[Bug 1609898] [NEW] dhclient incorrectly assumes a /64 ipv6 prefix

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[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]

None.  Non-standard (i.e. not /64) subnets served by dhcpv6 currently
are broken, this fixes that.

[Other info]

This is fixed in debian:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=684009

** Affects: isc-dhcp (Ubuntu)
     Importance: Medium
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Affects: isc-dhcp (Ubuntu Precise)
     Importance: Undecided
         Status: New

** Affects: isc-dhcp (Ubuntu Trusty)
     Importance: Undecided
         Status: New

** Affects: isc-dhcp (Ubuntu Xenial)
     Importance: Undecided
         Status: New

** Affects: isc-dhcp (Ubuntu Yakkety)
     Importance: Medium
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress


** Tags: patch
-- 
dhclient incorrectly assumes a /64 ipv6 prefix
https://bugs.launchpad.net/bugs/1609898
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