[Nut-upsuser] Question on simultaneous IPv4 and IPv6 "anyaddress" listening

[gene heskett]

On 8/5/23 08:25, Greg Troxel wrote:
> Jim Klimov via Nut-upsuser <nut-upsuser at alioth-lists.debian.net> writes:
> 
>>    I've recently found that at least on my test box the `LISTEN *` line had
>> only set up an IPv4 `0.0.0.0` listener but not an IPv6 `::0` listener for
>> `upsd`.
> 
> Interesting.  On one system I checked, I have 4 explicit directives for
> 127.0.0.1, ::1, and the LAN on v4/v6.  On another, I have an empty
> upsd.conf and it is listening:
> 
> nut      upsd        1047    4* internet stream tcp 127.0.0.1:3493
> nut      upsd        1047    5* internet6 stream tcp [::1]:3493
> 
>> In fact, at least on a "dual-stack" system, it seems impossible to
>> bind to both - so depending on binding order I either lose IPv6 or lose
>> IPv4 directly (but have it practically as IPv4-over-IPv6).
> 
> That is not intrinsic to a system that does v4 and v6.  It is about a
> misfeature which if turned on, when one binds to v6 also sets up a
> listener on v4 which connects as a mapped address.  These days, I view
> it as a bug for a system to be configuret hat way.  On NetBSD, from
> ip6(4):
> 
>       IPV6_V6ONLY int *
>               Get or set whether only IPv6 connections can be made to this
>               socket.  For wildcard sockets, this can restrict connections to
>               IPv6 only.
> 
> which is 1 on my system.
> 
>>    Given that `LISTEN *` support is in fact not documented explicitly (I
>> think), I am inclined to define it as listening to "any" on whatever
>> address families are available and supported by the NUT build, and somehow
>> ensuring that to the best of our capability (technical puzzles exist - see
>> GitHub issue).
> 
> It seems really obvious that * means anything, so agreed.
> 
> I think it's important that the default, if there are no LISTEN
> directives, be "listen on all localhost addresses of all address
> familes".  And probably there should be a way to say that explicitly,
> like "LISTEN localhost".
> 
> Practically, LISTEN localhost should:
> 
> #ifdef v6 at compile time
>    open a socket and bind to [::1]:3493
>      error log that v6 bind failed
> #endif
> 
>    open a socket and bind to 127.0.0.1:3493
>      if error:
>        if there is a v6 socket:
>          debug log that v4 bind failed, maybe, or maybe it's a real
>          error?  need to figure out if v6only=0 systems some try to map
>          this.  The point  being not to fight os/sysadmin choice even if
>          misguided :-)
>        else:
>          error log that v4 bind failed
> 
> and LISTEN * should
> 
> #ifdef v6 at compile time
>    open a socket and bind to INADDR6_ANY:3493
>      error log that v6 bind failed
> #endif
> 
>    open a socket and bind to INADDR_ANY:3493
>      if error:
>        if there is a v6 socket:
>          debug log that v4 bind failed
>        else:
>          error log that v4 bind failed
> 
As outlined here, that does resemble a good universal approach. However 
for me, who is likely 100 miles from the nearest working ipv6 address, 
the current situation seems to suffice.
The only problems I've had with bookworm are related to shifting all the 
system logging into journald, So it been very difficult to troubleshoot 
the new access problems with my raid10 based /home partition. And that 
has nothing to do with nut that I've observed..

My reliance on nut is nearly moot since a ups only has to survive 5 
seconds until my 20kw Kohler gets started.

>>    Detailed musing and logs are posted in
>> https://github.com/networkupstools/nut/issues/2012
>>
>>    Pro/Con ideas are welcome :)

Cheers, Gene Heskett.
-- 
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