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: r...@zoo.priv.at
--- /sbin/dhclient-script.orig 2013-05-27 23:00:32.000000000 +0200
+++ /sbin/dhclient-script 2014-01-10 17:08:13.000000000 +0100
@@ -344,9 +344,9 @@
;;
BOUND6|RENEW6|REBIND6)
- if [ "${new_ip6_address}" ] && [ "${new_ip6_prefixlen}" ]; then
+ if [ "${new_ip6_address}" ]; then
# set leased IP
- ip -6 addr add ${new_ip6_address}/${new_ip6_prefixlen} \
+ ip -6 addr add ${new_ip6_address}/128 \
dev ${interface} scope global
fi
--- client/dhc6.c.orig 2014-01-14 13:18:41.000000000 +0100
+++ client/dhc6.c 2014-01-14 13:19:06.000000000 +0100
@@ -3841,11 +3841,8 @@
piaddr(addr->address),
(unsigned) addr->plen);
} else {
- /* Current practice is that all subnets are /64's, but
- * some suspect this may not be permanent.
- */
client_envadd(client, prefix, "ip6_prefixlen",
- "%d", 64);
+ "%d", 128);
client_envadd(client, prefix, "ip6_address",
"%s", piaddr(addr->address));
}
