netdev - Re: [PATCH] arp: honour gratuitous ARP _replies

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Date:   Tue, 16 May 2017 08:01:34 -0700
From:   Ihar Hrachyshka <ihrachys@...hat.com>
To:     "David S. Miller" <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, Ihar Hrachyshka <ihrachys@...hat.com>
Subject: Re: [PATCH] arp: honour gratuitous ARP _replies_

On Mon, May 15, 2017 at 2:16 PM, Ihar Hrachyshka <ihrachys@...hat.com> wrote:
> When arp_accept is 1, gratuitous ARPs are supposed to override matching
> entries irrespective of whether they arrive during locktime. This was
> implemented in commit 56022a8fdd87 ("ipv4: arp: update neighbour address
> when a gratuitous arp is received and arp_accept is set")
>
> There is a glitch in the patch though. RFC 2002, section 4.6, "ARP,
> Proxy ARP, and Gratuitous ARP", defines gratuitous ARPs so that they can
> be either of Request or Reply type. Those Reply gratuitous ARPs can be
> triggered with standard tooling, for example, arping -A option does just
> that.
>
> This patch fixes the glitch, making both Request and Reply flavours of
> gratuitous ARPs to behave identically.
>
> As per RFC, if gratuitous ARPs are of Reply type, their Target Hardware
> Address field should also be set to the link-layer address to which this
> cache entry should be updated. The field is present in ARP over Ethernet
> but not in IEEE 1394. In this patch, I don't consider any broadcasted
> ARP replies as gratuitous if the field is not present, to conform the
> standard. It's not clear whether there is such a thing for IEEE 1394 as
> a gratuitous ARP reply; until it's cleared up, we will ignore such
> broadcasts. Note that they will still update existing ARP cache entries,
> assuming they arrive out of locktime time interval.
>
> Signed-off-by: Ihar Hrachyshka <ihrachys@...hat.com>

Please disregard this email, I forgot to update the patch version to
v2 and provide changelog. I posted (hopefully) correct version. Still
learning the process...

Ihar