[Dnsmasq-discuss] Clarification of prefix length field in dhcp-range

Quintus quintus at quintilianus.eu
Sat Oct 5 22:43:44 BST 2013

Am Sat, 5 Oct 2013 14:21:26 +0100
schrieb Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk>:

> Hi All,

Hi Kevin,

> dnsmasq2.67rc3 - possibly odd behaviour, probably I misunderstand :-)
> I have an interface that has a /64 on it.  dnsmasq.conf has amongst
> other things
> "
> dhcp-range=::100, ::F:FFFF:FFFF, constructor:br0, ra-names, 64, 12h
> enable-ra
> "

ra-names is useless in a DHCPv6 context as it tells clients to use
SLAAC (IPv6 stateless autoconfiguration) and *not* DHCPv6. You probably
want to remove it (enable-ra does the router advertisements for you).

> Now, if I change said interface to be a /48 (restarting dnsmasq), the
> logfile says it's built a constructed range as before, but it no
> longer does RA and in fact DHCPv6 requests get rebuffed with a 'no
> valid range'.  Shouldn't the ', 64' force this to behave as a /64 for
> the constructed range?  And shouldn't it do RA?

If you want dnsmasq to serve IPs from a /48 net, your *interface* needs
to have a /48 address assigned. You can’t have your network interface
have a /64 address and have dnsmasq delivering addresses with a
different prefix.

> Many thanks,
> Kevin

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