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Date:   Thu, 25 Apr 2019 12:30:33 +0200
From:   Adam Borowski <kilobyte@angband.pl>
To:     Nico Schottelius <nico-kernel-20190425@schottelius.org>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: How to turn off IPv4 without disabling IPv6
Message-ID: <20190425103033.GA20813@angband.pl>
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On Thu, Apr 25, 2019 at 11:32:38AM +0200, Nico Schottelius wrote:
> running some IPv6 only
> networks. The systems in the IPv6 only networks do not need any IPv4
> support anymore and thus for switches/routers we turned the support off.

> Today we tried to turn off IPv4 in the Linux kernel at compile time.
> But it seems that as soon as we turn off CONFIG_INET, CONFIG_IPV6 is
> automatically turned off as well.

Even if you don't want global nor even link-scope IPv4, way too many
programs assume that at least 127.0.0.1 (ie, lo) is working.  They can't be
reconfigured to use ::1 without patching and rebuilding.

> Coming back to my original question: is there a way or how would we turn
> off IPv4 support in the Linux kernel?

I believe this is not worth your time for today.

Just do what IPv6-haters do on stock modern distros: have no routes for the
other IP version configured; non-buggy programs will do the right thing.

This seems to work well.  Heck, I had a busy dev server with broken IPv4, I
didn't notice that for 1.5 years until I tried to pull something directly
from Github (which is still v4 only).

You're a network admin so you know far more than me wrt anything that goes
over the wire -- but as as a distro developer/user, I'd say there's a
considerable cost to have every of tens of thousands programs shipped by a
distro, and many more that are private to a company/university/etc, updated
to autodetect how to access "localhost" on a particular box.

That's an extra moving part where there was none before.  Complexity is bad.
Having the IPv4 stack built just for the lo interface simplifies things.


Meow!
-- 
⢀⣴⠾⠻⢶⣦⠀
⣾⠁⢠⠒⠀⣿⡁ Did ya know that typing "test -j8" instead of "ctest -j8"
⢿⡄⠘⠷⠚⠋⠀ will make your testsuite pass much faster, and fix bugs?
⠈⠳⣄⠀⠀⠀⠀