[Dnsmasq-discuss] dnsmasq and external DHCPv6 servers

[Eric Dittman]

On 1/6/22 11:21 AM, Simon Kelley wrote:
> On 06/01/2022 00:05, Eric Dittman via Dnsmasq-discuss wrote:
>> On 1/5/22 4:57 PM, Simon Kelley wrote:
>>> On 03/01/2022 21:55, Eric Dittman via Dnsmasq-discuss wrote:
>>>> I have a Ubiquiti UDM SE.  It uses dnsmasq for DNS, DHCP, and RAs.  I
>>>> have external DNS and DHCP servers so I want to send RA packets with
>>>> the M and O flags set and the A flag reset, but I can't figure out the
>>>> proper configuration to do that.  Right now just to get RA working on
>>>> the subnets for IPv6 I have the following:
>>>>
>>>> enable-ra
>>>> ra-param=*,high,0
>>>> domain=mydomain
>>>> dhcp-range=2001:470:xxxx::,ra-names,slaac
>>>> dhcp-range=2001:470:xxxx:10::,ra-names,slaac
>>>> dhcp-range=2001:470:xxxx:20::,ra-names,slaac
>>>> dhcp-range=2001:470:xxxx:40::,ra-names,slaac
>>>>
>>>> I'm getting autoconf IPv6 addresses but I'm not seeing any DHCPv6
>>>> packets on the external DHCPv6 servers (a pair per subnet).
>>>>
>>>> Any ideas how I can get this working?
>>>
>>> The dnsmasq RA code was designed to work with the dnsmasq DHCPv6 server,
>>> so there's no direct support for this. I think the closest you can get
>>> is to configure a static-addresses-only dhcp range
>>>
>>> dhcp-range=2001:470:xxxx:10::,static,slaac
>>>
>>> Which will not allocate any DHCPv6 leases, leaving the field open for
>>> you other DHCPv6 servers, but will set the M and O bits.
>>>
>>> Adding a new keyword to cover this circumstance is pretty
>>> straightforward.
>>
>> Thanks, Simon, that worked!  I really appreciate it.
>>
>> The only issue I'm having now is I'm getting the router link-local IPv6
>> address included along with the two IPv6 addresses I specify in the
>> DHCPv6 options for the DNS resolvers:
>>
>> nameserver 2001:470:xxxx:10::105
>> nameserver 2001:470:xxxx:10::106
>> nameserver fe80::300d:e6ff:fe85:e6e0
>> nameserver 10.0.10.105
>> nameserver 10.0.10.106
> 
> 
> Dnsmasq will include the router's link-local address as the DNS server
> IF no other addresses are configured with dhcp-option AND dnsmasq is
> configured to provide DNS.
> 
> If your case, since you're not using dnsmasq for DNS, disable DNS by
> setting
> 
> port=0
> 
> and that DNS server address will disappear.

Thanks again, Simon!

That took care of the DNS entry.  Ubiquiti has /etc/resolv.conf listing
127.0.0.1 and resolv.dnsmasq listing my nameservers, so disabling DNS
in dnsmasq disables that, so my script that adds the customipv6.conf
to the dnsmasq configuration directory will also need to update
resolv.conf.
-- 
Eric Dittman