[PROTON-2474] proton with cyrus-sasl queries DNS for short hostname

• Type: New Feature
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: proton-c-0.35.0
• Component/s: python-binding
• Assignee: Unassigned
• Reporter: Ken Dreyer
• Created: 08/Dec/21 21:30
• Updated: 13/Dec/21 18:30

Description

My python-qpid-proton 0.35.0 client hangs for about four seconds on every new connection operation to my broker. The problem is that the client tries to look up its own hostname's A and AAAA DNS records before completing the SASL exchange (and my DNS server takes a while for missing records.)

Steps to reproduce:

1. Set up a CentOS 8 or 9 host.
2. Install cyrus-sasl-devel
3. Install python-qpid-proton. Ensure it's dynamically linked to cyrus-sasl (/lib64/libsasl2.so.3).
4. Ensure the unqualified (short) hostname from the "hostname" command is not present in /etc/hosts.
5. Connect to a broker with ANONYMOUS SASL and no SSL.

Actual results:
Immediately after Proton receives the sasl.mechanisms message, but before Proton sends the sasl.init / sasl.outcome AMQP messages, it makes two DNS queries (A and AAAA) for my host's unqualified hostname.

The AAAA request takes a while to answer (3.5 seconds). This makes makes the pn_transport_push() call hang here:

-> exec(cmd, globals, locals)
  <string>(1)<module>()
  /home/vagrant/dns.py(51)<module>()
-> Container(HelloWorld()).run()
  /usr/lib64/python3.6/site-packages/proton/_reactor.py(180)run()
-> while self.process():
  /usr/lib64/python3.6/site-packages/proton/_reactor.py(245)process()
-> event.dispatch(self._global_handler)
  /usr/lib64/python3.6/site-packages/proton/_events.py(162)dispatch()
-> _dispatch(handler, type.method, self)
  /usr/lib64/python3.6/site-packages/proton/_events.py(125)_dispatch()
-> handler.on_unhandled(method, *args)
  /usr/lib64/python3.6/site-packages/proton/_reactor.py(876)on_unhandled()
-> event.dispatch(self.base)
  /usr/lib64/python3.6/site-packages/proton/_events.py(162)dispatch()
-> _dispatch(handler, type.method, self)
  /usr/lib64/python3.6/site-packages/proton/_events.py(123)_dispatch()
-> m(*args)
  /usr/lib64/python3.6/site-packages/proton/_handlers.py(1241)on_selectable_readable()
-> n = t.push(b)
  /usr/lib64/python3.6/site-packages/proton/_transport.py(304)push()
-> n = self._check(pn_transport_push(self._impl, binary))

Expected Results:

proton with cyrus-sasl should behave the same way it does without cyrus-sasl: it should authenticate quickly, without looking up the IP address of its own short hostname.

Extra information:

This only happens when I build against cyrus-sasl-devel. When I do not have cyrus-sasl-devel available, then the package from PyPI does not link against cyrus-sasl, and there is no DNS request. (The qpid-proton package in EPEL is built against cyrus-sasl, and that  is where I discovered this bug.)

If the client can resolve its unqualified (short) hostname to an IP, then there is no delay. For example, if my Proton client's hostname is myclient.example.com, I can fix the delay by adding the following entry to /etc/hosts:

127.0.0.1   myclient localhost

or by ensuring my DNS server will immediately return A or AAAA records for the unqualified "myclient" name.