Re: [v6ops] DHCPv6/SLAAC Make Hosts Confusing-//RE: new draft: draft-liu-bonica-v6ops-dhcpv6-slaac-problem

神明達哉 <jinmei@wide.ad.jp> Wed, 23 October 2013 19:12 UTC

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To: Mikael Abrahamsson <swmike@swm.pp.se>
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Cc: "v6ops@ietf.org" <v6ops@ietf.org>, "draft-liu-bonica-v6ops-dhcpv6-slaac-problem@tools.ietf.org" <draft-liu-bonica-v6ops-dhcpv6-slaac-problem@tools.ietf.org>
Subject: Re: [v6ops] DHCPv6/SLAAC Make Hosts Confusing-//RE: new draft: draft-liu-bonica-v6ops-dhcpv6-slaac-problem
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At Wed, 23 Oct 2013 20:50:43 +0200 (CEST),
Mikael Abrahamsson <swmike@swm.pp.se> wrote:

> > implementation dependent, and IIRC the ISC DHCPv6 client
> > unconditionally uses a /64 prefix when it configures a host's
> > interface with the address assigned via DHCPv6.
>
> Are you sure? I don't know what Ubuntu uses, but I have successfully
> gotten it to just get a single IA_NA + default gw, no /64 on the
> interface.

I'm pretty sure at least it behaved that way before.  I don't know what
the very latest implementation does in this regard, but at least
according to the source code it doesn't seem to change.  This is a
snippet of ISC DHCP 4.2.5:

    /* addr fields. */
    if (addr != NULL) {
        if ((ia != NULL) && (ia->ia_type == D6O_IA_PD)) {
[...]
        } 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);
            client_envadd(client, prefix, "ip6_address",
                      "%s", piaddr(addr->address));
        }

and, e.g., in scripts/linux:

  ${ip} -f inet6 addr add ${new_ip6_address}/${new_ip6_prefixlen} \
    dev ${interface} scope global

But a specific installation of ISC DHCP could use its own client
script, and it's possible that one such custom implementation just
ignores (new_)ip6_prefixlen given by dhclient.  (And, of course, I
don't even know whether your Ubuntu client uses the ISC implementation
in the first place).

--
JINMEI, Tatuya