[pkg-dhcp-devel] Bug#684009: isc-dhcp-client: dhclient must not assume a IPv6 prefix length of 64 when setting an address

[Ralf Schlatterbeck]

Note that dhclient does not itself configures the interface but instead
calls the shellscript /sbin/dhclient-script to do the work. So a quick
workaround is to patch that script to use a fixed netmask of /128
(patch attached).

The real fix is to hand a fixed /128 netmask to the dhclient-script from
the daemon. This patches C-code in dhclient (patch attached).

Note that the dhcpv6 protocol doesn't have an option for a netmask. So
it is always /128 and routing is left to icmpv6 router advertisements.

That also means that the option "accept_ra" of the dhcp method for the
INET6 address family in /etc/network/interfaces (see interfaces(5) man
page) probably should be on by default or completely removed. In
addition maybe a fixed netmask should be configurable (see excerpts from
RFC5942 below).

Just some more facts regarding this issue:
RFC 5942 is very clear about a DHCP client "inventing" a prefix:

RFC5942, p.7 under "Host Rules":
"""
   1.  The assignment of an IPv6 address -- whether through IPv6
       stateless address autoconfiguration [RFC4862], DHCPv6 [RFC3315],
       or manual configuration -- MUST NOT implicitly cause a prefix
       derived from that address to be treated as on-link and added to
       the Prefix List.  ...
"""

and on p.8 under the heading "Observed Incorrect Implementation Behavior":
"""
   ...  An address
   could be acquired through the DHCPv6 identity association for non-
   temporary addresses (IA_NA) option from [RFC3315] (which does not
   include a prefix length), or through manual configuration (if no
   prefix length is specified).  The host incorrectly assumes an
   invented prefix is on-link.  This invented prefix typically is a /64
   that was written by the developer of the operating system network
   module API to any IPv6 application as a "default" prefix length when
   a length isn't specified...
"""

I sincerely hope this gets fixed in the next release of dhcpd. Note that
I've also filed an upstream report with issue number #35178 (before I
knew about this debian report) and I'm surprised the currently scheduled
4.3.0a1 release doesn't yet have the fix.

Ralf
-- 
Ralf Schlatterbeck             email: ralf at zoo.priv.at
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