netdev - ARP updates and GARP

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Date:	Sun, 18 Apr 2010 23:54:19 -0700
From:	Sasha Levin <sasha@...sleep.com>
To:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: ARP updates and GARP

Hi,

We are currently testing IP fail-over on storage devices, and have observed an issue with the IP transfer from one device to another.

Assuming we have 2 storage devices A and B, and a server C which uses the storage, the scenario is:

1. Device A sends an ARP request which server C sees – server C updates it’s ARP table with the MAC of device A.
2. Device A fails, Device B takes over the IP and sends out a GARP.
3. Even though device C sees the GARP, it ignores it and keeps trying to communicate with device A until the entry is removed from its cache and a new ARP request is generated.

The code which causes this is located in arp_process@...t/ipv4/arp.c:

override = time_after(jiffies, n->updated + n->parms->locktime);

/* Broadcast replies and request packets
   do not assert neighbour reachability.
 */
if (arp->ar_op != htons(ARPOP_REPLY) ||
    skb->pkt_type != PACKET_HOST)
        state = NUD_STALE;
neigh_update(n, sha, state, override ? NEIGH_UPDATE_F_OVERRIDE : 0);
neigh_release(n);

According to the code, this scenario happens because the kernel ignores any ARP updates which happened in a short period after the previous ARP update. The reason which was stated in the comments is  “If several different ARP replies follows back-to-back, use the FIRST one. It is possible, if several proxy agents are active. Taking the first reply prevents arp trashing and chooses the fastest router.”.

This, however, doesn’t take into account GARPs which are not being sent by ARP proxies anyway and just ignores them too – causing a loss of communication for over a minute until the ARP cache refreshes.

Is there another reason for this rule? If not, Is possible to submit a patch which will take GARPs into account when ignoring ARP updates?

Thanks!

Sasha.
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