[Dnsmasq-discuss] override DNS server for particular hosts on IPv6

[Erik Edwards]

The line "enable-ra" is directing dnsmasq to act as a "router 
advertising daemon". The advertisement sent will include the RDNSS 
default address(es). This is how IPv6 works.

If the /_client_/ can ignore the RDNSS in the RA packet then the DHCPv6 
might have a chance of doing what you want. In Fedora, using 
NetworkManager, use the option to do IPv6 "Automatic (Address only)", 
then using dhcp6c, would be something to investigate. IIRC Windows can 
also do "Address only" via automatic/SLAAC configuration then use DHCPv6 
for all the other info. Android still has a ways to go to live happily 
in the IPv6 world, as of Android 10 - too many show stopping bugs. 
(keeps dropping and reconnecting to the WiFi). If your Android version 
will allow the granularity, the "Automatic (address only)" option may help.

On 3/25/21 3:50 AM, Hamish Moffatt wrote:
> I'm attempting to override the DNS server for a few hosts on both IPv4 
> and IPv6. I've tagged the hosts and applied dhcp-option to that tag 
> and it's working correctly for IPv4, but I'm not having much luck with 
> IPv6.
>
> I'll admit right now that I don't fully understand the use of DHCPv6 
> and how DNS servers are discovered, so it might be that this is just 
> not possible, at least for all types of clients.
>
> My dnsmasq is at ::1, and I want to use the DNS server at ::18 on the 
> same prefix. What I have is:
>
> dhcp-option=tag:adblock,6,192.168.42.18
> dhcp-option=tag:adblock,option6:dns-server,[<MY ULA>::18],[<MY GLA>::18]
>
> dhcp-range=set:lan,192.168.42.20,192.168.42.239,255.255.255.0,12h
> ra-param=br-lan,0,7200
> dhcp-range=set:lan,::1000,::ffff,constructor:br-lan,slaac,ra-names,12h
> dhcp-option=lan,option6:dns-server,[::]
>
> enable-ra
> quiet-ra
>
>
> What I see is that Android just has the <MY GLA>::1 address as its DNS 
> server. A Linux client using NetworkManager has <MY GLA>::1 as well as 
> the two override servers. Even when I set <MY GLA>::18 as the server 
> in the dhcp-option clause for the whole network, the devices still 
> learned the <MY GLA>::1 address.
>
> If I remove the dhcp-option clause for the whole network then Android 
> shows it's using the link local address for ::1 instead.
>
> How is Android always learning of the dnsmasq server itself as the DNS 
> server, and can I fix it?
>
>
>
> Thanks
>
> Hamish
>
>
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