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[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

** Also affects: linux (Ubuntu Xenial)
   Importance: Undecided
       Status: New

** Also affects: linux (Ubuntu Zesty)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1715812

Title:
  Neighbour confirmation broken, breaks ARP cache aging

Status in linux package in Ubuntu:
  Confirmed
Status in linux source package in Xenial:
  New
Status in linux source package in Zesty:
  New

Bug description:
  [SRU Justification]

  [Impact]
  A host can lose access to another host whose MAC address changes if they have active connections to other hosts that share a route. The ARP cache does not time out as expected - instead the old MAC address is continuously reconfirmed.

  [Fix]
  Apply series [1], which changes the algorithm for neighbour confirmation.
  That is, from upstream:
  51ce8bd4d17a net: pending_confirm is not used anymore 
  0dec879f636f net: use dst_confirm_neigh for UDP, RAW, ICMP, L2TP 
  63fca65d0863 net: add confirm_neigh method to dst_ops 
  c3a2e8370534 tcp: replace dst_confirm with sk_dst_confirm 
  c86a773c7802 sctp: add dst_pending_confirm flag 
  4ff0620354f2 net: add dst_pending_confirm flag to skbuff 
  9b8805a32559 sock: add sk_dst_pending_confirm flag 

  [Test case]
  Create 3 real or virtual systems, all hooked up to a switch.
  One system needs an active-backup bond with fail_over_mac=1 num_grat_arp=0.

  Put all the systems in the same subnet, e.g. 192.168.200.0/24

  Call the system with the bond A, and the other two systems B and C.

  On B, run in 3 shells:
   - netperf -t TCP_RR to C
   - ping -f A
   - watch 'ip -s neigh show 192.168.200.0/24'

  On A, cause the bond to fail over.

  Observe that:

   - without the patches, B intermittently fails to notice the change in
  A's MAC address. This presents as the ping failing and not recovering,
  and the arp table showing the old mac address never timing out and
  never being replace with a new mac address.

   - with the patches, the arp cache times out and B sends another mac
  probe and detects A's new address.

  It helps to use taskset to put ping and netperf on the same CPU, or
  use single-CPU vms.

  See [2] for more details.

  [References]
  [2] Original report: https://www.mail-archive.com/netdev@xxxxxxxxxxxxxxx/msg138762.html
  [1]: https://www.spinics.net/lists/linux-rdma/msg45907.html

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