dhclient-script does not respect ip6_prefixlen

[Scott Moser]

/sbin/dhclient-script does not seem to pay any attention to the prefixlen that is provided.
The result is that:
$ dhclient -6 -1 -v eth0
results in network configuration like this:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
    link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff
    inet6 fd42:83bb:a360:8010:2527:abdf:95cb:5e55/128 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::5054:ff:fe12:3456/64 scope link
       valid_lft forever preferred_lft forever

when it should have a /64 on the fd42 address, and subsequently no ability to reach other hosts on that network.

The fix seems simple enough:
  ip -6 addr add ${new_ip6_address}${new_ip6_prefixlen:+/${new_ip6_prefixlen}}
     dev ${interface} scope global

Ie, we just have to add '/${new_ip6_prefixlen}' if it is set.

This link seems relevant:

https://kb.isc.org/article/AA-01141/0/How-to-workaround-IPv6-prefix-length-issues-with-ISC-DHCP-clients.html

ProblemType: Bug
DistroRelease: Ubuntu 16.10
Package: isc-dhcp-client 4.3.3-5ubuntu15
ProcVersionSignature: Ubuntu 4.8.0-17.19-generic 4.8.0-rc7
Uname: Linux 4.8.0-17-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.3-0ubuntu7
Architecture: amd64
Date: Mon Oct 10 20:23:42 2016
DhclientLeases:

JournalErrors:
 Error: command ['journalctl', '-b', '--priority=warning', '--lines=1000'] failed with exit code 1: Hint: You are currently not seeing messages from other users and the system.
       Users in the 'systemd-journal' group can see all messages. Pass -q to
       turn off this notice.
 No journal files were opened due to insufficient permissions.
ProcEnviron:
 TERM=screen-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: isc-dhcp
UpgradeStatus: No upgrade log present (probably fresh install)

[LaMont Jones (lamont)]

The DHCP v6 protocol does not include prefixlen: On-link indication comes from router-advertisements, only.

So it's not so much that the script is ignoring it, as it is that dhclient is making up answers.