netdev - Re: [PATCH 2/2] ipvlan: always allow the broadcast MAC address

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Message-ID: <20150328193254.6c79784d@griffin>
Date:	Sat, 28 Mar 2015 19:32:54 +0100
From:	Jiri Benc <jbenc@...hat.com>
To:	Mahesh Bandewar <maheshb@...gle.com>
Cc:	Dan Williams <dcbw@...hat.com>,
	linux-netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/2] ipvlan: always allow the broadcast MAC address

On Fri, 27 Mar 2015 22:56:15 -0700, Mahesh Bandewar wrote:
> The current logic disables broadcast by default and enables only when
> an IPv4 address is added. If this is inverted and -
> enables broadcast by default but disables it when only IPv6
> address(es) is / are added. These links can have multiple addresses
> and hence have to be careful if any one of those is IPv4 then
> broadcast bit has to be set.

You'd have to be careful and ignore IPv6 link local addresses.
Those are added automatically whenever IPv6 is enabled and their
presence does not mean the network is not IPv4 only.

But I don't like such magic behavior. It would lead to DHCP sometimes
working and sometimes not in mixed v4/v6 environment depending on
whether DHCPv4 or SLAAC was faster.

Could we perhaps add a flag when creating ipvlan interface stating
whether IPv4 broadcast should be always enabled? Or, rather, the other
way round - whether it should be disabled by default. Call it "nodhcp"
or so.

Btw, speaking about IPv6 link local addresses, these actually do not
work with ipvlan correctly. I'm getting DAD failures if I have more
than one ipvlan interface, which is no wonder. This means that ipvlan
cannot work with IPv6 reliably by default (unless you take care of ll
address assignment and ensure all ipvlan interfaces get a different
one).

 Jiri

-- 
Jiri Benc
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