http.client and dns lookups

[Barry]

> On 1 Feb 2022, at 19:21, Michael Welle <mwe012008 at gmx.net> wrote:
> 
> Hello,
> 
> Barry <barry at barrys-emacs.org> writes:
> 
>>>> On 31 Jan 2022, at 19:22, Dieter Maurer <dieter at handshake.de> wrote:
>>> 
>>> Michael Welle wrote at 2022-1-30 09:18 +0100:
>>>> ...
>>> The machine this is running on regularly switches
>>>> its network configuration without restarting the Python application. Now
>>>> it turns out that the application is still using an old, outdated dns
>>>> server after such a network configuration switch.
>>> 
>>> It is unlikely that Python performs the host -> IP address translation
>>> itself. Almost surely, the translation is delegated to functions
>>> of the underlying C runtime library, e.g. "gethostbyname".
>> 
>> And glibc does not have the ability to reload the /etc/resolv.conf I recall.
>> If you rely on that you will need to change to using something like
>> dnsmasq or systemd-resolved so that resolv.conf uses localhost.
> hmmm, I don't know if that's all. For a test I just changed
> /etc/resolv.conf manually while the application was sleeping. After
> returning from sleep the application knew the change I had made in
> resolv.conf and, interestingly, an exception (name resolution failure,
> which is correct with the change I made) surfaces in the application.
> That doesn't happen when the 'real' issue occurs. Attaching strace to
> the Python process I can see that resolv.conf is stat'ed and open'ed. I
> guess now I'm more confused than before ;). There must be an additional
> condition that I'm missing.

That is good to know, last time I hit this was on centos 6 a little while ago.

Barry

> 
> Thanks for helping
> hmw
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