> On Apr 20, 2016, at 3:42 PM, Jonathan Vanasco <twisted-pyt...@2xlp.com> wrote:
>
>
> On Apr 20, 2016, at 1:31 PM, Jonathan Vanasco wrote:
>
>> A specific library was keeping the port open. I'm tracking down how/why
>> right now.
>
> So this was fun <sarcasm> thing to learn...
>
> An undocumented (yay) feature of python appears to be... python binds to a
> random port on all interfaces (0.0.0.0) once you send UDP data through it. I
> assume this is to allow for a response to come back.
This isn't so much a feature of Python as it is a feature of the BSD sockets
API. Sending traffic through a socket, whether it's TCP or UDP, has to bind a
client port. Given the nature of UDP, binding on all interfaces is the
expectation unless you specify.
I didn't have time to test a simple C program before sending this message, but
https://github.com/python/cpython/blob/master/Modules/socketmodule.c
<https://github.com/python/cpython/blob/master/Modules/socketmodule.c> only
calls "bind()" from sock_bind, not from send(), nor does
https://github.com/python/cpython/blob/master/Lib/socket.py
<https://github.com/python/cpython/blob/master/Lib/socket.py> engage in any
such shenanigans.
> We're using statsd for metrics in our twisted daemon and detect issues
> post-deployment.
>
> If you haven't used it, it's a node.js daemon from etsy that collects udp
> data and pipes it into python's graphite/carbon libraries. Then you get
> fancy graphics.
There's also a Twisted version :) https://pypi.python.org/pypi/txStatsD
<https://pypi.python.org/pypi/txStatsD>
txStatsD contains both server and client, so maybe you want to use that client
if you want better control over the UDP port.
> # this does nothing...
> sock = socket.socket(family, socket.SOCK_DGRAM)
> # but this binds to 0.0.0.0
> sock.sendto(data.encode('ascii'), addr)
>
> Sending data to the stats collector on 127.0.0.1:8125 inherently made python
> bind to 0.0.0.0, and on a port that seems to be in the 40000-60000 range.
That range is the ephemeral client port range
<https://en.wikipedia.org/wiki/Ephemeral_port
<https://en.wikipedia.org/wiki/Ephemeral_port>> so that's what would be
expected of an implicitly-bound socket.
> Since a socket to the stats collector is only created once for the process,
> Python holds that open the entire time.
If it needs to send UDP traffic, it needs to be able to receive UDP traffic as
well. You can bind it to a more specific interface, but you can't prevent the
port from opening to receive traffic.
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