev for completeness.

Cheers,

Roger