dhcpcd-discuss

Hi Roy,

Thanks for your comments and confirming that a /62 is really the way to 
go. That excerpt from RFC3633 really shed some light for me. I think 
your man pages properly document the requirements regarding SLA 0. There 
are two things about me that probably lead me to ignore the 
documentation: 1) I heavily rely on example to learn (and honestly an 
example of multi-interface delegation is pretty sparse on the internet), 
and 2) I have a firm grasp on subnetting, which lead me to be convinced 
that a /63 could be leveraged into two /64s.

My only suggestion is to provide an example of a multi-interface config 
in manpages.

-Ron

On 07/04/2015 03:24 PM, Roy Marples wrote:
> Hi Ron
>
> On Saturday 04 July 2015 11:28:09 Ron Angeles wrote:
> > I am currently using dhcpcd to delegate two prefixes to two interfaces
> > on the other side of my router. However, the math doesn't seem to add up
> > for me. The relevant config is as follows:
> >
> > ----
> > allowinterfaces eth0 eth1.10 eth1.11
> >
> > noipv4ll
> > noipv6rs
> > nodhcp
> > nodhcp6
> >
> > interface eth1.10
> > iaid 10
> >
> > interface eth1.11
> > iaid 11
> >
> > interface eth0
> > ipv6rs
> > dhcp
> > dhcp6
> > ia_pd 1/::/63 eth1.10/0/64 eth1.11/1/64
> > ----
> >
> > Since I am requesting only two /64s, I figure that I naturally should
> > only request a /63 from my ISP since it will be a perfect fit. However,
> > delegating this to my interfaces seems to hit a snag.
>
> Yes, it will hit a snag. Here's the exact wording of RFC3663 from section 12.1
>
>     When a requesting router subnets a delegated prefix, it must assign
>     additional bits to the prefix to generate unique, longer prefixes.
>     For example, if the requesting router in Figure 1 were delegated
>     3FFE:FFFF:0::/48, it might generate 3FFE:FFFF:0:1::/64 and
>     3FFE:FFFF:0:2::/64 for assignment to the two links in the subscriber
>     network.  If the requesting router were delegated 3FFE:FFFF:0::/48
>     and 3FFE:FFFF:5::/48, it might assign 3FFE:FFFF:0:1::/64 and
>     3FFE:FFFF:5:1::/64 to one of the links, and 3FFE:FFFF:0:2::/64 and
>     3FFE:FFFF:5:2::/64 for assignment to the other link.
>
>
> dhcpcd makes an exemption of the SLA 0 (ie no extra bits added) if it's
> assigned to exactly one interface.
> This allows it to work when the PD is a /64.
>
> The rationale is that the DHCP server doesn't know how many subnets are you
> going to create from the prefix and the RFC has an instruction to add a reject
> route for the prefix itself so that queries for unassigned subnets within your
> delegation to not go upstream. This in effect is the SLA 0.
>
> So the bottom line is you need to request a bigger prefix.
>
> > I must be missing something with my configuration, but it seems to me
> > that I am effectively restricted from reusing the first /64 of any
> > requested prefix. This in turn means that requesting a /63 prefix is the
> > equivalent of requesting a /64.
> >
> > As a workaround, I have since requested a /62 from my ISP since I can
> > use two of the three usable /64's.
>
> This isn't a workaround, it's actually the correct thing to do, as I described
> above.
>
> Can you suggest how I can update my man pages to make this more clear for
> future users?
>
> Thanks
>
> Roy