[Dnsmasq-discuss] Both IPv6 router + separate dnsmasq server do router advertisements - results in two IPv6 routes for clients

[Klaus Wissmann]

Hi all,

I’m pretty sure I’m overlooking something really simple but somehow I’m stuck.

Initially I had a fairly standard setup: my router at home does IPv6 router advertisements + DHCP + DNS etc., local hosts do slaac and stateless dhcpv6.

Due to several reasons I wanted to move DHCP + DNS away from the router to a separate server. So what I did was
• Switch off DHCPv6 on the router
• Keep Router Advertisements active on router (with O flag set)
• Set up dnsmasq on the new dedicated DNS/DHCP server (Debian 11, dnsmasq 2.85). dnsmasq config is here: https://pastebin.aw.net/?f55651e94ecc6755#GhRFdiYGdzPpurJOks4etAmXx7u3dxGvuJQBdemNWOk=

What is now happening is that both the router as well as the dnsmasq server send router advertisements and my local hosts end up with two default routes. If I completely leave out the dhcp-range statement for IPv6, then dnsmasq will not send any router advertisements but unfortunately the really nice ra-names functionality is also switched off.

Interestingly enough, my local Linux and MacOS don’t seem to be bothered at all by the two default routes while the Windows clients get confused sometimes (if I understand correctly depending on when they received which router advertisement).

I tried to dig into the source code a bit and if I prevent dnsmasq from executing send_ra_aliases() that seems to do the trick. But I have to admit that I don’t understand the source code thoroughly enough and I assume this will break something else...

So my question is: is there a way to stop dnsmasq from sending router advertisements while at the same still providing the ra-names functionality? 

Thanks,
 Klaus

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