How does dnsmasq get its upstream servers?

My ERX is supposed to act also as DNS server for the localnet. As such the clients in localnet get only the interface IP of the localnet interface of the ERX as DNS server.

Obviously the dnsmasq itself needs to ask upstream servers for non-localnet names. The uplink in the ERX is configured to do DHCP , the interface does both ipv4 and ipv6. Since its dynamic the uplink provides also the information about the to-be-used DNS server to the ERX.

How exactly is this information supposed to travel to the dnsmasq process? Sometimes I see the upstream ipv4#53 in dnsmasq.log when dnsmasq starts, but I never find it in any configuration file. I'm sure the uplink also provides info about the ipv6 address of the uplink DNS server. Is that info also supposed to travel to the dnsmasq process?

The thing is: sometimes the clients in localnet only get a 5(REFUSED) from the ERX for a "host www.ubnt.com erx-ipv4". And after about 30-60 minutes it magically starts to work, without any indication in the ERX syslogs.

I found a /etc/dnsmasq.d/dnsmasq-dhcp-conf.conf, but that file is always empty. What is it supposed to contain?