Where a service listens

A service that listens takes one argument that says where, and every library that listens reads it the same way. parse_listen turns it into the sockets it names, with no I/O; bind_listen binds them and returns a UDPEndpoint for each.

import socket
import netimps

netimps.parse_listen("*:67")
# (ListenAddress(address=IPv4Address('0.0.0.0'), port=67, interfaces=()),)

netimps.parse_listen("eth1:67,[::1]:69")
# (ListenAddress(address=IPv4Address('0.0.0.0'), port=67, interfaces=('eth1',)),
#  ListenAddress(address=IPv6Address('::1'), port=69, interfaces=()))

endpoints = netimps.bind_listen("127.0.0.1:0")
for endpoint in endpoints:
    datagram = endpoint.recv()
    if endpoint.admits(datagram):
        ...

What can be written

Written	Means
None, "*", "*:67", ":67"	every address (IPv4 unless family=socket.AF_INET6)
"127.0.0.1", "127.0.0.1:67", "[::1]:69", "::1"	one address, with a zone kept ("fe80::1%eth0")
"eth1", "Wi-Fi 2:67"	the wildcard, limited to that adapter
"aa-bb-cc-dd-ee-ff:67"	the wildcard, limited to every adapter carrying that MAC
("127.0.0.1", [67, 68])	a pair of host and ports, as a tuple or a list
["*:67", "[::1]:69"], "*:67,[::1]:69"	several of those

A host name is never resolved: "localhost" is an adapter name. A ListenAddress is itself accepted, so parse_listen(parse_listen(x)) is parse_listen(x). Default ports are the caller's: parse_listen(listen, default_ports=(67, 68)) gives each to a binding that names none.

What is bound

An address is one socket. A wildcard is one socket that reports the arrival interface, or one socket for each address of the host where that is not available or per_address=True asks for it. A wildcard limited to interfaces is one socket whose endpoint serves those adapters (endpoint.interfaces), bound to the device on Linux; elsewhere it receives from every interface and endpoint.admits(datagram) says whether a datagram arrived on one it serves. An IPv6 socket is IPv6 only on every platform, so "*" and "::" on one port are two sockets that do not overlap.

Receiving never filters. The caller asks admits, so it can count and report what it drops. Receive buffers, the receive loop and the default ports stay the caller's.