Add support for DNS SRV records to Mosquitto's mosquitto.py Code placed in the public domain, by Jan-Piet Mens, November 2013

mosquitto-dns-srv.diff

--- mosquitto.py.orig	2013-11-12 11:41:17.000000000 +0100
+++ mosquitto.py	2013-11-13 08:44:31.000000000 +0100
@@ -45,6 +45,11 @@
 import sys
 import threading
 import time
+HAVE_DNS = True
+try:
+    import dns.resolver
+except ImportError:
+    HAVE_DNS = False
 
 MOSQUITTO_MAJOR=1
 MOSQUITTO_MINOR=2
@@ -568,6 +573,45 @@
         self.connect_async(host, port, keepalive, bind_address)
         return self.reconnect()
 
+    def connect_srv(self, domain=None, keepalive=60, bind_address=""):
+        """Connect to a remote broker.
+
+        domain is the DNS domain to search for SRV records; if None,
+        try to determine local domain name.
+        keepalive and bind_address are as for connect()
+        """
+
+        if HAVE_DNS == False:
+            raise ValueError('No DNS resolver library found.')
+
+        if domain is None:
+            domain = socket.getfqdn()
+            domain = domain[domain.find('.') + 1:]
+
+        try:
+            rr = '_mqtt._tcp.%s' % domain
+            if self._ssl is not None:
+                rr = '_mqtts._tcp.%s' % domain
+            answers = []
+            for answer in dns.resolver.query(rr, dns.rdatatype.SRV):
+                address = (answer.target.to_text()[:-1], answer.port)
+                answers.append((address, answer.priority, answer.weight))
+        except (dns.resolver.NXDOMAIN, dns.resolver.NoAnswer, dns.resolver.NoNameservers):
+            raise ValueError("No answer/NXDOMAIN for SRV in %s" % (domain))
+
+        # FIXME: doesn't account for weight
+        for answer in answers:
+            address, prio, weight = answer
+            host, port = address
+
+            try:
+                # print "trying: ", host, port
+                return self.connect(host, port, keepalive, bind_address)
+            except:
+                pass
+
+        raise ValueError("No SRV hosts responded")
+
     def connect_async(self, host, port=1883, keepalive=60, bind_address=""):
         """Connect to a remote broker asynchronously. This is a non-blocking
         connect call that can be used with loop_start() to provide very quick

srv-sub.py

#!/usr/bin/python

import mosquitto

def on_connect(mosq, obj, rc):
    print "Connected to %s:%d" % (mosq._host, mosq._port)

def on_message(mosq, obj, msg):
    print(msg.topic+" "+str(msg.qos)+" "+str(msg.payload))

def on_subscribe(mosq, obj, mid, granted_qos):
    print("Subscribed: "+str(mid)+" "+str(granted_qos))

# If you want to use a specific client id, use
# mqttc = mosquitto.Mosquitto("client-id")
# but note that the client id must be unique on the broker. Leaving the client
# id parameter empty will generate a random id for you.
mqttc = mosquitto.Mosquitto()

mqttc.on_message = on_message
mqttc.on_connect = on_connect
mqttc.on_subscribe = on_subscribe
mqttc.connect_srv("mosquitto.org", 60)
mqttc.subscribe("$SYS/broker/version", 0)

rc = 0
while rc == 0:
    rc = mqttc.loop()

print("rc: "+str(rc))

IANA specifies secure-mqtt (not mqtts) for port 8883