netdev - Re: [patch net-next] ipv6: allow userspace to create address with IFLA_F

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Date:	Tue, 5 Nov 2013 21:57:44 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	David Miller <davem@...emloft.net>, jiri@...nulli.us,
	vyasevich@...il.com, netdev@...r.kernel.org, kuznet@....inr.ac.ru,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org, kaber@...sh.net,
	thaller@...hat.com, stephen@...workplumber.org
Subject: Re: [patch net-next] ipv6: allow userspace to create address with IFLA_F_TEMPORARY flag

On Tue, Nov 05, 2013 at 06:02:17PM +0100, Nicolas Dichtel wrote:
> Le 29/10/2013 13:40, Hannes Frederic Sowa a écrit :
> >On Tue, Oct 29, 2013 at 09:37:06AM -0000, David Laight wrote:
> >>>Note that you don't even need to put the DHCP protocol core into the
> >>>kernel to fix the promiscuous problem.  You just have to use the
> >>>current kernel interfaces correctly.
> >>>
> >>>It used to be the case a very long time ago that you couldn't even
> >>>receive broadcast UDP datagrams on a socket until an address was
> >>>configured on it.
> >>>
> >>>So everyone turns on promiscuous mode and uses RAW sockets or
> >>>AF_PACKET.
> >>>
> >>>Stupid?  yes.
> >>
> >>Not only that, but the dhcp client could use a normal UDP socket
> >>to keep the lease renewed - I suspect it has only ever needed
> >>to use the BPF interface (I didn't think it set promiscuous)
> >>when acquiring the initial lease.
> >
> >Yes, this is a very unfortunate situation. From my experience it is not 
> >that
> >easy to get a patch merged into isc-dhcp.
> >
> >It seems not that invasive to switch from af_packet to an udp socket with
> >SO_BROADCAST set.
> If I remember well, another problem is to be able to send these packets with
> 0.0.0.0 when another IP address is available on the system:
> 
> RFC2131
> 4.1 Constructing and sending DHCP messages
> ...
>    DHCP messages broadcast by a client prior to that client obtaining
>    its IP address must have the source address field in the IP header
>    set to 0.
> 
> We made a patch (never proposed upstream) to add a socket option to keep
> this 0.0.0.0 address.
> If people are interested, I can try to port it on net-next.

Yes, this indeed is a problem for a socket only dhcp client. I would make the
appropriate changes to isc-dhcp if you submit such a patch.

Thanks,

  Hannes

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