[Libguestfs] why do we have AI_ADDRCONFIG in nbdkit?

[Laszlo Ersek]

Hi,

we do we have have AI_ADDRCONFIG in nbdkit, from historical commit
999797badf48 ("Implement nbdkit server.", 2013-06-21)?

It prevents nbdkit from binding ::1, either automatically (when -i is
not specified at all), or in response to a manual request ("-i ::1"):

nbdkit: getaddrinfo: ::1: 32776: Address family for hostname not supported

Based on the explanation in
<https://bugzilla.redhat.com/show_bug.cgi?id=808147>, this behavior is
actually expected of getaddrinfo() when the host has no public IPv6
address configured.

So I think it's wrong for nbdkit to use AI_ADDRCONFIG -- why restrict
nbdkit to binding public IPv6 addresses, and exclude ::1?

The POSIX spec is terse on this:

https://pubs.opengroup.org/onlinepubs/9699919799/functions/freeaddrinfo.html

    If the AI_ADDRCONFIG flag is specified, IPv4 addresses shall be
    returned only if an IPv4 address is configured on the local system,
    [IP6] [Option Start]  and IPv6 addresses shall be returned only if
    an IPv6 address is configured on the local system. [Option End]

Ulrich Drepper's IPv6 programming intro says:

https://akkadia.org/drepper/userapi-ipv6.html

    AI_ADDRCONFIG

    This flag should always be set when the returned values are needed
    to make connections. If no specific protocol is requested, the Linux
    getaddrinfo implementation returns both IPv4 and IPv6 addresses.
    This can be less than optimal and is certainly slower if the machine
    has only interfaces for one protocol. These days there are still
    many systems which have no configured IPv6 address at all. In that
    case using an IPv6 address will always fail. Worse, it might cause
    the IPv6 kernel module to be loaded unnecessarily. Using
    AI_ADDRCONFIG avoids this by determining what protocols are
    supported by the currently configured network interfaces and return
    only addresses for those.

But in nbdkit's case, we're not making connections -- we're binding
interfaces and listening to / accepting connections. In
bind_tcpip_socket(), we have

  hints.ai_flags = AI_PASSIVE | AI_ADDRCONFIG;

... Here's another bit of info:

https://fedoraproject.org/wiki/QA/Networking/NameResolution/ADDRCONFIG

Note under "Proposed solutions":

https://fedoraproject.org/wiki/QA/Networking/NameResolution/ADDRCONFIG#Proposed_solutions

(1a) Remove all code that deals with AI_ADDRCONFIG, effectively
disabling it in the general getaddrinfo() code [then links a bugzilla
attachment for glibc, AFAICT!]

(2a) Remove AI_ADDRCONFIG in all software that uses it. Deprecate
AI_ADDRCONFIG and prevent/reject modifications that add it to any software.

My reason to ask about this: on my Fedora 34 host, which has no public
IPv6 address assigned, the connect-tcp6 test case always fails. It is
not possible for me to skip it (from the make check command line), and
this failure masks further test failures -- as it prevents the rest of
the test suite from running. I could create a local branch that simply
exits with code 77, but I think using AI_ADDRCONFIG is wrong in the
first place.

Thanks,
Laszlo