[Dnsmasq-discuss] DhcpV6 how to send prefixlen and default route

Simon Kelley simon at thekelleys.org.uk
Mon Nov 25 12:13:35 GMT 2013

On 25/11/13 11:57, Roy Marples wrote:
> On 23/11/2013 14:06, dnsmasq.bertrandb at dfgh.net wrote:
>> Hello,
>> Alwas some problems with my /97 prefix ...
>> on the client side dhclient say the prefix is /64
>> the router advertissement is ok with ridsc6 i see that i have /97 prefix
>> in dhcp no option for prefixlen seems possible
>> Is a trick exists ?
> DHCPv6 has no prefix or routing notation.
> As such any address not derived from a Prefix Delegation has to use /128
> for a prefix.
> dhcpcd does this and NetworkManager just dis-regards any dhclient
> returning information, also forcing a /128 prefix for the address.
> If you require your hosts to be on the same IPv6 subnet then you'll need
> to advertise the /64 (or whatever) prefix in the RA and turn off the
> ND_OPT_PI_FLAG_AUTO option. In rtadvd.conf(5) you can set
> pinfoflags="l", or rtadv I think the config option is AdvAutonomous off.
> I don't know the dnsmasq config option for this :/

ND_OPT_PI_FLAG_AUTO is SET if slaac, ra-names, or ra-stateless is 
specified, and CLEARED otherwise.

The intention is that on a a pure-DHCPv6 system, setting the dnsmasq 
configuration option enable-ra will send the RA packets required to make 
DHCPv6 work, (ie, the relevant prefixes are advertised, with the corrent 
prefix lengths and the ND_OPT_PI_FLAG_AUTO bit cleared.) If RA is being 
done elsewhere, enable-ra should be left unset.



> Thanks
> Roy
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