netdev - Re: [RFC] ipv6: always join solicited-node address

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Date:	Mon, 14 Oct 2013 02:55:08 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC] ipv6: always join solicited-node address

On Sun, Oct 13, 2013 at 07:24:24PM +0200, Bjørn Mork wrote:
> RFC 4861 section 7.2.1 "Interface Initialization" says:
> 
>    When a multicast-capable interface becomes enabled, the node MUST
>    join the all-nodes multicast address on that interface, as well as
>    the solicited-node multicast address corresponding to each of the IP
>    addresses assigned to the interface.
> 
> The current dependency on IFF_NOARP seems unwarranted. We need to
> listen on the solicited-node address whether or not we intend to
> initiate Neigbour Discovery ourselves.
> 
> This fixes a bug where Linux fails to respond to received Neigbour
> Solicitations on multicast capable links when IFF_NOARP is set.
> 
> Signed-off-by: Bjørn Mork <bjorn@...k.no>
> ---
> I am not at all sure about this... Comments are appreciated.
> 
> The observed problem is a MBIM mobile broadband modem sending NS
> to the host.  MBIM is a point-to-point USB protocol which does not
> have any L2 headers at all.  It can only transport IPv4 or IPv6
> packets.  So for IPv4 there is no question at all:  ARP just
> cannot be transported. The driver emulates an ethernet interface,
> setting IFF_NOARP to make sure the upper layers doesn't attempt
> to resolve the neighbours non-existing L2 addresses.
> 
> But then there is this modem which sends IPv6 Neigbour
> Solicitations to the host over the MBIM transport. The link
> layer addresses are meaningless, but it seems the modem still
> expects an answer.  Which we will not currently provide, because
> the NS is addressed to a solicited-node address we don't listen
> to.
> 
> So this patch seems like a quick-fix to that problem.  But it does
> change the semantics of IFF_NOARP, making us reply to NS even if
> this flag is set.  Which probably is wrong?

IFF_NOARP seems to be a bit messed up in ipv6. Your patch seems fine to
me but I would add protection to the ndisc_rcv and sending routines to
do nothing if IFF_NOARP is set for that interface.

So it would be possible that you could resolve this issue by just issuing
an "ip link set arp on dev <interface>" and won't have hassle with racing
interface initialization.

Is this a specific bug of the modem you are using or are all devices
powered by this driver like this?

Greetings,

  Hannes

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