netdev - IPV6 MTU

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Message-Id: <629c7820-2080-4e63-a3d3-b3dcffb2f6c8@www.fastmail.com>
Date:   Thu, 30 Dec 2021 22:44:50 -0500
From:   "Chris Hellberg" <chris@...ishellberg.com>
To:     netdev@...r.kernel.org
Subject: IPV6 MTU

Hello,

If we set an up or link-down interface MTU below MIN_IPV6_MTU, all traces of IPv6 dissappear from the interface stack (no sysctl variables or /proc/sys/net/ipv6/<if> exist). Restoring the MTU re-activates IPv6 again. For ease of user debugging when such a state exists (i.e. IPv6 would be active on this interface but-for MTU), is there any interest for an interface flag that could hint at this condition? Or some other kind of persistent indicator somewhere else appropriate?

If yes to the latter instead of an interface flag, would a sysctl be the best best place?

Thanks,

Chris