What is RDNSS?

RDNSS (Recursive DNS Server option) is an IPv6 Router Advertisement option that tells hosts which DNS resolvers to use, with no DHCPv6 needed.

With IPv6, a host can configure its own address from the prefix in a Router Advertisement (RA), with no DHCP server involved. An address alone is not much use without a resolver, though, and originally the only way to learn one was DHCPv6. The RDNSS option, standardised in RFC 8106, closes that gap: the router includes one or more resolver addresses, such as 2001:db8:53::53, with a lifetime saying how long the host may use them. A companion option, DNSSL, carries a DNS search list. Hosts pick both up from the same RA that gives them their prefix and default gateway, so a network can be fully configured with SLAAC and no stateful server at all.

RDNSS matters in practice because client support for DHCPv6 is uneven; Android, for example, has not implemented stateful DHCPv6 address assignment and relies on RAs. Many networks therefore send RDNSS and also run stateless DHCPv6 for clients that ask (the RA's "other configuration" flag tells hosts they may), so every operating system gets a resolver one way or the other. When IPv6 addresses work but names don't resolve over IPv6, a missing RDNSS option is a common cause. Keep the lifetime long enough that hosts don't drop the resolver between advertisements, and make sure the advertised address is a working resolver reachable from the LAN. See RA, SLAAC and NDP for the surrounding mechanism. The IPv6 on VyOS lesson configures Router Advertisements with an RDNSS option.