It's possible the device has decided to try DHCPv6 anyway - maybe
because it isn't hearing router advertisements. I hoped most devices wouldn't do this, but they're certainly allowed to.
Code:
18. DHCP Configuration Exchanges
A client initiates a message exchange with a server or servers to
acquire or update configuration information of interest. A client
has many reasons to initiate the configuration exchange. Some of the
more common ones are:
1. as part of the operating system configuration/bootstrap process,
2. when requested to do so by the application layer (through an
operating-system-specific API),
3. when a Router Advertisement indicates that DHCPv6 is available
for address configuration (see Section 4.2 of [RFC4861]),
4. as required to extend the lifetime of address(es) and/or
delegated prefix(es), using Renew and Rebind messages, or
5. upon the receipt of a Reconfigure message, when requested to do
so by a server.
I can see that there is an nvram variable
ipv6_dhcp6s_enable
referenced in the code but not 100% sure what it's doing. I can see some DHCPv6 logic is activated if either in stateful mode, or that nvram variable is 1.
You could put something at
/jffs/configs/dnamasq.conf.add
and it would be appended to the dnamasq config file. Not sure if there's something you can append to disable something already enabled.