A box that gives IPv6 announces its own DNS server in its router
advertisements: the devices may use it and go around Warda. With
Announce Warda as DNS server over IPv6 (PUT /api/v1/dns-services/ipv6,
meta ipv6_rdnss, off by default), Warda sends router advertisements with
the RDNSS option only (RFC 8106) on the interface of the default gateway,
every 200 seconds or so (valid 10 minutes) and in answer to the
solicitations of the devices (at most every 3 seconds): router lifetime 0
(never a default router), no prefix (no address changes). It announces its
local IPv6 address (ULA) first, else its global one, else its link-local
one, follows it when the prefix of the network changes (the old address
withdrawn), and withdraws it (lifetime 0) when turned off or stopped. The devices
keep the DNS server of the box too: the sure way is to have the box
announce Warda in place of its own server (see The IPv6 DNS of the
box; up to 0.7.9 the page said to turn it, or
IPv6, off in the box). Raw ICMPv6 socket (CAP_NET_RAW).