Python builds on MacOS do not expose the IPV6_RECVPKTINFO flag specified in [RFC3842], which is required for UDP protocols that need control over their servers' sending ports like [CoAP].

While I don't own Apple hardware and thus can't test it, the [nginx] code indicates that this API is available on OSX and is just gated behind `-D__APPLE_USE_RFC_3542`. Searching the web for that define indicates that other interpreted langues and applications use the flag as well (PHP, Ruby; PowerDNS, nmap, libcoap).

Please consider enabling this on future releases of Python on OSX.

[RFC3542]: https://tools.ietf.org/html/rfc3542
[CoAP]: https://github.com/chrysn/aiocoap/issues/69
[nginx]: http://hg.nginx.org/nginx/rev/9fb994513776