How to bind all interfaces dual-stack?

[jaraco]

I wish to write a server application that binds to all interfaces in a dual-stack IPv4/IPv6 environment.

I thought the example server would be doing that, as it appears to bind to '::0'. In other applications, like CherryPy, binding to that address makes the server available on all interfaces, IPv4 and IPv6, but in aiosmtpd, the server is only reachable on IPv6. As a result, when the application moves to an environment that only supports IPv4 (looking at you, Docker), the application is unreachable.

Reading up on the docs for create_server, I see that passing a None value for the host should bind to all interfaces on both stacks, and indeed, that's the behavior I see.

But aiosmtpd doesn't allow passing None due to the the localhost default behavior.

I did find one interface that worked, but I'm not happy with it:

	cont = controller.Controller(proxy, port=8025)
	# no really, I want all interfaces
	cont.hostname = None
	cont.start()

[jaraco]

One approach I'm considering is to use a value other than None as the sentinel for "hostname unspecified", have that value be translated into ::1.

But as I consider that, I realize the issue is broader. What if one specifies "::1" or relies on the default fallback - that binding also will be IPv6 only.

I'd really like to be able to bind to all interfaces in a dual-stack way.

[jaraco]

I do see that one may pass ['::1', '127.0.0.1'] for hostname, and that does bind to both stacks. Similarly, ['::0', '0.0.0.0'] works.

[jaraco]

Also, binding to 'localhost' will bind similarly to ['::1', '127.0.0.1'], but as far as I know, there's no way to simply specify "all interfaces".

Another solution I'm considering is for aiosmtpd to present a constant:

all_interfaces = ['::0', '0.0.0.0']
"hostname to bind to all interfaces"

And use that in the example server to draw attention to it.

[jaraco]

Aha! I just noticed that create_server also accepts '' to indicate all interfaces. And that does work in aiosmtpd. I think for now I'll update the docstring to reflect that as a recommendation.

[jaraco]

I've presented a PR, and that approach feels minimally invasive to address the concern.

[Mortal]

I think we can label this issue "rubber ducking" :-)

Thanks for the PR and for figuring this out!