netdev - Re: [PATCH net-next] ipv6: allow userspace to add IFA_F

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Message-ID: <20180226165619.GA10603@bistromath.localdomain>
Date:   Mon, 26 Feb 2018 17:56:19 +0100
From:   Sabrina Dubroca <sd@...asysnail.net>
To:     David Miller <davem@...emloft.net>
Cc:     dsahern@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] ipv6: allow userspace to add IFA_F_OPTIMISTIC
 addresses

2018-02-26, 10:57:11 -0500, David Miller wrote:
> From: Sabrina Dubroca <sd@...asysnail.net>
> Date: Mon, 26 Feb 2018 16:41:32 +0100
> 
> > What are you concerned about, if we let userspace set this flag?
> 
> I am concerned that the kernel is no longer in charge of making sure
> that all of the RFC rules are met in this area.

This can already happen with IFA_F_NODAD or net.ipv6.conf.*.accept_dad.
We'll send packets using non-unique addresses.

> Userland is now repsonsible for implementing correct behavior when it
> takes over this task, and therefore the kernel has no say in the
> matter of proper ipv6 neighbor discovery and addrconf behavior.

As an aside, that's also the case whenever userland uses packet
sockets.

> Unlike with things like DHCP, addrconf et al. in ipv6 are
> fundamentally defined aspects of the protocol suite.
> 
> This division of responsibility means that we will also run into
> situations where who (kernel or user) must take care of X or Y might
> be ambiguous or hard to pin down in certain circumstances.

I don't think it's ambiguous here, but I can add documentation.

> I really don't like this situation where a fundamental protocol is
> conditionally the responsibility of the kernel, it's really bad design
> decision overall.

I understand. But I think with this patch, userspace could rely on the
kernel's DAD, instead of having to perform DAD itself in order to
avoid the delay that non-optimistic DAD introduces.

-- 
Sabrina