Libguestfs - Jan 2022 - [nbdkit PATCH 2/2] server/sockets: get rid of AI_ADDRCONFIG

On Wed, Jan 19, 2022 at 01:46:08PM +0000, Richard W.M. Jones
wrote:
> On Tue, Jan 18, 2022 at 11:19:18AM -0600, Eric Blake wrote:
> > On Tue, Jan 18, 2022 at 02:48:33PM +0100, Laszlo Ersek wrote:
> > > +    case '4':
> > > +      tcpip_sock_af = AF_INET;
> > > +      break;
> > > +
> > > +    case '6':
> > > +      tcpip_sock_af = AF_INET6;
> > > +      break;
> > 
> > Thus, if the user uses nbdkit -46 (or -64), the last one specified
> > silently overrides the earlier one, rather than being diagnosed as
> > conflicting or explicitly permitting both.  The override effect
> > matches the long-option naming --ipv4-only, so I can live with it, but
> > I'm also open to the idea of explicitly adding code to diagnose
both
> > options at the same time as an error, if we think that's
friendlier
> > than silent override.
> 
> Is that actually an error?  -4 -6 says you want to listen on IPv4 and
> IPv6 which is redundant but not wrong.
Oh wait, actually it doesn't work like I expected.

Is it easy to make -4 -6 listen on both IPv4 & IPv6?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/

On 01/19/22 14:47, Richard W.M. Jones wrote:
> On Wed, Jan 19, 2022 at 01:46:08PM +0000, Richard W.M. Jones wrote:
>> On Tue, Jan 18, 2022 at 11:19:18AM -0600, Eric Blake wrote:
>>> On Tue, Jan 18, 2022 at 02:48:33PM +0100, Laszlo Ersek wrote:
>>>> +    case '4':
>>>> +      tcpip_sock_af = AF_INET;
>>>> +      break;
>>>> +
>>>> +    case '6':
>>>> +      tcpip_sock_af = AF_INET6;
>>>> +      break;
>>>
>>> Thus, if the user uses nbdkit -46 (or -64), the last one specified
>>> silently overrides the earlier one, rather than being diagnosed as
>>> conflicting or explicitly permitting both.  The override effect
>>> matches the long-option naming --ipv4-only, so I can live with it,
>>> but I'm also open to the idea of explicitly adding code to
diagnose
>>> both options at the same time as an error, if we think that's
>>> friendlier than silent override.
>>
>> Is that actually an error?  -4 -6 says you want to listen on IPv4 and
>> IPv6 which is redundant but not wrong.
>
> Oh wait, actually it doesn't work like I expected.
>
> Is it easy to make -4 -6 listen on both IPv4 & IPv6?
That's the default; in that case, just don't specify either.

-4 and -6 make AF_INET and AF_INET6 (respectively) the exclusive address
family within which the argument of option "-i" is resolved. IOW, -4
and
-6 are (intentionally) a very thin wrapper around hints.ai_family.

Here's some examples (with the "netstat -anp | grep nbdkit" output
pasted after the command line):

# most generic use: no -i, no -4, no -6: binds the wildcard address for
# both IPv4 and IPv6
$ nbdkit -f null
->
tcp        0      0 0.0.0.0:10809           0.0.0.0:*               LISTEN     
219346/nbdkit
tcp6       0      0 :::10809                :::*                    LISTEN     
219346/nbdkit

# no "-i", so bind the wildcard address, but restricted to IPv4 with
# "-4"
$ nbdkit -4 -f null
->
tcp        0      0 0.0.0.0:10809           0.0.0.0:*               LISTEN     
219393/nbdkit

# no "-i", so bind the wildcard address, but restricted to IPv6 with
# "-6"
$ nbdkit -6 -f null
->
tcp6       0      0 :::10809                :::*                    LISTEN     
219397/nbdkit

# "-i localhost" causes getaddrinfo() to find both the IPv4 and IPv6
# addresses (127.0.0.1 and ::1, respectively) in /etc/hosts
$ nbdkit -f -i localhost null
->
tcp        0      0 127.0.0.1:10809         0.0.0.0:*               LISTEN     
219386/nbdkit
tcp6       0      0 ::1:10809               :::*                    LISTEN     
219386/nbdkit

# same as above, but "-4" restricts the resolution to IPv4
$ nbdkit -i localhost -4 -f null
->
tcp        0      0 127.0.0.1:10809         0.0.0.0:*               LISTEN     
219402/nbdkit

# same as above, but "-6" restricts the resolution to IPv6
$ nbdkit -i localhost -6 -f null
->
tcp6       0      0 ::1:10809               :::*                    LISTEN     
219405/nbdkit

# "-i" is passed a numeric address, so the address family is inherent
$ nbdkit -i 127.0.0.1 -f null
->
tcp        0      0 127.0.0.1:10809         0.0.0.0:*               LISTEN     
219411/nbdkit

# same, but an IPv6 address is used
$ nbdkit -i ::1 -f null
->
tcp6       0      0 ::1:10809               :::*                    LISTEN     
219414/nbdkit

# repeat the above two, with -4 / -6 restrictions, which are no-ops in
# this case
$ nbdkit -i 127.0.0.1 -4 -f null
-> tcp        0      0 127.0.0.1:10809         0.0.0.0:*               LISTEN
219421/nbdkit

$ nbdkit -i ::1 -6 -f null
->
tcp6       0      0 ::1:10809               :::*                    LISTEN     
219424/nbdkit

# We can even attempt resolving the numeric addresses for the *wrong*
# address family:

$ nbdkit -f -i 127.0.0.1 -6 null
nbdkit: getaddrinfo: 127.0.0.1: 10809: Address family for hostname not supported

$ nbdkit -f -i ::1 -4 null
nbdkit: getaddrinfo: ::1: 10809: Address family for hostname not supported

These too are correct, because the numeric address 127.0.0.1 is
unresolvable in AF_INET6, and the numeric adress ::1 is unresolvable in
AF_INET.

Thanks
Laszlo