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This bug was fixed in the package isc-dhcp - 4.2.4-7ubuntu12.7

---------------
isc-dhcp (4.2.4-7ubuntu12.7) trusty; urgency=medium

  * Don't assume IPv6 prefix length of 64 (LP: #1609898).
    Pulled from debian commit c347ab8a43587164486ce1f104eedfd638594e59.

 -- Dan Streetman <dan.streetman@xxxxxxxxxxxxx>  Thu, 04 Aug 2016
13:07:23 -0400

** Changed in: isc-dhcp (Ubuntu Trusty)
       Status: Fix Committed => Fix Released

** Changed in: isc-dhcp (Ubuntu Xenial)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1609898

Title:
  dhclient incorrectly assumes a /64 ipv6 prefix

Status in isc-dhcp package in Ubuntu:
  Fix Committed
Status in network-manager package in Ubuntu:
  Fix Committed
Status in isc-dhcp source package in Precise:
  Fix Released
Status in network-manager source package in Precise:
  Invalid
Status in isc-dhcp source package in Trusty:
  Fix Released
Status in network-manager source package in Trusty:
  Triaged
Status in isc-dhcp source package in Xenial:
  Fix Released
Status in network-manager source package in Xenial:
  Triaged
Status in isc-dhcp source package in Yakkety:
  Fix Committed
Status in network-manager source package in Yakkety:
  Fix Committed
Status in isc-dhcp package in Debian:
  Fix Released

Bug description:
  [Impact]

  clients who get an ipv6 address from a dhcpv6 server assume the
  address has a /64 prefix, but that is not necessarily true, and if the
  subnet is different than /64 those clients will not be able to reach
  other addresses in that /64 prefix because the other systems are not
  on-link.  This /64 assumption of dhclient effectively breaks the
  client networking for certain addresses.

  [Test Case]

  Set up a server with two interface nics, and one client connected to
  each of those interfaces.  On the server, set up a ipv6 subnet on each
  interface, with a larger prefix than /64, e.g.:

  2001:db8:0:0:1::/96
  2001:db8:0:0:2::/96

  configure dhcpv6 on the server, to provide ipv6 addresses on each
  interface.  Set the server as the default ipv6 route for the clients.

  Allow the clients to get dhcpv6 ipv6 addresses from the server.  The
  clients will each get a ipv6 address with a /64 prefix, due to the bug
  in dhclient.

  Try to ping (or otherwise communicate) between the clients.  Since
  they have /64 prefixes, they think they are on-link with each other,
  but they are not, so they can't communicate.

  After the dhclient bug is fixed, repeat the above setup, and the
  clients will get /128 prefixes instead, and then will be able to
  communicate with each other, because they will route the traffic to
  each other through the server.

  [Regression Potential]

  Non-standard (i.e. not /64) subnets served by dhcpv6 currently are
  broken, this fixes that.

  However, any ipv6 network configurations that rely on the previous
  incorrect assumed /64 behavior (as described here:
  https://tools.ietf.org/html/rfc5942#section-5) in order to allow
  dhcpv6 clients to communicate with other systems inside the subnet,
  but do not use RA to also provide clients with the actual prefix len,
  will break.

  To clarify: a server that provides clients with dhcpv6 addresses, but
  does not also provide the prefix len via RA, will change behavior;
  previously, all clients on the subnet could talk directly to each
  other, with this update the clients cannot talk to each other
  directly; all traffic between clients will be routed through the
  default gateway.

  Further, if the network does not provide a default gateway - for
  example a local network that is intended only for traffic between
  local servers - the clients will not be able to talk to each other at
  all.

  Note that such configurations *are* broken configurations, that just
  happened to work before because of incorrect dhcp client behavior;
  such configurations must be updated to perform RA to provide the
  prefix len to clients.

  See comment 30 for details on how to configure radvd to restore
  network functionality.

  [Other Info]

  This is fixed in debian:

  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=684009

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