bug#55335: openssh-service no longer listens on IPv6

[Jack Hill]

On Sat, 14 May 2022, Ludovic Courtès wrote:

> Hi,
>
> Thinking about it, what do you think is the risk of using AF_INET6
> unconditionally?
>
> AFAICS it just works.  Is there a switch somewhere that might affect
> that behavior?

Yes, I beleive that it's in sysctl:

```
$ sysctl net.ipv6.bindv6only
net.ipv6.bindv6only = 0
```

If enabled, the v6 socket wouldn't work for v4. Disabled is the default on 
Guix System. I don't know what would happen if v6 were disabled entirely. 
Hopefully that's not something we have to worry about in 2022.

HTH,
Jack