Bind to both IPv4 and IPv6 if possible

[moggers87]

It appears that Salmon does not listen to IPv4 when run under Python 3:

salmon    3047   inboxen    6u     IPv6   102157        0t0        TCP localhost:8823 (LISTEN)

It's possible that it was only binding to IPv4 previously, but this is an unexpected change either way.

[moggers87]

This might be a Centos 6 issue - it binds to IPv4 on Fedora under both Pythons 2 and 3.

[moggers87]

Might be a result of https://bugs.python.org/issue14758

[moggers87]

Alright, so after a bit more reading:

IPv6 is only supported in Python 3 via smtpd; Python 2 explicitly sets AF_INET and Python 3 uses the first result from getaddrinfo (which could be AF_INET or AF_INET6)

lmtpd only supports AF_INET and AF_UNIX, i.e. no IPv6 support at all.

The lack of good IPv6 support over LMTP is not too much of an issue IMO, but the non-deterministic nature of IPv4-or-v6 in Python 3's smtpd is a problem and one we'll probably have to solve ourselves.

/me goes off a grumbles about core devs

[moggers87]

Also, check that we can support dual-stack IPv4/IPv6. pyftplib has some code we could borrow, assuming the license is compatible

[moggers87]

This is not actually a bug, it's just the way the underlying smtp library works.

[moggers87]

lmtpd now supports binding to IPv6, using the same method as Python3's smtpd