[Dnsmasq-discuss] Zeroing lifetime of ULA router advertisements

[Chris Webb]

I'm using dnsmasq 2.88 on a little 4G wwan router in a fairly  
standard/boring way.

The lan-facing interface br0 always has a v6 ULA address, but may  
sometimes also have a global v6 address with a default v6 route available.  
(I fail between carriers: whether I'm v4-only or v4+v6 depends on active  
carrier.)

My dnsmasq configuration has

   dhcp-range=set:lan,192.168.1.128,192.168.1.254,255.255.255.0,12h
   dhcp-range=set:lan,::,constructor:br0,ra-stateless,ra-names
   dhcp-option=tag:lan,option6:dns-server,[::]
   enable-ra

This works but always advertises a v6 default route, even when there's  
just the ULA address on br0. Conversely, adding

   ra-param=br0,0,0

offers the prefixes without a default route, even if there's a global  
address on br0. (Reading the man page, I thought adding ,64,0 or  
,64,deprecated to the v6 dhcp-range would do the same thing, but it  
doesn't seem to have any effect?)

Is there an easy way to have non-zero lifetime for advertisements for the  
global prefix, but drop the route lifetime to zero for advertisements for  
the ULA prefix? (I can hard-code the specific ULA prefix in dnsmasq.conf  
if necessary, but not the global prefix as that's dynamically delegated by  
the 4G carrier.)

As one attempt, I tried adding the router ULA address as deprecated with

   ip address add fd40:xxxx:xxxx::1/64 dev br0 preferred 0

but this just prevented client hosts from receiving ULA addresses at all.

Any hints would be very gratefully received!

Best wishes,

Chris.