[Dnsmasq-discuss] Why does the dnsmasq routing feature require a subnet prefix length of 64?

[Eric Fahlgren]

Yeah, some of the RFCs on v6 address formats hem and haw about how big the
network ID and interface ID parts are (probably written before actual
implementations were in place), but
https://www.rfc-editor.org/rfc/rfc4291#section-2.5.1 says quite
unequivocally:

   For all unicast addresses, except those that start with the binary
   value 000, Interface IDs are required to be 64 bits long...

Which drives a stake in the ground regarding how to partition those 128 bits.


On Tue, Jun 20, 2023 at 11:59 AM Petr Menšík <pemensik at redhat.com> wrote:

> I think that is required by SLAAC RFC, which adds another 2 bytes to 6
> bytes of hardware ethernet address.
>
> Which is in total 8 bytes, therefore 64 bits is required for it. Prefix
> cannot be higher, but can be lower in theory. There might be some
> implementation details now supporting lower prefix length in current
> implementation.
>
> Cheers,
> Petr
> On 15. 06. 23 12:07, renmingshuai via Dnsmasq-discuss wrote:
>
> When ra-only, slaac, or ra-stateless is configured in dhcp-range and the
> prefix len is set to a value other than 64, like this:
>
> “dhcp-range=2000:1000:1000:1000:1000:1000::, ra-stateless,120,infinite”
>
> the following error message is displayed:
>
> dnsmasq: prefix length must be exactly 64 for RA subnets at line 16 of
> /etc/dnsmasq.conf
>
> Why must the prefix length be 64? This may come from an RFC regulation or
> recommendation, but I didn't find it. Would you mind tell me the reason?
>
> --
> Petr Menšík
> Software Engineer, RHEL
> Red Hat, http://www.redhat.com/
> PGP: DFCF908DB7C87E8E529925BC4931CA5B6C9FC5CB
>
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